The Concise Valve Handbook, Volume I: Sizing and Construction [1 ed.] 9781947083660, 9781947083677

Table of contents :
Cover
Contents
List of Figures
List of Tables
Foreword
Chapter 1: Basic Principles
Chapter 2: Liquid Valve Sizing
Chapter 3: Gas Valve Sizing
Chapter 4: Valve Construction
Chapter 5: Valve Trim and Characterization
Chapter 6: Valve Selection
Glossary
Bibliography
About the Author
Index
Adpage
Backcover

Citation preview

The Concise Valve Handbook

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Research studies within the process industry routinely indicate

THE CONTENT

Michael A. Crabtree

functioning control systems. Furthermore, valves in general are consistently wrongly selected, regularly misapplied, and often incorrectly installed. This two-volume book comprises a comprehensive up-to-date body of knowledge that provides a total in-depth insight into valve and actuator technology—looking not just at control valves, but a whole host of other types including: check valves, shut-off valves, solenoid valves, and pressure relief valves. A methodology is presented to ensure the optimum selection of size, choice of body and trim materials, components, and ancillaries. Whilst studying the correct procedures for sizing, readers will also learn the correct procedures for calculating the spring ‘wind-up’ or ‘bench set’. Maintenance issues also include: testing for deadband/ hysteresis, stick-slip and non-linearity; on-line diagnostics; and signature analysis. Written in a detailed but understandable language, the two volumes are presented in a form suitable for both the beginner, with no prior knowledge of the subject, and the more advanced specialist.

THE TERMS • Perpetual access for a one time fee • No subscriptions or access fees • Unlimited concurrent usage • Downloadable PDFs • Free MARC records

AUTOMATION AND CONTROL COLLECTION

that the fluid control valve is responsible for 60 to 70% of poor-

For the last sixteen years, ‘Mick’ Crabtree, who holds an MSc in industrial flow measurement, has been involved in technical training and consultancy—running workshops on industrial instrumentation and networking throughout the world covering the fields of process control (loop tuning), process instrumentation, data communications, fieldbus, safety instrumentation systems (according to both ISA S84

The Concise Valve Handbook, Volume I

• Manufacturing Engineering • Mechanical & Chemical Engineering • Materials Science & Engineering • Civil & Environmental Engineering • Advanced Energy Technologies

Sizing and Construction, Volume I

CRABTREE

EBOOKS FOR THE ENGINEERING LIBRARY

The Concise Valve Handbook Sizing and Construction Volume I

and IEC 61508/61511), project management, on-line analysis, and technical writing and communications. This book represents some thirty years wealth of experiential

For further information, a free trial, or to order, contact:  [email protected]

knowledge gleaned by the author working within a wide variety of industries and from more than 6000 technicians and engineers who have attended the author’s workshops. ISBN: 978-1-94708-366-0

Michael A. Crabtree

The Concise Valve Handbook

The Concise Valve Handbook Sizing and Construction Volume I

Michael A. Crabtree

MOMENTUM PRESS, LLC, NEW YORK

The Concise Valve Handbook: Sizing and Construction, Volume I Copyright © Momentum Press®, LLC, 2018. All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted in any form or by any means—­ electronic, mechanical, photocopy, recording, or any other—except for brief quotations, not to exceed 400 words, without the prior permission of the publisher. First published by Momentum Press®, LLC 222 East 46th Street, New York, NY 10017 www.momentumpress.net ISBN-13: 978-1-94708-366-0 (print) ISBN-13: 978-1-94708-367-7 (e-book) Momentum Press Automation and Control Collection Cover and interior design by Exeter Premedia Services Private Ltd., Chennai, India 10 9 8 7 6 5 4 3 2 1 Printed in the United States of America

To my wife Pam—for her love and patience.

Abstract Research studies, within the process industry, routinely indicate that the final control element is responsible for 60% to 70% of poor-functioning control systems. Although valves themselves are consistently wrongly selected, ­regularly misapplied, and are often incorrectly installed, a large proportion of the blame may also be attributed to a number of associated ancillaries: the valve actuator, I/P converter, and positioner. Levelled at anyone working at a technical level in the process ­control industry, Volume I: Sizing and construction provides a total in-depth insight into valve technology. While studying both liquid and gas valve sizing, the guide also presents a methodology to ensure the optimum selection of type, size, body and trim materials, components, and ancillaries—­covering: control valves, check valves, shut-off valves, and solenoid valves. Volume II: Actuation, Maintenance, and Safety Relief, takes an in-depth look at actuators and positioners. This volume also explores a variety of maintenance and diagnostic issues including: testing for dead-band/hysteresis, stick-slip and non-linearity; on-line diagnostics; ­ signature analysis; and correct procedures for calculating the spring “wind-up” or “bench set.” A complete section is also devoted to the whole field of safety relief devices. Lastly, this volume covers a number of topics which are all too often  ignored: acoustics; water hammer; and even classification of ­stainless steel.

KeyWords ball valves, butterfly and plug valves, cavitation and flashing, characterization, construction, gas sizing, globe valves, leakage, liquid sizing, ­material selection, noise treatment, trim

Contents List of Figures

xv

List of Tables

xxv

Foreword

xxvii

Volume I 1   Basic Principles

1

1.1  The Final Control Element as Part of the Control Loop

2

1.2  Basic Theory

3

1.3  Equation of Continuity

3

1.4  Bernoulli’s Equation

5

1.5  Choked Flow

8

1.6  Pressure Recovery

9

1.7  Turndown Ratio and Rangeability

11

1.8  Velocity Profiles

12

1.9  Reynolds Number

13

1.10 Flashing and Cavitation

14

1.11 Flashing

15

1.12 Cavitation

16

1.13 Leakage Classification

18

1.14 Isolation Valve Leakage Classification

21

2   Liquid Valve Sizing

23

2.1  Practical Considerations

23

2.2  Application of Formulae

24

2.3  Sizing Example 1

27

x  •  Contents

2.4  Piping Geometry Factor

29

2.5  Sizing Example 2

31

3   Gas Valve Sizing

33

3.1  Pressure Drop Mechanism

33

3.2  Specific Heat Ratio Factor

38

3.3  Gas Expansion Factor

40

3.4  Valve Sizing

41

3.5  Sizing Example 1

43

4   Valve Construction

47

4.1  Globe Valve

48

4.2  Bonnet Assembly

49

4.3  PTFE (Teflon)

49

4.4  Laminated Graphite

50

4.5  Extended Bonnet

52

4.6  Bellows Seal Bonnet

52

4.7  Valve Trim

54

4.8  Guiding

55

4.9  Post-guiding

55

4.10 Top- and Bottom-guided Double Seat

56

4.11 Single-ported Balanced Globe Valve

57

4.12 Cage-guiding

58

4.13 Split Body Globe

59

4.14 Angle Is

60

4.15 Needle Valve

61

4.16 Bar Stock Body Valve

61

4.17 Gate Valve

61

4.18 Wedge Gate

62

4.19 Slab Valve

64

4.20 Expanding Gate Valve

65

4.21 Knife Edge Gate Valve

67

4.22 Pinch Valve

69

4.23 Diaphragm Valve

72

4.24 Rotary Control Valves

74

Contents   •   xi

4.25 Ball Valve

75

4.26 Trunnion Ball Valve

77

4.27 Characterized Ball Segment Valve

81

4.28 Butterfly Valve

82

4.29 Plug Valve

84

4.30 Eccentric Plug Valve

86

4.31 Check Valves

88

4.32 Valve Sizes and Pipe Schedules

88

4.33 Material Selection

90

4.34 Corrosion

90

4.35 Erosion

94

4.36 End Connections

94

4.37 Screwed End Connections

94

4.38 Flanged End Connections

95

4.39 Hub End Body

96

4.40 Welded End Connections

96

4.41 Lap Joint Flange

98

4.42 Flangeless Connections

99

4.43 Grayloc® Connector 5   Valve Trim and Characterization

100 103

5.1  Inherent Characteristics

103

5.2  Linear Inherent Flow Characteristic

103

5.3  Equal Percentage Inherent Flow Characteristic

104

5.4  Quick Opening Inherent Flow Characteristic

104

5.5  Modified Percentage Inherent Flow Characteristic

105

5.6  Characteristic Profiling

105

5.7  Installed Characteristics

105

5.8  Cavitation Control

108

5.9  Reducing Cavitation

110

5.10 Eliminating Cavitation

112

5.11 Noise Sources

113

5.12 Mechanical Noise

115

5.13 Hydrodynamic Noise

116

xii  •  Contents

5.14 Aerodynamic Noise

117

5.15 Noise Prediction

117

5.16 Noise control

117

5.17 Path Treatment

118

5.18 Insulation

119

5.19 Silencers

120

5.20 Source Treatment

120

5.21 Velocity Control

120

6   Valve Selection

123

Glossary

129

Bibliography

131

About the Author

133

Index

135

Volume II 7    Valve Actuators and Positioners 7.1  Pneumatic Control 7.2  Flapper–Nozzle Assembly 7.3  I/P Converter 7.4  Diaphragm Actuators 7.5  Springless Diaphragm 7.6  Advantages and Disadvantages of Diaphragm Actuators 7.7  Cylinder Actuators 7.8  Spool Block 7.9  Electro-Hydraulic Actuation 7.10 Electric Actuation 7.11 Torque Limiting 7.12 Hammer-Blow Mechanism 7.13 Solenoid Valve 7.14 Digital Actuators 7.15 Transfer Mechanisms 7.16 Valve Positioners 7.17 Positioner Guidelines

141 141 141 142 144 145 147 147 149 149 150 152 153 153 155 157 161 163

Contents   •   xiii

8   Valve Testing and Diagnostics

167

  8.1  Deadband and Hysteresis

167

  8.2  Testing Procedures

169

 8.3  Online Diagnostics

173

 8.4  Electronic Torque Monitoring

176

9   Valve Maintenance and Repair

179

 9.1   In-Line Repairs

180

 9.2   Repairs Under Pressure

180

  9.3   Repairs on Drained Systems

181

 9.4   Packing Replacement

181

  9.5   Replacing or Refinishing Seat Rings

181

 9.6   Other In-Line Repairs

182

 9.7   In-Line Post-Repair Procedures

182

 9.8   Shop Repairs

182

 9.9   Actuator Bench Set

183

 9.10  Spring Calculations

184

10  Safety Relief Valves

187

  10.1  History

187

  10.2  Definitions

190

  10.3  Weight-Loaded Pressure/Vacuum Relief Valves

191

 10.4  Spring-Loaded Relief Valves

192

 10.5  Applications

194

 10.6  Limitations

194

 10.7  Safety Valves

194

  10.8  Basic Operation: Lifting

196

  10.9  Basic Operation: Reseating

198

  10.10 Conventional Safety Relief Valves

200

  10.11 Balanced Safety Relief Valves

204

  10.12 Bellows-Type Balanced Safety Valve

204

  10.13 Piston-Type Balanced Safety Valve

206

  10.14 Non-Reclosing Pressure Relief Devices

211

  10.15 Conventional Rupture Disc

215

  10.16 Scored Tension-Loaded Rupture Disc

215

xiv  •  Contents

  10.17 Composite Rupture Disc

216

  10.18 Graphite Rupture Disc

217

  10.19 Burst Disc Applications and Installation Practices

218

  10.20 Performance Tolerance

219

  10.21 Maximum Operating Pressure

221

  10.22 Cyclic/Pulsating Duties

221

  10.23 Case A: 276 kPa (g) or Higher

222

  10.24 Case B: Lower than 276 kPa (g)

223

  10.25 Standards

223

Appendix A: J–T Valve

225

Appendix B: Basic Acoustics

227

Appendix C: Block and Bleed

241

Appendix D: Water Hammer

245

Appendix E: Stainless Steel

255

Glossary

263

Bibliography

265

About the Author

267

Index

269

List of Figures Figure 1.1. The four basic elements of a control system: process, transducer (sensing element plus transmitter), final control element, and controller.

2

Figure 1.2. Velocity and pressure distribution of a fluid flowing through a restriction (e.g., a valve orifice).

4

Figure 1.3. To allow the same amount of liquid to pass the velocity must increase.

4

Figure 1.4. Actual flow versus √∆P.

9

Figure 1.5. Streamlined valves dissipate less energy, and therefore have a higher pressure recovery than less streamlined valves.10 Figure 1.6. Streamlined valves have higher velocities at the vena contracta with subsequent lower pressures.

10

Figure 1.7. Comparison of rangeability and turndown.

11

Figure 1.8. A flat ‘ideal’ velocity profile.

12

Figure 1.9. A laminar ‘parabolic’ velocity profile.

12

Figure 1.10. A turbulent velocity profile.

13

Figure 1.11. Flashing occurs when the downstream pressure is at or below the fluid vapor pressure.

14

Figure 1.12. Damage due to flashing appears smooth and polished.

15

Figure 1.13. Cavitation is caused by the pressure dropping to the vapor pressure of the fluid and rising to a higher pressure further downstream.

16

Figure 1.14. Cavitation damage occurs when the bubbles collapse on or near solid surfaces within the valve or piping and material is chipped away.

16

Figure 1.15. Cavitation damage shows up as very dull, rough, and pitted—sponge-like in appearance.

17

xvi  •   List of Figures

Figure 1.16. Typical liquid noise characteristic (courtesy Fluids handling: Principles & Practice).

18

Figure 2.1. Flow is only proportional to √∆P within the subcritical region.26 Figure 2.2. Data on Fisher Emerson easy-e® ES (flow up) cage-guided globe valve having a linear characteristic with a valve opening to 100% of its total travel.

29

Figure 3.1. Initially, there is a straight-line relationship between the mass flow (w) and the pressure drop ratio (x).

35

Figure 3.2. As the pressure drop ratio (x) increases, there will come a point where the mass flow will no longer ­continue to increase at the same rate and will start to deviate from the straight line.

35

Figure 3.3. This deviation will continue until no further increase in the ‘pressure drop ratio’ will yield any additional flow. This is termed ‘choked flow’ and the point at which it occurs is the called terminal pressure drop ratio (XT). 35 Figure 3.4. At low flows, as the velocity increases, the pressure decreases and reaches a minimum at the vena contracta. 36 Figure 3.5. At some point, before choked flow occurs, the flow rate is still increasing and the velocity at the vena contracta becomes sonic. 37 Figure 3.6. As the pressure drop ratio increases, the vena contracta moves toward the physical restriction and its cross-­ sectional increases. When it backs up to the physical ­restriction, no further increase in flow is possible and is said to be choked.

37

Figure 3.7. Test performed on a globe valve set at 70% open.

38

Figure 3.8. Performance of two 50 mm valves having almost identical CV values of 60 when 70% open—one a ­linear globe valve and the other a high performance ­butterfly valve.

39

Figure 3.9. XT not only varies with different styles of valves, but also with the valve opening.

39

Figure 3.10. Data on Fisher Emerson easy-e ES (flow up) cage-guided globe valve having a linear characteristic with a valve opening to 100% of its total travel.

45

Figure 4.1. Simple classification of valve types.

48

®

List of Figures   •   xvii

Figure 4.2. A typical globe valve (courtesy Emerson–Fisher).

48

Figure 4.3. A typical packing box assembly comprising a number of plastic or composition washers that are retained by a packing flange. 50 Figure 4.4. Typical standard V-ring packing arrangement.

50

Figure 4.5. Typical double graphite packing arrangement.

51

Figure 4.6. Electrochemical series showing the electrode potentials referenced to hydrogen. 52 Figure 4.7. Extended bonnet in which the stuffing box is located further away from the process medium allows the ­process medium temperature to be extended up to 800°C or more (courtesy Emerson–Fisher).

52

Figure 4.8. Bellows stem seals are designed to eliminate valve leakage and normally supplement conventional packing. 53 Figure 4.9. The bellows are available as either a formed or welded construction.

53

Figure 4.10. In the seat retainer, the seat ring is clamped into the body by the bonnet and seat retainer (courtesy Valtek ­Control Products).

55

Figure 4.11. Single seat top-guided contact valve.

56

Figure 4.12. A double seat bottom-guided globe valve.

57

Figure 4.13. In the single-ported balance globe valve, the valve plug is made as a piston having an internal passage that permits the fluid pressure to communicate to both sides of the plug.

58

Figure 4.14. Cage-guided control valve.

59

Figure 4.15. The split-body, stem-guided globe caters for difficult flows with high viscosity.

59

Figure 4.16. Single-seat angle valve.

60

Figure 4.17. Split-body needle valve (courtesy Fisher Rosemount).

61

Figure 4.18. Bar stock bodies are often specified for corrosive applications (courtesy Emerson–Fisher).

62

Figure 4.19. A typical gate valve comprising a disk that slides up and down between seats.

63

Figure 4.20. The gate faces can be in the form of a wedge shape, sliding up and down between tapered seats.

63

Figure 4.21. (a) One-piece solid wedge. (b) Flex wedge.

64

xviii  •   List of Figures

Figure 4.22. In the slab valve, the gate is pushed against the seal by the flow pressure.

64

Figure 4.23. The moving section comprises a slab-shaped gate with a single port drilled through it.

65

Figure 4.24. The expanding gate valve comprises two segmented assemblies that are attached to each other by either a spring or lever mechanism.

65

Figure 4.25. When the gate is in its final upper (open) position, with the front segment against the upper stop, the lower back angles are in contact with each other (courtesy Daniel Valves).66 Figure 4.26. Exaggerated view showing how the opposing forces cause the gate and segment assembly to expand, sealing against both seats. 66 Figure 4.27. When the gate is in its final lower (closed) position, with the front segment against the bottom stop, the upper back angles are in contact with each other (courtesy Daniel Valves).

67

Figure 4.28. Exaggerated view showing how the opposing forces cause the gate and segment assembly to expand, ­sealing against both seats.

67

Figure 4.29. The knife edge gate valve is an excellent valve for service that requires either full or no flow.

68

Figure 4.30. The V-insert gate valve.

69

Figure 4.31. Flow/travel characteristics for different variations of the sliding gate valve.

69

Figure 4.32. Sliding gate regulator valve makes use of slotted movable disc and a stationary slotted plate (courtesy Jordan Valve).

70

Figure 4.33. (a) When throttled open, the orifices of the disc align with the openings of the plate to allow the required flow to pass through the slots. (b) When the valve is closed, the disc and plate form a solid barrier to flow (courtesy Jordan Valve). 70 Figure 4.34. Sliding gate valve using a stationary plate and a rotating disk.

71

Figure 4.35. (a) The pinch valve comprises a rubber hose, which ­normally provides full flow. (b) When pinched together, the flow stops. 71

List of Figures   •   xix

Figure 4.36. Double vise mechanism in which the clamps move together and pinch the valve closed at the center line (a) open and (b) closed.

72

Figure 4.37. In the air-operated pinch valve, the valve body acts as a built-in actuator, requiring an actuation pressure of ­approximately 2 to 3 bar over line pressure for closure (courtesy Red Valve).

72

Figure 4.38. In the Saunders patent, flow through the body is over a transverse weir (a) and the valve is closed by means of a flexible rubber or synthetic dome-shaped diaphragm (b) (courtesy Crane Process Flow Technologies Ltd.)

73

Figure 4.39. Straight-through diaphragm valves, exemplified by the Saunders full bore K/KB type, have a smooth non-­ ­turbulent body design and are designed for m ­ inimum flow resistance while allowing rodding out and easy ­cleaning (courtesy Crane Process Flow Technologies Ltd.).

74

Figure 4.40. In the ‘floating’ ball valve, the ball is completely supported by two seats that provide bearing support to the ball s­ egment, with fully closed to fully open ­performed by a 90° rotation of the plug segment.

75

Figure 4.41. In the split-body design, the split between the body and cap is off-center so that the stuffing box is left intact when the ball or seating is removed.

76

Figure 4.42. End-entry design in which the body comprises a single piece, with the ball retained by an insert screwed into the body at one end.

76

Figure 4.43. The trunnion ball valve distributes excess hydraulic load into the valve body, rather than through the seating ­(courtesy ITT ­Industries, Inc.). 77 Figure 4.44. Leakage integrity is ensured through the use of floating spring-loaded seats that are kept in contact with the ball even in the absence of line pressure. 78 Figure 4.45. As the line pressure increases the seat area creates a piston effect that forces the seat against the ball. Sealant injection points allow emergency sealant injection to be carried out in the event of seat-insert or stem-seal damage that could lead to external or ­internal  leakage.

78

xx  •   List of Figures

Figure 4.46. In the event that leakage past the seals does occur, and in order to cater for double block-and-bleed a­ pplications, a body vent allows the body cavity to be vented to ­atmosphere. 79 Figure 4.47. (a) In its fully open position, the ball valve presents a single circular orifice to the flowing medium. (b) As the ball is rotated toward its closed position, the shape of the opening changes to become two identical elliptical ­orifices that offer two equal restrictions in series. 80 (c) Closed ­position. Figure 4.48. Pathway fitted with parallel perforated attenuator plates that produce a smooth gradual pressure reduction across the valve that minimizes velocity, noise 80 generation, and cavitation (courtesy Neles-Metso). Figure 4.49. Characterized V-notch ball valve in which the opening between the ball and seal is modified to provide different 81 flow characteristics (courtesy Emerson–Fisher). Figure 4.50. Different flow characteristics of the characterized ball valve.81 Figure 4.51. Ball segment valve with centrally mounted shaft (courtesy Somas Instrument AB).

82

Figure 4.52. Ball segment valve with eccentrically mounted shaft, ­allowing pressure between the segment and seat to be increased by increasing torque (courtesy Somas ­Instrument AB).

82

Figure 4.53. Butterfly valve comprises a circular disc-shaped damper mounted on a shaft. 83 Figure 4.54. Conventional center-disc butterfly valve.

83

Figure 4.55. Offset disc, or high performance butterfly valve.

84

Figure 4.56. (a) conventional disc shape (b) the fishtail disc (courtesy Emerson–Fisher).

84

Figure 4.57. Flow characteristics of different-shaped butterfly discs. 85 Figure 4.58. Basic plug valve.

85

Figure 4.59. Camflex: eccentric rotary plug valve (courtesy ­ Masoneilan).86 Figure 4.60. Once seating occurs, a positive seal between plug and seat is achieved by the elastic deformation of the plug arms (courtesy Masoneilan).

87

List of Figures   •   xxi

Figure 4.61. In the hinged ‘swing check valve,’ the sealing disc is attached to a hinge, which is free to rotate around the hinge pin.

88

Figure 4.62. The pinch check valve feature: a typical 30-year ­operational lifespan, a low headloss; they do not rust or ­corrode; they are not affected by UV; and their flexibility allows them to compress around trapped ­solids (courtesy Tideflex).

89

Figure 4.63. Screwed end valve connections with tapered female thread.94 Figure 4.64. Maximum pressure rating versus temperature for ­carbon steel flanges (ANSI B16.5).

95

Figure 4.65. Flat-face flanged end connection.

96

Figure 4.66. The raised-face flange has a circular raised face.

96

Figure 4.67. Hub end or separable flange body.

97

Figure 4.68. Socket welding end.

97

Figure 4.69. Butt welding ends.

97

Figure 4.70. The lap joint flange is used in conjunction with a lap joint stub end that is butt-welded onto the process pipeline (courtesy Coastal Flange Inc.).

98

Figure 4.71. Flangeless valves are held between flanges by long through-bolts.99 Figure 4.72. The Grayloc® connector comprises three components: a metal seal ring, a two-part clamp assembly, and two hubs (courtesy Oceaneering International). 100 Figure 4.73. (a) The clamp assembly fits over the two hubs and forces them against the seal ring rib. (b) As the hubs are drawn together, the seal ring lips deflect against the inner sealing surfaces of the hubs (courtesy Oceaneering International).

101

Figure 5.1. Inherent flow characteristics.

104

Figure 5.2. Quick opening flow characteristic uses a simple disc shaped plug (courtesy Fisher Rosemount).

105

Figure 5.3. Plug outlines used to obtain (from left to right) equal percentage; linear; and quick-opening flow ­characteristic ­(photographs courtesy of Mitech).

106

xxii  •   List of Figures

Figure 5.4. For cage-guided trim, flow characterization is determined by the shape of windows in the cylindrical cage (a) equal percentage (b) linear and (c) quick-opening flow ­characteristics. 106 Figure 5.5. Differential pressure drop is actually distributed across both the valve and other parts of the system. 106 Figure 5.6. Installed linear flow characteristic.

107

Figure 5.7. Installed equal percentage flow characteristic.

108

Figure 5.8. Trim selection guide—liquid applications (courtesy Mitech).109 Figure 5.9. Plug hard facing variations: from left to right: seat surface; full contour; lower guide area; and full ­contour and lower guide (courtesy Valtek Control ­Products).

110

Figure 5.10. High differential pressure across the valve can lead to cavitation.

110

Figure 5.11. Cavitation across the valve can be avoided by installing a choke. 111 Figure 5.12. Cavitation control seat retainer (courtesy Mitech).

111

Figure 5.13. Mitech’s ZZ seat retaining energy dissipating device for small diameter valves and pressures above 40 bar (courtesy Mitech).

112

Figure 5.14. Disk stack design of globe valve trim (courtesy Mitech).113 Figure 5.15. Some relative noise levels for common sounds and ­activities.

114

Figure 5.16. Typical liquid noise characteristic (courtesy Fluids ­handling: Principles Practice).

116

Figure 5.17. There are two main elements to noise treatment: path treatment and source treatment.

118

Figure 5.18. Cut-away section of an in-line silencer making use of an inlet ­diffuser to break up turbulence. (courtesy Valtek ­Control Valves, Flowserve Corporation).

120

Figure 5.19. Simple low noise retainer device (courtesy Mitech).

121

Figure 5.20. Simple diffuser plate (courtesy Mitech).

121

List of Figures   •   xxiii

Figure 5.21. Tortuous path in which the fluid undergoes a series of contractions, expansions, direction changes and splits in axial, radial and circumferential directions (courtesy Emerson-Fisher).122 Figure 6.1. Control valve selection guide for gases and liquids (courtesy Mitech).

126

Figure 6.2. Control valve body selection guide (courtesy CSD Controls).

127

Figure 6.3. Control valve trim selection guide (courtesy CSD Controls).

127

Figure 6.4. Actuator and accessory selection guide (courtesy CSD Controls).

128

List of Tables Table 1.1. Comparison of the flow coefficient, CV, of a number of different types of valve at different sizes

8

Table 1.2. Typical numerical values of FL for some different valve styles

11

Table 1.3. Control valve seat leakage classifications according to ANSI/FCI 70-2-1991

19

Table 1.4. Class VI maximum seat leakage according to ANSI/FCI 70-2-1991

20

Table 1.5. Leakage rates for different-sized isolation valves based on API 598

21

Table 2.1. Saturated vapor pressure levels for water at different temperatures

28

Table 3.1. Results of test show that XT (the choked value of x) remains constant at 0.72 in each instance

38

Table 4.1. Preferred series of ANSI and DN pipe sizes

89

Table 4.2. Example of some pipe schedules based on ASME standards B36.10M and B36.19M

91

Table 4.3. Some of the most common cast materials

93

Table 4.4. Preferred series of ANSI class and PN pressures

95

Table 5.1. Valve characteristic selection guide

108

Table 5.2. Permissible personnel noise level exposure and time levels according to OSHA

114

Table 5.3. Typical pipe wall attenuation (dBA) for a carbon steel pipe according to different pipe schedules and pipe sizes

119

xxvi  •   List of Tables

Table 6.1. Comparison table: rating from 1 down to 5 (1 = highest rating 5 = poorest rating) (courtesy Crane Process Flow Technologies Ltd.)

124

Table 6.2. Comparison of different control valves where: 1 = good; 2 = average; 3 = poor; and x = not suitable (courtesy Mitech)

125

Foreword In this book, ‘The Concise Valve Handbook—Part 1. Sizing and construction,’ I have made use of a building-block approach, presenting material in a form suitable for two distinct classes of reader: the beginner, with no prior knowledge of the subject and the more advanced specialist. The complete text is suitable for the advanced reader. However, those parts of the text that involve a mathematical treatment, which are not required by the beginner, are indicated by a mark ► at the beginning and ◄ at the end. Consequently, for the beginner, the text may be read, with full understanding, by ignoring the marked sections. I offer no apologies for my preference for metric-based measurement: the SI system. Apart from the United States, only two other countries in the world still adhere to the fps system (foot-pound-second)—the so-called imperial system first defined in the British Weights and Measures Act of 1824—Myanmar (Burma) and Liberia. I have tried to mix it up as far as possible, and I have got a units conversion table right in the front of the book. But, for the moment, just try for the following: 1 bar = 100 kPa ≈ 1 atmosphere ≈ 14.7 psi 1 inch = 25.4 mm 20°C = 68 °F 100°C = 212 °F And lastly, while I have made some compromises (analog instead of analogue; program instead of programme), I reserve my right to spell according to the British system:

xxviii  •  Foreword

English Metre Litre Fibre Colour Vapour

United States Meter Liter Fiber Color Vapor

Unit Conversions Quantity Distance

Area

Volume

Mass Force Pressure

SI 25.5 mm 1 millimetre 1m 1m 0.9144 m 1 square metre (m2) 1 square metre (m2) 1 square metre ­millimetre (mm2) 1 cubic metre (m3) 1 cubic metre (m3) 0.02832 m3 1 litre 1 litre 1 litre 3.785 litres 1 kg 454 g 1N 4.448 N 1 bar 1 kPa (kN/m2) 6.895 kPa 1 psi

United States customary 1 in 0.03937 in 39.37 in 3.281 ft 1 yd 1550 in2 10.76 ft2 0.00155 in2 61.02 in3 35.31 ft3 1 ft3 61.02 in3 0.03531 ft3 0.2642 gal 1 gal 2.205 lb 1 lb 0.2248 lbf 1 lbf 14.504 lbf/in2 (psi) 0.145 lbf/m2 (psi) 1 psi 0.0361 inches H2O (in WC)

Foreword   •   xxix

Quantity Temperature Flow rate

SI K °C 1 m3/h 1 kg/h

United States customary 1.800 °R 1.8 °C + 32 = °F 4.403 gal/min (gpm) 2.205 lb/h

CHAPTER 1

Basic Principles In any process control loop, the final control element is the mechanism that changes the value of the manipulated variable in response to the output signal from the control unit. The final control element comprises the actuator, with its associated and linkage and positioner, and the final control element proper—valve, pump, transformer, motor, variable speed drive, and so on. It is axiomatic that many of the problems associated with control loop performance can also be laid at the door of the final control element. Fluid properties can vary enormously from industry to industry. The fluid may be toxic, flammable, abrasive, radioactive, explosive, or corrosive; it may be single-phase (clean gas, water, or oil) or multi-phase (e.g., slurries or dust-laden gases). The pipe carrying the fluid may vary from less than 1 mm to many in diameter. The fluid temperature may vary from close to absolute zero to several hundred degrees Celsius, and the pressure may vary from high vacuum to high pressures. Many different types of valves have been developed to suit these variations in fluid properties and flow applications. However, only a few valves have found widespread application, and no one single valve can be used for all applications. The basic purpose of a control valve is to control the flow of a medium in a pipe, either turning it on or off or varying it continuously. However, a control valve designed primarily to throttle energy is not necessarily designed for shut-off purposes, and these two requirements often have to be balanced or realized in separate systems. In the fairly certain knowledge that prefaces or forewords are rarely read, I’m taking this opportunity to invite you to go back and peruse the foreword since there are a couple of fairly important points that you really need to understand.

2  •   The Concise Valve Handbook

1.1 The Final Control Element as Part of the Control Loop Routinely, research studies within the process industry indicate that the final control element is responsible for 60% to 70% of poor-functioning control systems. The problems lie not just with the valve itself, but also with the valve actuators, I/P converters, and positioners. However, probably the majority of problems can be attributed to oversized valves and undersized actuators. The first step in successful application is to gain an understanding of the basics of a control system. As shown in Figure 1.1, the four basic elements of any control system comprise: • • • •

process; transducer (sensing element plus transmitter); final control element; and controller.

Again referring to Figure 1.1, the controlled variable is called the Process Demand (PD) or Manipulated Variable (MV), or simply the OP, while the measured variable is called the Process Demand. If the PD is subject to a step change, by how much will the PV change? This is determined by what is called the process gain (KP) and is given by dividing the percentage change in the PV by the percentage change made by the PD: Equation Chapter (Next) Section 1 Kp =



PV (%)  PD (%)

Controller Process Demand (PD) Process Variable (PV)

Process Transducer (sensor + transmitter)

Final control element

Figure 1.1.  The four basic elements of a control system: process, transducer (sensing element plus transmitter), final control element, and controller.

(1.1)

Basic Principles   •  3

Thus, for example, if we make a step change of 20% to the PD and the PV also changes by 20%, then the process gain (KP) is 1. However, if the PV only changes by 10%, then the process gain (KP) is 0.5. Alternatively, if the PV changes by 60%, then the process gain (KP) is 2. Generally, the process gain should lie between the 0.5 and 2.0. If it is less than 0.5, then typically, the transmitter span is too wide for good control. If the process gain is greater than 2, this is usually an indication that the control valve is oversized.

1.2 Basic Theory A control valve can be simply represented as a restriction in a pipeline that creates a pressure drop, or head loss, that bears a relationship to the flow rate. Because the flow entering the restriction is equal to the flow exiting the restriction, a reduction in cross-sectional flow area at the restriction must yield an increase in fluid velocity. The velocity increase is accompanied by a proportional decrease in pressure that results from an energy tradeoff where potential energy is converted to kinetic energy. The point of minimum cross-sectional flow area, maximum velocity, and minimum pressure is called the vena contracta. This physical phenomenon is based on two well-known equations: the equation of continuity and Bernoulli’s equation. Fluid flow through a valve orifice is illustrated in Figure 1.2. Assume that a fluid of density ρ flowing in the pipe of area A1 has a mean velocity v1 at a line pressure P1. It then flows through the restriction of area A2 where the mean velocity increases to v2 and the pressure falls to P2.

1.3 Equation of Continuity The equation of continuity states that, for an incompressible fluid, the volume flow rate, Q, must be constant. Very simply, this indicates that when a liquid flows through a restriction, then in order to allow the same amount of liquid to pass (to achieve a constant flow rate), the velocity must increase (Figure 1.3). ►Mathematically:

Q = v1.A1 = v2.A2

(1.2)

4  •   The Concise Valve Handbook V1 P1

VVC PVC

V3 P2

Vena Contracta Unrecoverable pressure loss

Pressure

∆P Velocity

Distance

Figure 1.2.  Velocity and pressure distribution of a fluid flowing through a restriction (e.g., a valve orifice).

QOUT = v2. A2

Velocity

QIN = v1. A1

Distance

Figure 1.3.  To allow the same amount of ­liquid to pass the velocity must increase.

where: v1 and v2 and A1 and A2 are the velocities and cross-sectional areas of the pipe at points 1 and 2, respectively.

Basic Principles   •  5

1.4 Bernoulli’s Equation In its simplest form, Bernoulli’s equation states that, under steady flow conditions, the total energy (pressure + kinetic + gravitational) per unit mass of an ideal fluid (i.e., one having a constant density and zero viscosity) remains constant along a flow line.

v12 P1 v 22 P2  + = + ρ 2 ρ 2

(1.3)

where: v = velocity at a point in the streamline; P = pressure at that point; ρ = fluid density; g = acceleration due to gravity; z = level of the point above some arbitrary horizontal reference plane with the positive z-direction in the direction opposite to the gravitational acceleration; and k = constant In the restricted section of the flow stream, the kinetic energy (dynamic pressure) increases due to the increase in velocity, and the potential energy (static pressure) decreases. Relating this to the conservation of energy at two points in the fluid flow, then:

v12 P1 v 22 P2  + = + ρ 2 ρ 2

(1.4)

Multiplying through by ρ gives:

1 1 ρ .v12 + P1 = .ρ.v 22 + P2  2 2

(1.5)

1 1 P1 − P2 = .ρ.v 22 − .ρ.v12  2 2

(1.6)

or: or:

∆P =

1 1 ρ .v 22 − .ρ.v12  2 2

(1.7)

6  •   The Concise Valve Handbook

where:

∆P = P1 − P2 

(1.8)

Now from the continuity equation (1.2), we can derive:

v1 =

Q  A1

(1.9)

v2 =

Q  A2

(1.10)

and: substituting in (1.7): 2



2

1  Q 1  Q ∆P = .ρ.  − .ρ.     2  A2  2  A1 

(1.11)

Solving for Q:



Q = A2 .

2∆P ρ A  1−  2   A1 

2



(1.12)

Since it is more convenient to work in terms of the diameters of the restriction (d) and the ID (inside diameter (D)) of the pipe, we can ­substitute for:

A1 =

À.D 2  4

(1.13)

A2 =

π.d 2  4

(1.14)

and: to give:



Q=

π.d . 4 2

2.∆P ρ  d 1−    D

4



(1.15)

Basic Principles   •  7

The term:

1  d 1−    D

4



(1.16)

is called the ‘Velocity of Approach Factor’ (EV), and by substituting in (1.15), we have:

Q = E v .d 2 .

2.∆P  ρ

(1.17)

Unfortunately, equation (1.17) only applies to perfectly laminar, inviscid flows. In order to take into account the effects of viscosity and turbulence, a term called the discharge coefficient Cd is introduced that marginally reduces the flow rate (Q). The full equation for an incomprehensible fluid thus becomes:

Q = Cd .E v .d 2 .

2∆P  ρ

(1.18)

In valve technology, this equation has been modified as follows:

Q = Cv

∆P  SG f

(1.19)

Q = flow rate; CV = valve flow coefficient; ∆P = differential pressure (P1–P2); and SG = specific gravity of fluid (water at 60°F = 1.0). The valve flow coefficient, CV, is an index used to measure the capacity of a control valve. CV is determined experimentally, using water as the test fluid, for each style and size of valve with the valve either fully open or at a given valve opening, usually stated as a percentage of maximum travel. Numerically, CV is defined as “the number of US gallons per minutes of water at 60°F that will pass through a given flow restriction, with a pressure drop of 1 psi across the valve.” CV is thus an index that allows the liquid capacities of different valves to be compared under a standard set of conditions. Table 1.1 compares the flow coefficient of a number of different types of valve at different sizes.

8  •   The Concise Valve Handbook

Table 1.1.  Comparison of the flow coefficient, CV, of a number of different types of valve at different sizes

Valve type Globe: cage-guided Globe: stem-guided Full ball V-notch ball Butterfly: standard Butterfly: high performance

Cv (typical at 100% open) Valve Size 50 mm 100 mm 200 mm 70 240 850 45 180 N/A N/A 500 2,200 175 600 1,820 90 520 2,820 90 490 2,170

The metric equivalent of CV is KV, defined as “the number of cubic metres per hour of water at 15°C that would pass through a valve with a 1 bar pressure drop across it.” Although many valve manufacturers now list the KV values in their data sheets, CV still remains the almost universal standard, despite its imperial background. Nonetheless, many users prefer to work with metric standards and make use of CV:

Q = 0.87 C v

∆P  SG f

(1.20)

where the volumetric flow rate (Q) is expressed in m3/hr and the differential pressure drop (∆P) is expressed in bars.

1.5 Choked Flow Equation (1.19) implies that, for a given valve, simply increasing the pressure differential across the valve can continually increase the flow. In reality, this relationship only holds true for a limited range. This is illustrated in Figure 1.4 that shows a typical plot of actual flow versus √∆P through a flow restriction. This indicates that flow is only proportional to √∆P within the sub-critical flow region. If the differential pressure is further increased, a point is reached where no further flow increase occurs, despite increasing the differential pressure. This is termed as choked flow (also known as the critical flow) and is the maximum flow rate possible through that valve. Decreasing the downstream pressure will not result in an increased flow

Basic Principles   •  9 Sub-critical

Critical

Flow

Choked flow Incipient cavitation

Cv

∆P

Figure 1.4.  Actual flow versus √∆P.

rate, although the valve can handle the higher pressure drop with no detrimental effects. In gases, choked flow occurs when the velocity reaches the speed of sound (Mach 1). For liquids, the speed of sound is extremely high, and practically speaking, incompressible fluids do not choke. In practice, however, as the differential pressure is increased and the velocity increases, the pressure at the vena contracta decreases. If the vena contracta pressure falls to below the vapor pressure of the liquid, partial vaporization occurs and the sonic velocity of the resultant liquid/vapor mixture falls dramatically. At this point, choked flow occurs. The practical consideration of choked flow is that, when calculating the CV required for a particular application, only the choked pressure drop can be used in the formulae and not the actual pressure drop. This results in a larger CV requirement than would otherwise be the case. If choked flow is not taken into account, it is possible to select a valve that is too small.

1.6 Pressure Recovery The ability of the control valve to reconvert kinetic energy downstream of the restriction back into pressure is a characteristic known as pressure recovery. The degree of pressure recovery varies from valve to valve. Considering two valves with equal flow, a streamlined valve will dissipate less energy through the restriction and will, therefore, have more energy downstream for recovery to a higher pressure. Conversely, in a less streamlined valve, larger amounts of energy are dissipated through

10  •   The Concise Valve Handbook

the restriction, and therefore, less energy will be available downstream for recovery to a higher pressure (Figure 1.5). Alternatively, streamlined valves produce relatively higher velocities through their restriction than do less streamlined, restrictive valves. Velocity, being inversely proportional to pressure, suggests lower pressure at the vena contracta with high-recovery streamlined valves (Figure 1.6). The amount of pressure recovery varies with valve style and stroke and is a function of the upstream, vena contracta, and downstream pressures. It is quantified by what is called the pressure recovery coefficient FL—a dimensionless expression of the pressure recovery ratio in a control valve that is mathematically represented as:

FL =

P1 − P2  P1 − Pvc

(1.21)

P1 Pressure

High recovery P2 Low recovery PVC

Distance

Figure 1.5.  Streamlined valves dissipate less energy, and therefore have a higher pressure recovery than less streamlined valves.

Pressure

P1 Low recovery

P2

PVC High recovery Distance

Figure 1.6.  Streamlined valves have higher velocities at the vena contracta with subsequent lower pressures.

Basic Principles   •  11

Table 1.2.  Typical numerical values of FL for some different valve styles

FL FL2

Cage-guided Standard globe globe 0.9 0.85 0.81 0.72

Disk 60° 0.75 0.56

Disk 90° 0.5 0.25

Ball 90° 0.6 0.36

where: P1 = upstream pressure; PVC = vena contracta pressure; and P2 = downstream pressure. Evaluation of the preceding expression suggests that high-recovery valves will result in less pressure drop, P1–P2. Therefore, high-recovery valves have a low value of FL, and low recovery valves have high values of FL—where FL is always less than 1.0. FL is determined by laboratory tests and cataloged by most valve manufacturers for use in more precise determination of valve capacity during critical flow. It also is useful in predicting damaging phenomena such as cavitation. Some typical numerical values of FL are given in Table 1.2. Note: FL is the ISA nomenclature and that sometimes the symbol Cf is used. Fisher Rosemount formerly made use of the symbol Km where:

FL = K m 

(1.22)

1.7 Turndown Ratio and Rangeability In our discussions on CV, it should be noted that two definitions are ­frequently used and confused. Turndown relates to the application and is the ratio of the calculated CV at maximum conditions to the calculated CV at minimum Rangeability applies to the valve and is the ratio of the CV of the valve fully open to the minimum CV at which it can control. On this basis, the rangeability of the selected valve must exceed the turndown requirements of the application. This is illustrated in Figure 1.7. Calculated CV at min

Calculated CV at max

Turndown of application Min. controllable C

CV fully open Rangeability of selected valve

Figure 1.7.  Comparison of rangeability and turndown.

12  •   The Concise Valve Handbook

1.8 Velocity Profiles One of the most important fluid characteristics affecting valve performance is the shape of the velocity profile in the direction of flow since the predictions used in sizing calculations are based on a fully developed turbulent flow. In a frictionless pipe in which there is no retardation at the pipe walls, a flat ‘ideal’ velocity profile would result (Figure 1.8) in which all the fluid particles move at the same velocity. Real fluids, however, do not ‘slip’ at a solid boundary, but are held to the surface by the adhesive force between the fluid molecules and those of the pipe. Consequently, at the fluid/pipe boundary, there is no relative motion between the fluid and the solid. At low flow rates, the fluid particles will move in straight lines in a laminar manner, with each fluid layer flowing smoothly past adjacent layers with no mixing between the fluid particles in the various layers. As a result, the flow velocity increases from zero, at the pipe walls, to a maximum value at the center of the pipe, and a velocity gradient exists across the pipe. The shape of a fully developed velocity profile for such a laminar flow is parabolic, as shown in Figure 1.9, with the velocity at the center being equal to twice the mean flow velocity. Clearly, this concentration of velocity at the center of the pipe can compromise the flow computation if not corrected for.

Figure 1.8.  A flat ‘ideal’ velocity profile.

Figure 1.9.  A laminar ‘parabolic’ velocity profile.

Basic Principles   •  13

Figure 1.10.  A turbulent velocity profile.

For a given pipe and liquid, as the flow rate increases, the flow of a fluid will cease to be laminar and becomes turbulent. In turbulent flow, the paths of the individual particles of fluid are no longer straight, but intertwine and cross each other in a disorderly manner so that thorough mixing of the fluid takes place. As shown in Figure 1.10, the velocity profile for turbulent flow is flatter than for laminar flow, and thus closer approximates to the ‘ideal’ or ‘one dimensional’ flow.

1.9 Reynolds Number The onset of turbulence is often abrupt, and in order to be able to predict the type of flow present in a pipe, for any application, use is made of the Reynolds number, Re—a dimensionless number given by:

Re =

ρ .V.D  µ

(1.23)

where: ρ = density of fluid (kg/m3); µ = viscosity of fluid (Pa.s); v = mean flow velocity (m/s); and D = diameter of pipe (m). In practice, the Reynolds number is a ratio of the viscous and inertial forces. If the viscous forces dominate (Re < 2,000), the flow is laminar, and if the inertial forces dominate (Re > 3,000), the flow is turbulent. At Re 2,000 to 3,000, the flow is said to be transitional. The major significance of changes in the flow regime between turbulent and laminar flow is that, for turbulent flow, the pressure loss is proportional to the square of velocity and in laminar flow the losses are linearly proportional to the velocity. This means that, for equivalent flow

14  •   The Concise Valve Handbook

rates, the differential pressure across the valve will be different for each flow regime. As stated earlier, the predictions used in sizing calculations are based on a fully developed turbulent flow in which inertial forces dominate. If, due to a change in viscosity, the flow regime changes, then the predicted calculations are no longer valid. As a result, a correction factor can be applied where: CVR = FV .CV 



(1.24)

where: CVR = required CV; FV = Reynolds number factor; and CV = valve flow coefficient.

1.10 Flashing and Cavitation Flashing and cavitation are two related physical phenomena and are the most common causes of control valve failure. Flashing and cavitation cause structural damage to the valve and adjacent piping, and in order to reduce or compensate for these undesirable effects, it is important to understand the changes that occur to the medium as it passes through the valve. Flashing and cavitation only occur within liquids and takes place whenever the internal pressure of the liquid falls below the vapor pressure. This is illustrated in Figure 1.11 that shows a pressure gradient curve through a valve where PV represents the vapor pressure of the flowing

Pressure

P1

Vapour pressure PV P2 PVC Distance

Figure 1.11.  Flashing occurs when the downstream pressure is at or below the fluid vapor pressure.

Basic Principles   •  15

fluid. As the liquid passes through the restriction, there is a decrease in pressure. When the pressure at the vena contracta reaches the vapor pressure of the fluid, vapor bubbles begin to form. In effect, the liquid is said to be boiling. In water, at 100°C, this occurs at normal atmospheric pressure of 1.01 bar. However, at 20°C, the pressure would need to be reduced to 23.8 mbar before vaporization occurs.

1.11 Flashing If, as shown in Figure 1.11, the outlet pressure (P2) of the valve is at or below the vapor (PV), the vapor bubbles remain intact and proceed further downstream. This is known as flashing. The vaporization of liquid causes a large increase in volume, and therefore higher overall fluid velocity. Liquid droplets suspended in a high-velocity vapor flow stream impinging on metallic surfaces can cause physical damage to carbon steel and cast iron. The damage is smooth and polished, similar to erosion (Figure 1.12). The majority of flashing damage occurs at the point of highest velocity—at or near the seat line of the valve plug or seat ring. The process is two-stepped: a corrosion film forms at the surface that is subsequently ‘swept’ away by the high-velocity liquid flow. This cycle is then repeated. The noise associated with flashing is a high-pitched hissing sound, similar to that of sand passing through the valve.

Figure 1.12.  Damage due to flashing appears smooth and polished.

16  •   The Concise Valve Handbook

1.12 Cavitation If, further downstream, the outlet pressure (P3) of the valve recovers to a point above the vapor pressure of the fluid, the vapor bubbles will collapse (Figure 1.13). This two-stage phenomenon, vapor bubble formation and their subsequent collapse, is known as cavitation. Implosion of the vapor bubbles produces large pressure shocks due to microjet or spherical shock waves. If the vapor bubbles are close to or in contact with a solid wall, pressure shocks of the order of 100,000 bar (1.5 million psi) are generated, producing both noise and physical damage. If the bubbles collapse on or near solid surfaces, material is chipped away (Figure 1.14). The amount of damage in a short period of time can be P1

Pressure

P2 Vapour pressure PV

PVC Distance

Figure 1.13.  Cavitation is caused by the pressure dropping to the vapor pressure of the fluid and rising to a higher pressure further downstream.

Figure 1.14.  Cavitation damage occurs when the bubbles collapse on or near solid surfaces within the valve or piping and material is chipped away.

Basic Principles   •  17

extensive and eventually prevents the control valve from performing its intended function. Because cavitation damage occurs where the bubbles collapse, the effects will be evident downstream of the restriction and will show up as very dull, rough, and pitted—sponge-like in appearance (Figure 1.15). Corrosion, often mistaken as cavitation, takes on a similar appearance with its affected area being more generalized or widespread. The noise associated with cavitation is a high-pitched hissing sound—usually accompanied by vibration—similar to that of gravel passing through the valve. The noise levels produced are rarely a problem to the surrounding environment, but can be used as a good indication of the severity of the cavitation (Figure 1.16). Should cavitation be allowed to continue, the next manifestation will be a loss of seat tightness as the damage starts to affect the seating surfaces. Further use will cause the normal controlling position to progressively reduce, as the valve has to move toward the closed position to compensate for the wear taking place. As devastating as cavitation damage is, it is fortunate that its occurrence is associated with few process fluids. Hydrocarbon mixtures, such as gasoline, do not have a fixed vapor pressure and will ‘boil’ over a relatively wide temperature range. Gasoline boils from 40°C to 200°C. It is felt that this results in an apparent buffeting effect, protecting the body wall and other vital valve components. On the other hand, high surface tension associated with water enhances the damage potential due to the high related implosion stresses. In addition to collapse in the vicinity of surfaces, the potential for damage is high when flowing fluid:

Figure 1.15.  Cavitation damage shows up as very dull, rough, and pitted—sponge-like in appearance.

18  •   The Concise Valve Handbook 110

100

SPL dBA

90

80

70

Full cavitation Incipient cavitation

60

50 0.02

0.04

0.06

0.1

0.2 ΔP P1 − P 2

0.3

0.4

Flashing

0.6 0.8 1.0

Figure 1.16.  Typical liquid noise characteristic (courtesy Fluids handling: Principles & Practice).

• has well-defined vapor pressure; • has high surface tension; and • is not a mixture.

1.13 Leakage Classification At the beginning of this chapter, we stated that a control valve designed primarily to throttle energy is not necessarily designed for shut-off purposes. In order to quantify the ability of a control valve to provide adequate shut-off, when closed, the American National Standards Institute (ANSI) has produced a set of standardized testing procedures for control valves. The ANSI/FCI 70-2-1991 standard uses six different classifications to describe the valves seat leakage capabilities. These are outlined in Tables 1.3 and 1.4. What do these figures mean in practical terms? From equation (1.20), we had:

Q = 0.87 C v

∆P  SG f

(1.25)

Basic Principles   •  19

Table 1.3.  Control valve seat leakage classifications according to ANSI/ FCI 70-2-1991 Maximum permitted Test Test Class leakage medium pressures Testing procedure I No test required provided that both supplier and user agree II 0.5% of Air or 3 to 4 bar or Pressure applied to valve inlet, rated water maximum with outlet open operating capacity at 10 to to atmosphere or differential, 52°C III 0.1% of connected to a low whichever rated head loss measuring is lower capacity device, with full IV 0.01% normal closing of rated thrust provided by capacity actuator. -9 V 5 × 10 l/s Water at Maximum Pressure applied to 10 to rated valve inlet after of water 52°C differential filling entire body per mm pressure cavity and connected orifice across piping with water per bar valve plug and stroking valve differential plug closed. Use net specified maximum actuator thrust, but no more, even if available during test. Allow time for leakage flow to stabilize. Pressure applied Air or 3.5 bar or VI Not to to valve inlet. nitrogen maximum exceed Actuator should be amounts at 10 to operating differential, adjusted to operating given, 52°C conditions specified according whichever with full normal to port is lower closing thrust diameter applied to valve plug seat. Allow time for leakage flow to stabilize and use suitable measuring device.

20  •   The Concise Valve Handbook

Table 1.4.  Class VI maximum seat leakage according to ANSI/FCI 70-2-1991 Nominal port ­diameter ins mm 1 25 1½ 38 2 51 2½ 64 3 76 4 102 6 152 8 203

Bubbles per minute(1) ml per minute Bubbles per minute 0.15 1 0.30 2 0.45 3 0.60 4 0.90 6 1.70 11 4.00 27 6.75 45

Bubbles per minute as tabulated are a suggested alternative based on a suitably calibrated measuring device; in this case, a 6.3 mm OD × 0.8 mm wall tube submerged in water to a depth of from 3 to 6 mm. The tube end shall be cut square and smooth with no chamfer or burrs, and the tube axis shall be perpendicular to the surface of the water. Other apparatus may be constructed and the number of bubbles per minute may differ from those shown as long as they correctly indicate the flow in ml per minute. (1)

Assume a 50 mm globe valve with a CV of 50. With a differential pressure of 3.5 bar, the maximum water flow rate is:

Q = 0.87.50

3.5 = 81m3 /hr  1

(1.26)

The question now is, how long would it take for a valve meeting ANSI requirements to fill a 10- litre (about 2.6 gallons) bucket? Let us look at ANSI II, that is, 0.5% of rated capacity: 0.5% of 81 m3/hr = 0.405 m3/hr = 405 l/hr = 6.75 l/min So, time taken to fill a 10 l bucket would be 1.5 minutes. Not very good, is it? What about ANSI III, that is, 0.1% of rated capacity? 0.1% of 81 m3/hr = 0.081 m3/hr = 81 l/hr = 1.35 l/min

Basic Principles   •  21

So, the time taken to fill a 10 l bucket would be nearly 7.5 minutes. Still not very good. And, ANSI IV, that is, 0.01% of rated capacity? 0.01% of 81 m3/hr = 0.0081 m3/hr = 8.1 l/hr = 0.135 l/min Here, the time taken to fill a 10 l bucket would be just over 74 minutes. And finally, ANSI V, that is, 5 × 10-9 l/s of water per mm orifice per bar differential 5 × 10-9 × 50 × 3.5 × 60 × 1000 ml/min = 0.0525 ml/min = 132 days 4.5 hours

1.14 Isolation Valve Leakage Classification It should be noted that the foregoing ANSI seat leakage classes apply only to control valves and not to isolation valves. An isolation valve is designed solely for that purpose: to stop the flow of a gas or liquid. Consequently, it is intended for use only in the fully open or closed positions, either to divert the process media or to isolate it. While control valves would generally be actuator-operated, isolation valves can be actuator- or manually operated. Obviously, the standards for seat leakage rates for isolation valves are more rigorous than for control valves and are generally governed by API 598. This states that, for all resilient-seated valves, there shall be no leakage at all during the specified test duration, with the period ranging from 15 to 120 s, dependent on the size of the valve. For liquids, 0 drops means no visible leakage, while for gas, 0 bubbles means less than one bubble during the test (Table 1.5). Table 1.5.  Leakage rates for different-sized isolation valves based on API 598 Valve size (NPS) ≤2 2½–6 8–12 ≥ 14

Resilientseated valves 0 0 0 0

Metal-seated valves Liquid test (drops Gas test per min) (bubbles per min) 0 0 12 24 20 40 2/min/inch NPS 4/min/inch NPS

CHAPTER 2

Liquid Valve Sizing The single biggest problem in the selection of valves and their associated actuators is that the valves tend to be oversized and the actuator undersized. This is mainly due to inaccurate input conditions and to the various safety margins that are added during the selection process.

2.1 Practical Considerations At first glance, it might be thought that using an oversized valve should not be serious—apart from the extra cost. However, oversized valves are a problem for four main reasons: 1. In an oversized valve, the valve is throttling near the closed position where, particularly in rotary valves, seal friction is likely to be greater. 2. The combined valve and process gain is high, so that the controller gain will need to be reduced to avoid instability problems with the loop. This could result in the loop becoming unstable and cycling. 3. Because the valve is operating near the seat, the high-velocity flow between the plug and seat may result in excessive wear. 4. At near closure, a severely oversized valve tends to act more like a quick opening valve and the valve tends to reach system capacity at relatively low travel. In modern processes, valve sizing is normally undertaken using a PC-based software package. When use is made of a properly designed package and when the fluid properties and service conditions are accurately entered, valve sizing can be both accurate and simple. However, users should be aware that simply entering a series of figures into a

24  •   The Concise Valve Handbook

proprietary software package, without an understanding of the calculation process, may lead to errors. It is important to be aware that valve sizing is only as good as the data entered. For example, in order to determine the CV of a valve, the data required would, typically, be: • • • • • • • •

Media: e.g., water Valve style: e.g., general purpose 50 to 500 mm Valve size Flow rate Upstream pressure Downstream pressure Upstream pipe diameter Downstream pipe diameter

Again, the results would vary from program to program, but typically, would include: • • • •

CV Flow rate for incipient cavitation Flow rate for critical cavitation Anticipated aerodynamic noise

Even in ostensibly straightforward ON/OFF control applications, it would be a mistake to underrate the importance of correct valve sizing procedures. By using a valve sizing program, the user is ‘disciplined’ into taking into account all of the parameters associated with the process (including viscosity, corrosion, and abrasion), and thus minimize the risk of damage to the valve due to incorrect application. Valve sizing programs are necessarily proprietary, and at one time, vendors charged a considerable price for the privilege of using their ­programs. Now, programs are freely available from a wide range of range of manufactures.

2.2 Application of Formulae At the basic level, what are we trying to achieve? Very simply, for a given process and valve style, we need to determine the CV. We then need to test and see whether, for liquids, the medium is likely to flash or cavitate.

Liquid Valve Sizing   •  25

Unlike an isolation valve, a control valve cannot be sized according to the line size, but must be sized according to the requirement of the process conditions. The most significant factors are: • Fluid • Flow rate • Pressure drop To be able to compare the requirement of an application with the capacity of a valve, we use the CV. It is important to remember that the formulae and the valve CV values are not exact, but are to be used as a guide. The most commonly used formulae are those supported by the ISA and the IEC. In our original definition, we had: Equation Chapter (Next) Section 2

Q = Cv

∆P  SG f

(2.1)

where: Q = flow rate; CV = valve coefficient; DP = differential pressure; and SG = specific gravity of the fluid CV is the quantity of water in U.S. gallons at 60°F that will pass through the valve each minute with a 1 psi pressure drop across it. The metric equivalent of CV is designated KV value is in m3/hr with 1 bar pressure drop:

C v = 1.16 ⋅ K V 

(2.2)

Thus, in order to work directly in metric, we modify the formula to:

Q = 0.865 ⋅ CV

∆P  SG f

(2.3)

The first question to ask ourselves is: what are we are trying to achieve when we size a valve? The answer is that, very simply, we are trying to choose a valve having a CV that is commensurate with the desired operating conditions.

26  •   The Concise Valve Handbook

At first glance, we might be led into thinking that we can basically make use of the formula given in equation (2.3). Transposing:

CV = 1.16 ⋅ Q ⋅

SG f  ∆P

(2.4)

Would this be appropriate? Referring to Figure 2.1, we can see that flow is only proportional to DÖP within the subcritical flow region. Because it is obviously unrealistic to increase the differential pressure beyond the ‘choked flow pressure drop,’ we simply need to ensure that we choose a valve of sufficient capacity, so that we are operating in the subcritical region. Clearly, the formula given in equation (2.4) does not indicate whether the valve is operating in the subcritical or critical regions. In order to do so, we need to know at what point the ‘choked pressure drop’ (DPchoked) occurs. To this effect, the formula is modified to:

CV = 1.16 ⋅ Q ⋅

SG f  ∆Pe

(2.5)

where: DPe = effective differential pressure (bar) The effective differential pressure (DPe) is very simply the smaller of the ‘actual pressure drop’ (DP) and the ‘choked pressure drop’ (DPchoked): ∆P = P1 − P2 



Sub-critical

(2.6)

Critical

Flow

Choked flow Incipient cavitation

Cv ∆Pchoked ∆P

Figure 2.1.  Flow is only proportional to √∆P within the subcritical region.

Liquid Valve Sizing   •  27

and the ‘choked pressure drop’ (DPchoked) is calculated from:

∆Pchoked = FL2 ⋅ (P1 − Pv ) 

(2.7)

where: FL = Pressure recovery factor P1 = Inlet pressure PV = Vapor pressure level

2.3 Sizing Example 1 Let us look at a very simple sizing example for an Emerson Fisher easy-e® 150 mm ES (flow up) cage-guided globe valve having a linear characteristic with a valve opening to 100% of its total travel. Fluid = P1 = P2 = T = Q = C V = FL =

Water 15 bara 2 bara 25°C 700 m3/hr 417 (from datasheet) 0.81 (from datasheet)

The first thing to remember is that all the pressures used in equations 2.5 and 2.6 are referenced to absolute pressure (with normal atmospheric pressure standardized at 1.013 bar) Firstly, we need to determine which is the smallest value: the ‘actual pressure drop’ (DP) or the ‘choked pressure drop’ (DPchoked). DP = P1 − P2 = 15 − 2 = 13.0 bar In order to calculate (DPchoked): DPchoked = FL2 (P1 − PV) We already know the value of the ‘Pressure Recovery Value’ (FL = 0.81) of the valve from the datasheet. However, we still need to determine the ‘Vapour Pressure Level’ (PV) for the medium. Table 2.1 shows the saturated vapor pressure levels for water at different temperatures. From this, we can ascertain that the saturated vapor pressure level (PV) at 25°C is 3.17 kPa, which expressed in bars is 0.0317 bar.

28  •   The Concise Valve Handbook

Table 2.1.  Saturated vapor pressure levels for water at different temperatures Temperature (°C) −10 0 10 15 20 25 30 50 100

Saturated vapor pressure (Pa) 2.60 * 102 6.11 * 102 1.23 * 103 1.71 * 103 2.33 * 103 3.17 * 103 4.24 * 103 1.23 * 104 1.01 * 105

Now, substituting: DPchoked = FL2 (P1 − PV) = 0.812 * (15.0 − 0.0317) = 0.66 * 14.9683 = 9.9 bar Taking the lower of these two figures and substituting in the equation in a 1.14, we obtain: CV = 1.16 ⋅ Q ⋅

1 SG = 1.16 ⋅ 600 ⋅ = 221 ∆Pe 9.9

So, it would seem that the valve we chose, having a CV of 417, would fit the bill and we would be working well within the subcritical linear area without the possibility of cavitation. Just as a quick check it would also pay us to determine the actual maximum velocity (v) through the valve: Velocity (v) Flow rate (Q) Valve bore Area of valve bore Velocity

= Flow/Area = Q/A = 600 m3/hr = 0.166 m3/s = 150 mm = 752 * p = 17672 mm2 = 0.018 m2 = 0.166/0.018 = 9.3 m/s

Liquid Valve Sizing   •  29

Figure 2.2.  Data on Fisher Emerson easy-e® ES (flow up) cage-guided globe valve having a linear characteristic with a valve opening to 100% of its total travel.

We might consider this velocity as near to the upper limit because a well-used rule of thumb is that the maximum velocity should be = 10 m/s.

2.4 Piping Geometry Factor In reality, the formulae given in equations 2.3 and 2.5 are somewhat ­simplified and can be rewritten as:

Q = N1 ⋅ FP ⋅ CV ⋅

∆Pe  SG f

(2.8)

30  •   The Concise Valve Handbook

which may be rearranged as:

CV =

Q



∆Pe N1 ⋅ FP ⋅ SG f

(2.9)

where: N1 = unit conversion factor (1.0 for fps (U.S. gals/min and psi) and 0.865 (m3/hr and bar (A)) FP = piping geometry factor The piping geometry factor (FP) should be used if fittings such as reducers, elbows, or tees are attached directly to the inlet and outlet connections of the control valve. Several manufacturers have determined the FP factors experimentally for their valves. ►For FP values not listed, use may be made of the following equation:

Fp =

1 ∑ K  CV  ⋅ 1+ N 2  d 2 

2



(2.10)

where: ∑K = K1 + K2 + KB1 − KB2 K1 = upstream resistance coefficient K2 = downstream resistance coefficient KB1 = inlet Bernoulli coefficient KB2 = outlet Bernoulli coefficient N2 = constant (0.00214 for mm or 890 for inches) The most commonly used fitting is the short-length concentric reducer, and the upstream and downstream resistance coefficients (K1 and K2) are given by: 2



 d2  K1 = 0.5 1 − 2    D 



 d2  K 2 = 1.0 1 − 2    D 

(2.11)

2

where: d = nominal valve size D = internal diameter of piping

(2.12)

Liquid Valve Sizing   •  31

If the inlet and outlet fitting are identical then: 2

 d2  K1 + K 2 = 1.5 1 − 2    D 



(2.13)

Furthermore, if the inlet and outlet Bernoulli coefficients KB1 and KB2 are the same (KB1 = KB2), they cancel out. If they are different, then: 4

 d K B1 or K B2 = 1 −     D



(2.14)

2.5 Sizing Example 2 Assume that it is required to fit the valve used in Example 1 in a 200 mm pipeline. This would involve the use of two concentric reducers from 200 mm down to 150 mm on the inlet side and from 150 mm up to 200 mm on the outlet side. As the inlet and outlet fittings are identical and KB1 = KB2, and they cancel out, then:  d2  K1 + K 2 = 1.5 1 − 2   D 

2

 1502  K1 + K 2 = 1.5 1 − 2  200 

2

∑ K = 1.025 And, from equation 2.10, we get: 1

Fp = 1+

Fp =

∑ K  CV  ⋅ N 2  d 2 

2

1 1.025  417  1+ ⋅  0.00214  1502 

2

= 0.9266

32  •   The Concise Valve Handbook

CV =

600 0.865 ⋅ 0.9266 ⋅

9.9 1

= 238.8

From the calculation, it appears that the valve we chose, with a CV of 417, would still fit the bill.

CHAPTER 3

Gas Valve Sizing To date, our discussions have all related to incompressible fluids whose flow rate depends only on the difference between the inlet and outlet ­pressures. As we have seen, the flow is the same whether the system ­pressure is low or high, so long as the difference (∆P) between the inlet and outlet pressures is the same. However, because gas is compressible, flow calculations are slightly more complex because density changes with pressure. This means that valves with the same CV rating and different shape could have radically different gas flow characteristics. Consequently, by itself, the valve flow coefficient CV years is insufficient to describe both liquid and gas flow through valves over the full range of pressure drops. For a given steady-state flow path, the mass flow rate (density × velocity × flow area) cannot change. As a result, an expanding gas must accelerate to higher velocities to maintain the mass flow rate. Previously we saw that for liquids, flow is only proportional to √∆P within the sub-critical flow region, and that if the differential pressure is increased even further, choked flow occurs—a point where no further flow increase occurs despite increasing the differential pressure.

3.1 Pressure Drop Mechanism In the previous chapter, we saw that, for incompressible fluids, we had the following (simplified) equation: Equation Chapter (Next) Section 3

Q = CV

∆P  SG

(3.1)

34  •   The Concise Valve Handbook

It therefore follows that:

Q α CV

∆P  ρ

(3.2)

Further, in order to determine the mass flow, we can multiply through by density (ρ) to give:

Q α CV ∆P.ρ 

(3.3)

One of the first differences in gas flow is that, instead of using pressure drop alone, as for incompressible liquids, we make use of the ‘pressure drop ratio’ (x = ∆P/P1), which takes into account gas expansion:

∆P = x P1

(3.4)

∆P = x.P1 

(3.5)

or:

Now substituting in equation (3.3) and using the symbol normally used for gas mass flow (w):

w α CV x .P1.ρ 

(3.6)

wα x 

(3.7)

or simplifying:

As illustrated in Figure 3.1, this relationship indicates that there is a straight-line correlation between the mass flow (w) and the pressure drop ratio (x). However, as the pressure drop ratio (x) increases, there will come a point where the mass flow will no longer continue to increase at the same rate and will start to deviate from the straight line (Figure 3.2). This deviation will continue until further increase in the ‘pressure drop ratio’ will not yield any additional flow. This is termed ‘choked flow’ and the point at which it occurs is the called terminal pressure drop ratio (XT) (Figure 3.3).

Mass flow (w)

Gas Valve Sizing   •  35

∆P P1

Mass flow (w)

Figure 3.1.  Initially, there is a straight-line relationship between the mass flow (w) and the pressure drop ratio (x).

∆P P1

Mass flow (w)

Figure 3.2.  As the pressure drop ratio (x) increases, there will come a point where the mass flow will no longer continue to increase at the same rate and will start to deviate from the straight line.

Choked Terminal pressure drop ratio XT

∆P P1

Figure 3.3.  This deviation will continue until no further increase in the ‘pressure drop ratio’ will yield any additional flow. This is termed ‘choked flow’ and the point at which it occurs is the called terminal pressure drop ratio (XT).

36  •   The Concise Valve Handbook

So, exactly what causes this deviation? In order to answer this question and find out why the deviation occurs, we need to remember that, at low flows, as the velocity increases, the pressure decreases and reaches a minimum at the vena contracta (Figure 3.4). As the pressure decreases at the vena contracta, the density also decreases, and thus, we can deduce from equation (3.7), which shows that, at the vena contracta, the mass flow rate is proportional to the square root of the density: wα ρ 



(3.8)

then, as the density falls, so does the mass flow rate. However, at some point before choked flow occurs, the flow rate is still increasing and the velocity at the vena contracta becomes sonic (Figure 3.5). When the flow rate reaches a maximum at the restriction, the vena contracta is downstream of the restriction and has a smaller cross-­sectional area. Consequently, the downstream flow rate can still increase. As the pressure drop ratio increases even further, the vena contracta starts moving toward the physical restriction (Figure 3.6). As the vena contracta moves backward toward the physical restriction, its cross-sectional increases. Even so, the downstream flow rate can still increase. Finally, when the vena contracta backs up to the physical restriction, no further increase in flow is possible and is said to be choked.

Vena contracta

Pressure

Velocity

Figure 3.4.  At low flows, as the velocity increases, the pressure decreases and reaches a minimum at the vena contracta.

Mass flow (w)

Gas Valve Sizing   •  37

Choked Sonic velocity

XT

∆P P1

Figure 3.5.  At some point, before choked flow occurs, the flow rate is still increasing and the velocity at the vena contracta becomes sonic.

Increasing vena contracta enlargement

Figure 3.6.  As the pressure drop ratio increases, the vena contracta moves toward the physical restriction and its cross-sectional increases. When it backs up to the physical restriction, no further increase in flow is possible and is said to be choked.

We have seen that choked flow occurs at a point called the terminal pressure drop ratio (XT). Like CV, the XT is a nondimensional quantity determined by the manufacturer (using air as the medium) and listed in the valve datasheet. Figure 3.7 illustrates a test performed on a globe valve set at 70% open. Using V1, the inlet pressure P1 is set firstly at 7 bar. V2 is then adjusted to increase ∆P from zero until the flow chokes at 5 bar. With the inlet pressure P1 set at 14 bar, V2 is again adjusted to increase ∆P until the flow chokes; in this case, at 10 bar. This procedure is repeated, but in this case, with the inlet pressure P1 set at 70 bar and V2 again adjusted until the flow chokes: in this case, at 50 bar. The results are shown in Table 3.1, which clearly show that since XT (the choked value of x) is ∆P/P1, it remains constant at 0.72 in each instance.

38  •   The Concise Valve Handbook

V1

Pressure source

P1

P2

V2

Globe valve 70% open

Flow

Figure 3.7.  Test performed on a globe valve set at 70% open.

Table 3.1.  Results of test show that XT (the choked value of x) remains constant at 0.72 in each instance P1 (bar) ∆PT (choked) XT

7 5 0.7

14 10 0.7

70 50 0.7

Indeed, given a particular style of control valve at a specific opening (in this case 70%), the pressure drop ratio at which flow becomes choked is a constant. It is, thus, possible to predict that, for an inlet pressure of 28 bar, the flow would choke at a differential pressure (∆PT) of 20 bar. For air, the terminal pressure drop (∆PT) at which gas flow becomes fully choked is thus:

∆PT = P1.X T 

(3.9)

Different valve styles will have different values of XT. Figure 3.8, for example, shows the performance of two 50 mm valves having almost identical CV values of 60 when 70% open—one a linear globe valve and the other a high performance butterfly valve. Clearly, the pressure drop ratio (XT) at which the flow will choke is very much different between the two styles. Figure 3.9 also shows not only how XT varies with different styles of valves, but also how it varies with the valve opening.

3.2 Specific Heat Ratio Factor The published values of XT are based on tests using air as the test medium. Consequently, it is necessary to compensate for gases having different

Gas Valve Sizing   •  39 Globe valve 70% open High-performance butterfly valve 70% open Flow

Choked

Choked XT = 0.4 CV = 60

XT = 0.7 CV = 60

∆P P1

Figure 3.8.  Performance of two 50 mm valves having almost identical CV values of 60 when 70% open—one a linear globe valve and the other a high performance butterfly valve. Globe 0.8 0.7

Eccentric plug

XT

0.6 0.5 0.4

Ball

0.3 0.2

HP butterfly

0.1 0

0

10 20 30 40 50 60 70 80 90 100 % Open

Figure 3.9.  XT not only varies with different styles of valves, but also with the valve opening.

ultrasonic velocities from that of air. This compensation is carried out by multiplying the published values of XT by the specific heat ratio factor (Fk). In turn, the specific heat ratio factor (Fk) is calculated by dividing the ratio of specific heats (k) of the gas under consideration by the ratio of specific heats of air (which is 1.4): Specific Heat Ratio Factor (FK ) = 

Ratio of Specific Heats of gas (k) Ratio of Specific Heats of air (1.4) (3.10)

40  •   The Concise Valve Handbook

Thus, for air:

FK =

1.4 = 1.00  1.4

(3.11)

For gases other than air, Fk is likely to be greater than or less than 1.00. Indeed, in reality, with few exceptions (e.g., hydrogen, helium, and argon), the specific heat ratio of most gases found in industry is lesser than air, and thus Fk will be less than 1.00. The specific heat ratio of natural gas (methane), for example, is 1.32 so that:

FK =

1.32 = 0.943  1.4

(3.12)

Consequently, the compensated calculation for determining the terminal pressure drop (∆PT) at which the gas flow becomes fully choked, for gases other than air, is:

PT = P1.FK .X T 

(3.13)



PT = FK .X T  P1

(3.14)

where: ∆PT = terminal pressure drop (bar or psi) P1 = upstream absolute pressure (bara or psia) XT = pressure drop ratio factor Fk = specific heat ratio factor = k/1.4 where: k = ratio of specific heats

3.3 Gas Expansion Factor We saw earlier how the pressure drop mechanism for gases is extremely complex, and accordingly, determination of the density (ρVC) at the vena contracta would be difficult. As a result, the ISA/IEC control valve gas equations make use of the upstream density (ρ1)—which is much easier to determine. In order to account for the variations at the vena contracta that include: density (ρVC), enlargement, and position variation, use is made of a compensating element called the gas expansion factor (Y).

Gas Valve Sizing   •  41

At a fixed valve opening (CV constant), the gas expansion factor (Y) is a linear function of the pressure drop ratio (x) given by:

Y = 1−

x  3FK . x T

(3.15)

where: Y = gas expansion factor x = pressure drop ratio (∆P/P1) P1 = upstream absolute pressure (bara or psia) xT = pressure drop ratio factor Fk = specific heat ratio factor = γ/1.4 From equation (3.15), as X approaches, Y approaches 1.0. Furthermore, from equation (3.15), it may be seen that the conditions for choked flow are fulfilled when X is equal to the product of Fk and XT so that:

Y = 1−

x 1 = 1 − = 0.667  3.Fk .x T 3

(3.16)

In order, therefore, to ensure that the conditions for choked flow are not exceeded, the gas expansion factor (Y) lies between 1.0 and 0.667.

3.4 Valve Sizing The formulae for determining the valve coefficient (CV) for compressible fluids is dependent on whether the flow rate is specified in volumetric flow rate units or mass flow rate units. In turn, the appropriate formula is then determined by which parameters of the gas have been specified: the specific gravity (SGg), the molecular weight (M), or the specific weight (γ1). This gives rise to a choice between the following formulae: For volumetric flow rates, we can choose either of the following: If the specific gravity (SGg) has been specified:

q

CV =

N 7 ⋅ Fp ⋅ P1 ⋅ Y

x SG g ⋅ T1 ⋅ Z



(3.17)

If the molecular weight (M) of the gas has been specified:

CV =

q x N 7 ⋅ Fp ⋅ P1 ⋅ Y M ⋅ T1 ⋅ Z



(3.18)

42  •   The Concise Valve Handbook

For mass flow rates, we can choose either of the following: If the specific weight (γ1) has been specified:

CV =

w



N 6 ⋅ Fp ⋅ P1 ⋅ Y x⋅ P1⋅ ³ 1

(3.19)

If the molecular weight (M) of the gas has been specified:

CV =

w N8 ⋅ Fp ⋅ P1 ⋅ Y

x⋅ M T1 ⋅ Z



(3.20)

where: CV = valve coefficient (dimensionless) 1 Fp = piping geometry factor γ1 = specific weight of gas M = molecular weight of gas (dimensionless) N6 = unit conversion factor (27.3 for SI units [kg/h][bar][kg/m3] (63.3 for fps units [lb/h][psi][lb/ft3] N7 = unit conversion factor (417 for SI units [m3/h][bar][degrees K] (1360 for fps units [scfh][psi][degrees R] N8 = unit conversion factor (94.8 for SI units [kg/h][bar][degrees K] (19.3 for fps units [lb/h][psi][degrees R] N9 = unit conversion factor (2240 for SI units [m3/h][bar][degrees K] (7320 for fps units [scfh][psi][degrees R] P1 = upstream absolute pressure (bara or psia) q = volumetric flow rate (m3/h or scfh) SGg = specific gravity of gas T1 = upstream temperature (K or °R) w = mass flow rate (kg/h or lb/h) x = pressure drop ratio (∆P/P1) Y = expansion factor 2 Z = compressibility factor 1 Fp: As for incompressible fluids, the piping geometry factor is a correction that accounts for pressure losses due to fittings such as reducers, elbows, or tees attached directly to the inlet and outlet connections of the valve. If there are no fittings, Fp has a value of 1.0. Several manufacturers

Gas Valve Sizing   •  43

have determined Fp factors for their valves which are available in the relevant datasheets. For Fp values not listed, use may be made of the equation (2.8) in Section 2.3. 2 Z: the compressibility factor modifies the ideal gas law to account for the real gas behavior and is defined as:

Z=

P⋅ V  R ⋅T

(3.21)

where: P = pressure V = molar volume of the gas T = temperature R = gas constant Essentially, the compressibility factor (Z) is the ratio of the volume occupied by a real gas to the volume occupied by it under the same temperature and pressure conditions if it were ideal. For an ideal gas, Z = 1 and deviations from ideal behavior become more significant the closer a gas is to a phase change, the lower the temperature or the larger the pressure. Compressibility charts are available that plot Z as a function of pressure at constant temperature.

3.5 Sizing Example 1 Determine the size for a Fisher Emerson easy-e® ES (flow up) cage-guided globe valve having a linear characteristic with a valve opening to 100% of its total travel. Assume that the valve and line size are equal. Fluid = Natural gas P1 = 15 bara P2 = 2 bara ∆P = 13 bar x = ∆P/P1 = 0.8667 T = 25 °C = 298.15 K SGg = 0.60 k = 1.31 q = 120,000 m3/hr Z = 0.8 The first step is to determine which equation to use and the appropriate unit conversion factor. Because the specific gravity of the gas (SGg)

44  •   The Concise Valve Handbook

has been specified, together with the volumetric flow rate (q = m3/h), we need to make use of equation (3.6), and thus, the unit conversion factor would be N7 = 417 (for SI units). At this stage, the valve and line size were assumed to be equal so that FP = 1.0. The next task is to determine the expansion factor (Y) from equation (3:15): Y = 1−

x 3 ⋅ F³ ⋅ x T

In turn, we now need to determine values for Fγ and xT. Since Fγ (specific heat ratio factor) = γ/1.4 and γ (ratio of specific heats) is given for the gas as 1.31, then: Fγ = k/1.4 = 1.31/1.4 = 0.94 Now, we have to go in to an iterative mode and choose a valve size. On the basis that we have a 150 mm line, we will assume, for the moment, a 150 mm valve. From the datasheet, we see that an Emerson Fisher easy-e® ES (flow up) cage-guided globe valve having a linear characteristic with a valve opening to 100% of its total travel (Figure 3.10), has an xT value of 0.745. We also determined, at the outset, that: x = ∆P/P1 =0.8667 We also know that the conditions for critical pressure drop are realized when the calculated value of x equal or exceeds the appropriate value of Fγ.xT, then these values should be compared: Fγ.xT = (0.94).(0.745) = 0.697 Clearly, the pressure drop ratio, x = 0.8667 exceeds this calculated critical value (0.697) and choked flow conditions are indicated. On this basis: Y = 0.667, and x = Fγ.xT = 0.697. And lastly, the only remaining factor is Z (given as 0.8).

Gas Valve Sizing   •  45

Figure 3.10.  Data on Fisher Emerson easy-e® ES (flow up) cage-guided globe valve having a linear characteristic with a valve opening to 100% of its total travel.

Looking at equation (3.17), we now have enough to solve for the required CV. CV =

q N 7 ⋅ Fp ⋅ P1 ⋅ Y

x SG g ⋅ T1 ⋅ Z

46  •   The Concise Valve Handbook

CV =

120000 417 ⋅ 1.0 ⋅15 ⋅ 0.667 ⋅

0.8667 0.6 ⋅ 298.15 ⋅ 0.8

= 369.6

The preceding result indicates that the valve is adequately sized (rated CV from the datasheet = 417 (Figure 3.10)).

CHAPTER 4

Valve Construction As stated in Chapter 1, a control valve is designed primarily to throttle energy and is not necessarily suitable for shut-off purposes. Consequently, these two requirements often have to be balanced or realized in separate systems. In essence, any control assembly comprises three elements: the valve body, trim, and topwork or actuator assembly. The valve itself, thus, really only comprises the body components (body, bonnet, packing, and gasket) and the trim, which includes all the removable internal parts of a valve that come in contact with the flowing fluid. Because there are many types of control valve body sub-assemblies, one of the basic problems in discussing valves is how to classify them. Probably, the simplest method is to look at the two major categories: reciprocating and rotary motion valves (Figure 4.1). Reciprocating motion (or sliding stem) valves are those where the valve stem, which acts directly on the controlling mechanism (e.g., the plug), traverses linearly, usually up and down. Examples of sliding stem valves include: • • • • • • •

globe valves bar stock valves needle valves angle valves gate valves pinch valves Rotary motion valves are those whose stems move in a rotating fashion. Examples include: • ball valves • butterfly valves • plug valves

48  •   The Concise Valve Handbook Valve type Rotary motion

Linear motion

Globe

Globe Angle

Gate

Diaphragm

Multiple orifice

Ball

Pinch or clamp

Butterfly

Cylinder type

Segmented ball

Tapered

Full ball

Knife

Plug

Eccentric sphere

Figure 4.1.  Simple classification of valve types.

4.1 Globe Valve The globe valve is one of the most common types of body style for sliding-stem valves. Figure 4.2 illustrates a typical globe valve, which takes its name from the general shape, or more particularly, the globular-shaped cavity around the port region. The main advantages of the globe valve include: rugged construction; linear relationship between the control signal and valve stem movements; Valve plug stem Packing flange Packing Packing box

Bonnet

Bonnet gasket

Valve plug

Cage

Cage gasket

Seat ring

Valve body

Seat ring gasket

Figure 4.2.  A typical globe valve (courtesy Emerson–Fisher).

Valve Construction   •  49

a wide range of trims and accessories allowing the flow characteristics to be tailored to the application; availability of anti-cavitation trim for liquid applications and noise attenuation trim for gaseous applications; a wide range of designs for corrosive, abrasive, high-temperature, and high-pressure applications; and relative small values for dead band and hysteresis. A major disadvantage of the globe valve is that, at larger sizes, it is expensive compared with other styles. In addition, it has a relatively high mass and a lower capacity than, for example, ball of butterfly valves. Globe valve sizes most commonly range from 0.5 to 16 inch (DN 15 to 400), although some manufacturers produce globe valves up to 48 inch (DN 1,200) or even larger. In order to gain some insight into the choices faced by the user, it is necessary to look a little closer at the different components that go to make up the complete valve.

4.2 Bonnet Assembly The bonnet assembly includes that part through which the valve plug stem moves and the sealing or packing box. The packing box (sometimes referred to as the stuffing box) contains the packing elements that prevent egress of the process medium fluid. At the same time, it must allow the valve plug stem to move as freely as possible. Typically, the packing box comprises a number of plastic or composition washers that are retained by a packing flange (Figure 4.3). Adjustment of the packing flange stud bolts compresses the packing, causing it to press tighter against the stem. A wide variety of materials is available for use as packing, with the choice determined on the basis of the composition, temperature, and pressure ratings of the process medium. Generally, however, packing selection is based on the process medium temperature—with PTFE used for temperatures below 250°C and graphite composition used for higher temperatures. In addition, the increasing need to take into account national regulatory requirements for fugitive emissions has also lead to the development of a number of enhanced seals from a number of manufacturers.

4.3 PTFE (Teflon) With an upper temperature limitation of about 250°C, PTFE packing comprises a nest of spring-loaded V-shaped rings that require no lubrication

50  •   The Concise Valve Handbook Stud Nut Packing flange Packing follower Stem Chevron packing Packing spring

Figure 4.3.  A typical packing box assembly comprising a number of plastic or composition washers that are retained by a packing flange.

Packing follower Female adaptor V-rings Male adaptor Washer spring Washer Lower wiper

Figure 4.4. Typical standard V-ring packing arrangement.

(Figure 4.4). Although inert to most chemicals, PTFE requires an extremely smooth stem finish (0.1 µm) and will leak if the stem or packing surfaces are damaged.

4.4 Laminated Graphite Suitable for temperatures from about –30°C up to 650°C laminated graphite packing is suitable for most fluids and may also be used in high-radiation

Valve Construction   •  51

Packing follower Filament ring Laminated rings Lantern ring

Sacrificial Zinc washers zinc washers

Figure 4.5.  Typical double graphite packing arrangement.

environments. The packing comprises light-weight graphite/carbon fibers, twisted together and interlock braided and contains a special lubricant to provide a bearing film and prevent wicking (Figure 4.5). Designed for severe service packing, laminated graphite can handle strong acids and alkali solutions, except fuming nitric acid, oleum, and fluorine. Other benefits include: suitable for high shaft speeds, lubrication not required, easy to use, inexpensive, readily available, and tolerant to abuse. On the down side, graphite packing produces high stem friction with resultant hysteresis. This is particularly true for high temperature and/or high-pressure service. In such cases, an external lubricant may be required, in which the packing space is filled with a sticky lubricant, and a compression screw is used to force the grease into the packing area. Note should also be taken of the sacrificial zinc washers. In the presence of a suitable electrolyte, an electrochemical reaction could take place between two dissimilar metals having different electrode potentials. This can result in galvanic corrosion and subsequent removal of material from some of the important metallic components of the valve. The solution lies in the use of a sacrificial anode—a third metal introduced into the electrochemical circuit—that is more willing to surrender its electrons than the other two. A commonly use metal is zinc, which, as shown in Figure 4.6, has a negative electrode potential (referenced to hydrogen) compared with many other metals. This sacrificial anode may take the form of either a zinc washer or zinc-dusted graphite rings.

52  •   The Concise Valve Handbook Fluorine

+ 2.87 V

Gold Bromine Silver

+ 1.50 V + 1.09 V + 0.80 V

Copper Hydrogen Nickel

+ 0.34 V + 0.00 V − 0.25 V

Zinc

− 0.76 V

Potassium Lithium

− 2.93 V − 3.05 V

Increasing oxidizing strength with tendency to form negative ions

Increasing reducing strength with tendency to form positive ions

Figure 4.6.  Electrochemical series showing the electrode potentials referenced to hydrogen.

4.5 Extended Bonnet Where excessive heat or cold is likely to inhibit valve performance, use can be made of an extended bonnet (Figure 4.7) in which the stuffing box is located at the top of the bonnet, further away from the process medium. In addition, the bonnet may be provided with radiating fins to further dissipate the heat. This allows the process medium temperature to be extended up to 800°C or more.

Extension bonnet

4.6 Bellows Seal Bonnet Also known as zero-leak or emission-free valves, bellows stem seals are designed to eliminate valve leakage and normally supplement conventional packing. Comprising an accordion-like tube, the bellows are welded to the valve bonnet at one end and the stem at the other (Figure 4.8). The bellows is expanded or compressed depending upon the movement of the stem.

Figure 4.7.  Extended bonnet in which the stuffing box is located further away from the process medium allows the process medium temperature to be extended up to 800°C or more (courtesy Emerson–Fisher).

Valve Construction   •  53

Anti-twist pin and slot Blind plug for leakage detection Weld Bellows Process medium

Weld

Figure 4.8.  Bellows stem seals are designed to eliminate valve leakage and normally supplement conventional packing.

The bellows are available as either a formed or welded construction (Figure 4.9). Formed-type bellows are typically produced by forcing a thin metal tube (typically phosphor bronze) to expand under hydraulic pressure inside a bellows-shaped mold. This forces the tube to assume the convoluted shape of the mold with rounded and widely spaced folds. Welded bellows or diaphragm bellows are manufactured from thin stainless steel washer-like plates that are welded together at each both their inner and outer circumferences, thus creating a number of individually formed diaphragms that are combined to form the complete bellows structure. Individual diaphragm plates

Formed bellows

Welds

Welded bellows

Figure 4.9.  The bellows are available as either a formed or welded construction.

54  •   The Concise Valve Handbook

The bonnet is normally provided with a leakage detection hole. Subsequently, if the bellows sustains damage, any subsequent leakage may be detected by connecting an appropriate pressure sensor or gas detection system. In order to prevent damage to the bellows, an anti-twist prevention measure, in the form of a rotation prevention pin, is also incorporated. The lifespan of a bellows seal is up to 1,000,000 stroke cycles, dependent on construction. While welded bellows have a longer lifespan than formed bellows, this must be offset by the increased cost. Much depends on the expected stroke length and estimated number of stroke cycles. In addition, the lifespan is also affected by a number of factors including: • • • • •

pulsations temperature cycles vibration water hammer shock excessive hunting

4.7 Valve Trim The valve trim includes all the removable internal parts of the valve that come in contact with the flowing fluid. This includes the plug, seat ring, seat retainer (if used), cage (if used), and stem. The valve plug, together with the seat ring, provides a variable restriction that determines the flow rate of the process medium. The plug, which is directly positioned by the actuator, also determines the flow characteristics of the valve—the relationship between the valve position and the flow rate. The seat ring may be secured by a number of methods: welded, threaded, seat retainer, and cage retainer. A welded seat ring affords an extremely leak-tight fitting, but can only be replaced during refurbishment of the valve. On the other hand, corrosion of the threads on a screwed-in type seat can also render the seat immovable. Corrosion of the screw threads can also produce leakage. Further, vibration can lead to looseness and unscrewing of the screw threads. The seat retainer shown in Figure 4.10 avoids the difficulties associated with welded and threaded seats. As shown, the seat ring is clamped into the body by the bonnet and seat retainer. When the bonnet is fully installed, force is transmitted through the seat retainer to secure the seat ring in position. The body, seat retainer, and seat ring are all machined

Valve Construction   •  55

Stem guide Bonnet

Seat retainer

Plug Seat ring

Figure 4.10.  In the seat retainer, the seat ring is clamped into the body by the bonnet and seat retainer (courtesy Valtek Control Products).

to close tolerances to provide the proper gasket compression. Unlike the ­bonnet, the seat ring does not always bottom in the body, allowing this small clearance to compensate for manufacturing tolerances and ­thermal expansion. The result is that removal of the seat is easy, even under extremely corrosive conditions. In the cage-guided control valve, previously illustrated in Figure 4.2, movement of the valve plug is guided by a cylindrical cage that secures the seat ring in position in a manner identical to the seat retainer. Again, the cage and seat ring are self-aligning.

4.8 Guiding Sliding stem valves, and in particular, globe valves, are categorized according to the plug guiding system used. The guide provides support and positioning of the valve plug over the full range of travel. Since accurate guiding is essential for proper alignment with the seat ring, wear or failure of the guide will detract from efficient control of the process fluid.

4.9 Post-guiding As shown in Figure 4.11, the post is larger in diameter than the stem and extends from the stem into the valve body, connecting directly to the valve plug. While the post supports the plug from bearing loads, the narrower stem providing positioning control.

56  •   The Concise Valve Handbook

Valve stem Post guide bushing

Post Valve plug

Seat ring

Figure 4.11.  Single seat top-guided contact valve.

The single seat top-guided configuration shown in Figure 4.11 is perhaps the most popular general service control valve. Its rugged heavy top-guiding provides sufficient support to ensure plug stability. Although the plug is unbalanced, requiring larger actuators to overcome unbalance forces, tight shut-off, ANSI Class IV with ANSI Class V optional, is achievable. For ease of maintenance, the screwed-in seat ring is replaced by a cage retained ring and gasket, commonly known as quick change trim. This simplicity of design and construction makes it applicable on high-temperature and corrosive services.

4.10 Top- and Bottom-guided Double Seat The design of this type of valve (Figure 4.12) originated from its early use as a regulator and features top and bottom guides, which minimize vibration. The advantage of this double-seated construction lies in the reduction of the required actuator force. This is because the hydrostatic effect of the fluid pressures, acting on each of the two seats, tends to cancel each other out. In practice, the lower of the two ports has a smaller diameter to allow withdrawal of the smaller plug through the larger (upper) port. In addition, because the fluid passing the lower seat (tending to close the plug) has a tendency to ‘suck’ the plug into the seat, it creates a dynamic imbalance

Valve Construction   •  57

Top guiding bush

Note: Smaller diameter of lower plug

Valve plugs

Bottom guiding bush

Figure 4.12.  A double seat bottom-guided globe valve.

between this force and the differential pressure acting across the upper plug area. This dynamic imbalance can be as high as 40% of the equivalent single-seat hydrostatic force. The cost for such a valve is considerably higher than a top-guided valve. A further disadvantage is that debris, accumulating in the bottom guide area, leads to valve sticking and inoperability. One of the main problems, however, is the difficulty in mating both plugs at the same time. This, together with temperature variations, yield fairly high leak rates, ANSI Class II, and as a result, block valves are required to isolate the pipeline. This design offers CV ratio of 50:1, which is limited by the plug-toseat clearance area built-in for ease of assembly and for high-temperature service. The tortuous path through the body does not lend itself very well to slurries or any fluid with entrained particles.

4.11 Single-ported Balanced Globe Valve Where actuator forces may become an issue, particularly in larger-sized valves, use is often made of the single-ported balance globe valve. As shown in Figure 4.13, the valve plug is made as a piston having an internal passage that permits the fluid pressure to communicate to both sides of the

58  •   The Concise Valve Handbook

Balancing ports Cage Piston ring

Piston

Figure 4.13.  In the single-ported balance globe valve, the valve plug is made as a piston having an internal passage that permits the fluid pressure to communicate to both sides of the plug.

plug. One or more rings, located in a groove near the top of the piston, seal the upper plug area against the outlet portion of the valve. The balanced design tends to cancel out the hydrostatic forces acting on the plug and leads to a great reduction in the required actuator forces. However, the seat leakage is considerably increased.

4.12 Cage-guiding In the unbalanced cage-guided control valve (Figure 4.14), the outside diameter of the valve plug is in close proximity to the inside wall surface of the cylindrical cage throughout the travel range. Since the cage and seat ring are self-aligning and the plug is guided along the inside diameter of the cage, vibration is reduced and the problem of sided loads is reduced, improving plug stability. Cage-guiding is used primarily for high pressure, high temperature, and where tight shut-off is required and for flashing condensate applications normally found in power generation facilities. Shut-off is achieved by a piston seal wrapped around the plug that can be of Ni-resist (cast iron) (ANSI Class II), or ‘0’ ring or graphite rings (ANSI Class IV). Cage-guiding yields approximately 20% greater capacity than topguided single-seat valves. Although cavitation and noise are handled effectively with this design, its tortuous path through body and cage does not permit its use on slurry services. Its complex design (multiple

Valve Construction   •  59 Valve plug

Cage

Seat ring gasket

Seat ring

Figure 4.14.  Cage-guided control valve.

parts) makes it approximately 15% higher in cost than single-seat valves. Further, it is not well-suited for corrosive services

4.13 Split Body Globe Specially designed for the chemical industry, the split-body, stem-guided globe valve (Figure 4.15) may be constructed of corrosion-resistant alloys, and yet remain inexpensive. Catering for difficult flows with high viscosity, the valve features a streamlined internal flow passage that is smooth and free of pockets to prevent material build-up and minimize fouling. The split body globe valve also permits easy maintenance with the clamped-in seat ring allowing ‘in line’ trim replacement. By separating the split body and removing the lower body adapter, the seat ring is removed, and the plug can be unscrewed from the actuator stem.

Valve plug stem

Packing

Seat ring

Figure 4.15.  The split-body, stemguided globe caters for difficult flows with high viscosity.

60  •   The Concise Valve Handbook

Stem-guiding avoids the accumulation of solids that commonly collect behind conventional valve plugs guided by plug post and bushing. The requirement for low-cost or special alloy bodies is facilitated through the use of separable flanges that are not in contact with the medium and which may be constructed of carbon steel. The principal disadvantage of this design is its limited pressure drop capability due to the stem-guiding. This produces a long plug overhang, which may be subject to vibration. Further, the split body introduces another gasket joint, which is subject to leakage

4.14 Angle is In the single-seat angle valve (Figure 4.16), the exit flow is 90° to the inlet flow and can be used to eliminate an elbow. The angle valve is particularly suited for slurries since its streamlined flow path reduces fluid directional changes and erosive effects of abrasive slurries. Further, because there is little restriction to the outflow, if flashing or cavitation does occur, its effects are felt further downstream, saving not only in maintenance, but also in degradation in valve performance. Its heavy top-guided construction provides sufficient support for plug stability. Some designs offer quick-change features, which improve maintenance capability, thus minimizing plant downtime. Venturi seat rings are often used where significant increases in capacity can be realized. Unfortunately, noise and cavitation potential are increased. The unit cost is approximately 20% greater than single-seat globe valves.

Figure 4.16. Single-seat angle valve.

Valve Construction   •  61

4.15 Needle Valve Needle valves are used for precise metering of very small flows. The splitbody needle valve shown in Figure 4.17 is functionally similar to an angle valve, but permits a finer adjustment of flow. The end of the stem is pointed like a needle and fits accurately into the needle seat. The seat is typically metal, although elastomeric seats have been used for very fine adjustments.

4.16 Bar Stock Body Valve Bar stock bodies (Figure 4.18) can be machined from any metallic bar stock material, or even plastic, and are often specified for corrosive applications. When exotic metal alloys are required, a bar stock body is normally less expensive than a body produced from a casting. Bar stock bodies are available in a variety of trims and are also available in angle-valve style.

4.17 Gate Valve Although specifically designed to operate for use in isolation applications (either fully opened or fully closed), the gate valve is, nonetheless, available in a variety of forms suitable for modulating control.

Figure 4.17. Split-body needle valve (courtesy Fisher Rosemount).

62  •   The Concise Valve Handbook

Figure 4.18.  Bar stock bodies are often specified for corrosive applications (courtesy ­Emerson–Fisher).

The gate valve is an excellent valve for service that requires either full or no flow. When fully open, the gate valve has no flow restriction, with the flow area at the point of control being equal to the full cross-sectional area of the line. Since flow is straight through the line, pressure drop across a gate valve is only about 1/50 of that of a globe valve of comparable size. One of the most or widely used types of gate valve, illustrated in Figure 4.19, comprises a disk that slides up and down between seats. The gate faces can be parallel or form a wedge shape, sliding up and down between tapered seats, as shown in Figure 4.20.

4.18 Wedge Gate When closing the valve, the wedging action forces the gate tightly against both seats, making the valve tight against flow in either direction. The gate valve illustrated in Figure 4.20 makes use of a one-piece solid wedge as shown in Figure 4.21(a). The solid wedge gate is the simplest, most economical style and is also the most resistant to corrosion and vibration. However, sealing is dependent on precise machining, and it becomes more difficult to obtain a good fit on a larger valve sizes. Furthermore, its rigidity does not allow

Valve Construction   •  63

Hand wheel Bonnet Gasket

Stem

Body Disk

Figure 4.19.  A typical gate valve comprising a disk that slides up and down between seats.

Bonnet Stem Body

Wedgeshaped disk

Tapered seats

Figure 4.20.  The gate faces can be in the form of a wedge shape, sliding up and down between tapered seats.

it to accommodate to seat distortion and makes it prone to sticking when subjected to temperature extremes. These problems are largely overcome in the flexible wedge gate (Figure 4.21 (b)) in which a circumferential groove is cast or machined around its perimeter. This groove produces the flexibility that allows the seating surfaces to move independently and adapt to minor machining inaccuracies.

64  •   The Concise Valve Handbook

(a)

(b)

Figure 4.21.  (a) One-piece solid wedge. (b) Flex wedge.

The gate’s ability to flex also minimizes sticking due to large temperature changes or pipeline distortion. Consequently, the flexible wedge gate is now standard for large steel valves. A general drawback of wedge gate valves is that the wedging action that forces the gate tightly against both seats may also trap some of the process medium between them. If left for some period of time, the compressed medium can create a ‘sticking’ action, making it difficult to open the gate.

4.19 Slab Valve This drawback is overcome in the slab valve, shown in Figure 4.22, in which the gate is pushed against the seal by the flow pressure. Manual override Actuator Return spring

Stem Gate Gate bore Process bore Valve cavity

Figure 4.22.  In the slab valve, the gate is pushed against the seal by the flow pressure.

Valve Construction   •  65

The moving section comprises a slab-shaped gate (Figure 4.23) with a single port drilled through it. A variety of finishes and materials of construction (including ceramic) are available for corrosive and erosive conditions. Generally, wedge gate valves seal better than single slab gates, at a low pressures.

4.20 Expanding Gate Valve

Figure 4.23. The moving section comprises a slabshaped gate with a single port drilled through it.

The expanding gate valve provides a tight mechanical seat seal, both upstream and downstream, and under high and low differential pressure conditions, by mechanically expanding the gate and segment assembly against the seats. The gate comprises two segmented assemblies that are centralized with respect to each other by either a spring (Figure 4.24) or lever mechanism. When the gate is in its final upper (open) position (Figure 4.25), with the front segment against the upper stop, the lower back angles are in contact with each other.

Figure 4.24.  The expanding gate valve comprises two segmented assemblies that are attached to each other by either a spring or lever mechanism.

66  •   The Concise Valve Handbook

Stem

Top stop

Body

Bottom stop

Figure 4.25.  When the gate is in its final upper (open) position, with the front segment against the upper stop, the lower back angles are in contact with each other (courtesy Daniel Valves).

This is illustrated in greater (exaggerated) detail in Figure 4.26 and shows how the gate and segment assembly expand, sealing against both seats and protecting the seat facing from the flow line and isolating the body cavity from the flow bore. While the valve is opening or closing, the gate–segment assembly is collapsed, with both back angles services in contact. In this case, the centralizing mechanism prevents relative movement between the gate and segment, allowing the gate and segment assembly to travel freely without sticking or wedging. When the gate moves to its final lower (closed) position (Figure 4.27), with the front segment against the bottom stop, the upper back angles of the gate and segment now move into contact with each other.

Force due to stem

Force due to top stop

Exaggerated view of open position

Figure 4.26.  Exaggerated view showing how the opposing forces cause the gate and segment assembly to expand, sealing against both seats.

Valve Construction   •  67 Stem

Body

Top stop

Force due to stem

Force due to bottom stop

Bottom stop

Exaggerated view of closed position

Figure 4.27.  When the gate is in its final lower (closed) position, with the front segment against the bottom stop, the upper back angles are in contact with each other (courtesy Daniel Valves).

Figure 4.28.  Exaggerated view showing how the opposing forces cause the gate and segment assembly to expand, sealing against both seats.

This is again illustrated in greater (exaggerated) detail in Figure 4.28 and shows how the gate and segment assembly expand, sealing against both seats and forming a tight mechanical seal.

4.21 Knife Edge Gate Valve In essence, the knife edge gate valve (Figure 4.29) comprises a blade, supported by guides, that moves down across the bore of the valve to close off flow of the medium. A major advantage of the knife edge gate valve is that it can be left open or closed on a variety of water, gas, and chemical duties for long periods of time, with the sure knowledge of satisfactory operation when required. However, while the gate valve can be usually used in a fully open or fully closed position, close regulation of flow is not possible. This is because throttling only occurs when the valve is in an almost shut ­position, where most of the flow reduction occurs. At this point, the small ­crescent-shaped aperture causes a high flow velocity that can erode seat faces.

68  •   The Concise Valve Handbook Valve stem Blade guides

Blade V-insert

Figure 4.29.  The knife edge gate valve is an excellent valve for service that requires either full or no flow.

Furthermore, repeated movement of the disc near the point of closure against upstream pressure can create a drag between the seat on the downstream side and may gall or score the seat faces. In addition, the high-velocity flowing liquid impinging against a partially open disc or wedge produces vibration that can damage seating surfaces and score the downstream side. A variation on the conventional gate valve is the V-insert (Figure 4.30) that modifies the flow/travel characteristics from quick opening to that shown in Figure 4.31. Another adaptation that caters to close regulation is the sliding gate seat illustrated in Figure 4.32, which makes use of slotted movable disc, and a stationary slotted plate. When throttled open, the orifices of the disc align with the openings of the plate to allow the required flow to pass through the slots (Figure 4.33 (a)). When the valve is closed, the disc and plate form a solid barrier to flow, with the upstream pressure and retaining guide combining to keep the disc and plate in constant contact (Figure 4.33 (b)). Because the disc and plate remain in constant contact, chatter is eliminated. Valve noise is also reduced through the use of straight-through flow passage, which reduces turbulence, and multiple orifices that divide the flow into smaller components.

Valve Construction   •  69 Valve stem Blade guides

Blade V-insert

Figure 4.30.  The V-insert gate valve.

lg

ate

100 na tio en nv

60

Co

Percentage flow

80

40 20

rt

se

in

V-

20 40 60 Percentage travel

80

100

Figure 4.31.  Flow/travel characteristics for different variations of the sliding gate valve.

A further variation on this scheme is shown in Figure 4.34, which again makes use of a stationary plate. Here, however, a rotating disk ­covers or uncovers two holes that form the flow aperture.

4.22 Pinch Valve Basically, the pinch valve comprises a rubber hose or sleeve (Figure 4.35), which is clamped in a pipeline and pinched together to stop the flow. When

70  •   The Concise Valve Handbook Slotted movable disc

Slotted stationary plate

Figure 4.32.  Sliding gate regulator valve makes use of slotted movable disc and a stationary slotted plate (courtesy Jordan Valve). Slotted stationary plate

Slotted movable disc (a)

Actuating mechanism

(b)

Figure 4.33.  (a) When throttled open, the orifices of the disc align with the openings of the plate to allow the required flow to pass through the slots. (b) When the valve is closed, the disc and plate form a solid barrier to flow (courtesy Jordan Valve).

fully open, it is similar to a straight-through rubber-lined pipe, and when closed, gives complete isolation. Simple and cost-effective, the pinch valve has an excellent resistance to abrasion, and because of this, it is ideal for use on slurries. Because there are no cavities, seals, glands, or seats, there is little wear. Further, turbulence is minimal, thus also increasing the life of the valve.

Valve Construction   •  71

Open

Throttling

Closed

Figure 4.34.  Sliding gate valve using a stationary plate and a rotating disk. Resilient rubber pipe

(a) Flow shut-off occurs when pipe is pinched

(b)

Figure 4.35.  (a) The pinch valve comprises a rubber hose, which normally provides full flow. (b) When pinched together, the flow stops.

Typically, the closing device in a pinch valve is a double vise mechanism (Figure 4.36) where the clamps move together and pinch the valve closed at the center line. Pinch valves may be used very effectively as control valves in a number of applications where abrasion, sewerage, and corrosion are factors. However, it should be noted that very high forces are required to operate pinch valves and keep them closed. A medium pressure of 6 bar, for example, requires a force of over around 8 tons to close and seal a 300 mm (12”) pinch valve. Generally, therefore, their use in control applications should be restricted to low pressures (about 4 bar) and low velocities. In another form, the air-operated pinch valve illustrated in Figure 4.37, the valve body acts as a built-in actuator. Modulating the air pressure within the annular space between the body and sleeve can open, throttle, or close the valve. An actuation pressure of approximately 2 to 3 bar over line pressure is required for closure.

72  •   The Concise Valve Handbook Valve stem

Vice clamps actuated by valve stem Resilient rubber pipe

(a)

(b)

Figure 4.36.  Double vise mechanism in which the clamps move together and pinch the valve closed at the center line (a) open and (b) closed.

Figure 4.37.  In the air-operated pinch valve, the valve body acts as a built-in actuator, requiring an actuation pressure of approximately 2 to 3 bar over line pressure for closure (courtesy Red Valve).

Air-operated pinch valves are available in line sizes up to 700 mm (28”) diameter at working pressures of 3.5 bar.

4.23 Diaphragm Valve Diaphragm valves make use of a resilient diaphragm, which is used to effect closure. The principal advantage of this type of valve is that the stem seal is eliminated. Diaphragm valves are used primarily for handling viscous fluids, slurries, or corrosive fluids. They can be used to throttle flow, but because of the large shut-off area, low-flow throttling characteristics are not good. The effective operating temperature range is limited by

Valve Construction   •  73

the properties of the diaphragm and run from –50°C to 230°C. Pressure ratings run to 20 bar. A good example of the diaphragm valve is the Saunders patent valve (Figure 4.38) that was originally developed in South Africa to handle slurries on the gold mines—a service for which it is ideally suited. Flow through the body is over a transverse weir, and the valve is closed by means of a flexible rubber or synthetic dome-shaped diaphragm. Like pinch valves, the limiting factor is the high stem thrust required to depress the diaphragm at high pressures. Equally suited to on/off or flow control applications, a diaphragm valve will handle positive pressures or high vacuum. Due to the wide range of material options, it will handle almost all applications within its temperature and pressure ranges (175°C max. and 16 bar max.), and as such, is used in almost every industry on both corrosive and abrasive applications. All working parts are isolated from the line media, which enhances its reliability. Maintenance is a simple task with the body remaining in the pipe, the valve thus being ‘field serviceable.’ It has a linear flow characteristic that makes it well-suited to throttling or modulating duties at modest differential pressures. On/off and control automation is possible with the use of modern compact actuators and accessories. With a wide variety of diaphragms and linings available, almost any corrosive fluid can be handled with confidence. However, line pressures are extremely limited, and characteristics are unpredictable beyond 50% open.

Valve stem Resilient flexible diaphragm

Transverse weir

Figure 4.38.  In the Saunders patent, flow through the body is over a transverse weir (a) and the valve is closed by means of a flexible rubber or synthetic dome-shaped diaphragm (b) (courtesy Crane Process Flow Technologies Ltd.)

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Figure 4.39.  Straight-through diaphragm valves, exemplified by the Saunders full bore K/KB type, have a smooth non-turbulent body design and are designed for minimum flow resistance while allowing rodding out and easy cleaning (courtesy Crane Process Flow ­Technologies Ltd.).

Straight-through diaphragm valves (Figure 4.39) have a much longer stroke and require a more flexible diaphragm than the weir-type, restricting the diaphragm and material to elastomers. Because of their high flexibility and large area, high vacuum will tend to balloon the diaphragm into the flow passage. The degree of ballooning varies according to the model type and manufacturer, producing either a small and acceptable reduction in flow area or a collapse of the diaphragm. In the latter case, the bonnet must be evacuated to balance the pressure on the diaphragm.

4.24 Rotary Control Valves Rotary control valves have become increasingly popular in recent years due to their low weight, simplicity of design, high capacity, and reliable and friction-free packing. And, because of their comparative simplicity, rotary valves are also lower in price, have higher reliability, and are easier to maintain. Another major advantage of the rotary valve when compared with the sliding stem is that the stem rotates about a fixed axis. This eliminates the alternate wetting and exposure to air to which the sliding stem is subject, thereby minimizing corrosion along the stem in the packing box area. This permits the use of simple, and in many cases, single-depth stuffing boxes.

Valve Construction   •  75

4.25 Ball Valve Originally designed for on/off service in high capacity, moderate pressure applications, the role of the ball valve has gradually been expanded, and it is now being used to replace the globe valves in many control applications. Ball valves are available in a wide range of body constructions and sizes, ranging from as small as 6 mm (½”) up to 1,200 mm (48”) and even higher. In its simplest form, the ‘floating’ ball or ‘full-ball’ valve (Figure 4.40) comprises a solid ball having a straight hole drilled or cast through its center, which rotates perpendicular to the flow stream. The ball is mounted between two PTFE seat rings that support the ball in the valve body and provide bearing support to the ball segment. A closely machined ‘spade’ or blade on the stem fits in a slot machined in the ball. The stem passes through the body of the valve and will have a sealing arrangement to withstand the pressure of the media flowing through the valve. When turned through 90° to its closed position (hence, the term ‘quarter turn’ valve), the full media pressure acts on the side of the ball, forcing it against the rear seat to provide extremely high sealing, typically up to ANSI Class 6. The floating ball valve is available in a number of body designs that determine how the ball is inserted into the body. In the split-body design, shown in Figure 4.41, the split between the body and cap is off-center so that the stuffing box is left intact when the ball or seating is removed.

Forward seating seal

Rear seating seal

Open

Closed

Figure 4.40.  In the ‘floating’ ball valve, the ball is completely supported by two seats that provide bearing support to the ball segment, with fully closed to fully open performed by a 90° rotation of the plug segment.

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Figure 4.41.  In the split-body design, the split between the body and cap is off-center so that the stuffing box is left intact when the ball or seating is removed.

Other designs include an end-entry design (Figure 4.42), in which the body comprises a single piece, with the ball retained by an insert screwed into the body at one end; a top-entry design in which the ball is inserted from top; and a three-piece design in which the ball and seating is retained in position by two end caps that are secured to the body by bolts passing through all three sections. The downside of the floating ball is that much higher torques are required to seat and unseat the valve than required in the mid-position,

Screwed insert

Figure 4.42.  End-entry design in which the body comprises a single piece, with the ball retained by an insert screwed into the body at one end.

Valve Construction   •  77

the so-called ‘run’ torque. As a result, while floating ball valves are ideal for on/off duties, they are not suited for modulating duties where the high friction levels (particularly on the rear seating) lead to rapid wear and damage.

4.26 Trunnion Ball Valve This problem is overcome in the trunnion ball valve, illustrated in Figure 4.43, where the ball is fabricated with an integral shaft (or trunnion) that supports and centers the ball within the body. In this manner, the medium pressure forces are transferred to the trunnions and bearings, rather than to the seals. Since the ball is fixed and does not move with medium pressure, leakage integrity is ensured through the use of floating spring-loaded seats that are kept in contact with the ball, even in the absence of line pressure (Figure 4.44). As shown in more detail in Figure 4.45, as the line pressure increases, the seat area creates a piston effect that forces the seat against the ball. Thus, the higher the line pressure, the greater the piston action. In high-pressure large-sized ball valves use, it is often made of a lubricated ball in which a special sealant is injected between the sealing faces. This also allows emergency sealant injection to be carried out in the event of seat-insert or stem-seal damage that could lead to external or internal leakage.

Top load bearings

Forward seating seal

Top trunnion

Rear seating seal Bottom trunnion

Bottom load bearings

Figure 4.43.  The trunnion ball valve distributes excess hydraulic load into the valve body, rather than through the seating (courtesy ITT Industries, Inc.).

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Upstream

Downstream

Figure 4.44.  Leakage integrity is ensured through the use of floating spring-loaded seats that are kept in contact with the ball even in the absence of line pressure.

Figure 4.45.  As the line pressure increases the seat area creates a piston effect that forces the seat against the ball. Sealant injection points allow emergency sealant injection to be carried out in the event of seat-insert or stem-seal damage that could lead to external or internal leakage.

A well-designed trunnion-mounted ball valve has the capability of providing both upstream and downstream sealing integrity so that minimal pressure-build occurs in the body cavity. In the event that leakage past the seals does occur, and in order to cater for double block and bleed applications*, a body vent (Figure 4.46) allows the body cavity to be vented to atmosphere or safe disposal. *See Appendices for further information on double block and bleed systems. The diagrams of ball valves illustrated so far show what is termed a full port design in which the diameter of the ball port is the same diameter

Valve Construction   •  79 Valve body

Process fluid

Figure 4.46.  In the event that leakage past the seals does occur, and in order to cater for double block-and-bleed applications, a body vent allows the body cavity to be vented to atmosphere.

as the inside diameter of the pipe and body, thus offering virtually no resistance to fluid flow. Flow is unrestricted, but the valve is larger and more expensive, so this is only used where free flow is required, for example, in pipelines, which require pigging. In cases where size and weight of the valve are an issue, use may be made of a regular port ball, in which the diameter of the port may be reduced down to 75% of the full port diameter, reducing the weight substantially. On much smaller valves, requiring a much smaller ball diameter, use is made of a reduced port ball where the port diameter is typically one pipe size smaller than the valve’s pipe size. In its fully open position, the ball valve presents a single circular ­orifice to the flowing medium (Figure 4.47 (a)). However, as the ball is rotated toward its closed position, the shape of the opening changes to become two identical elliptical orifices that offer two equal restrictions in series (Figure 4.47 (b)). Finally, when the valve is fully closed (Figure 4.47 (c)), flow is completely blocked. The pathway, or ‘waterway,’ between these two restrictions allows the flow to partially recover. The overall result is an inherent equal-­ percentage flow characteristic. The streamlined design of the ball valve yields very high capacity, and accordingly, greater noise generation and cavitation potential.

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(a)

(b)

(c)

Figure 4.47.  (a) In its fully open position, the ball valve presents a single circular orifice to the flowing medium. (b) As the ball is rotated toward its closed position, the shape of the opening changes to become two identical elliptical orifices that offer two equal restrictions in series. (c) Closed position.

Figure 4.48.  Pathway fitted with parallel perforated attenuator plates that produce a smooth gradual pressure reduction across the valve that minimizes velocity, noise generation, and cavitation (courtesy Neles-Metso).

In some ball valve designs, the pathway is fitted with parallel perforated attenuator plates that produce a smooth gradual pressure reduction across the valve that minimizes velocity, noise generation, and cavitation (Figure 4.48). Ball valves are available in a wide range of body constructions and sizes, ranging from as small as 6 mm (½”) up to 1200 mm (48”) and even higher.

Valve Construction   •  81

4.27 Characterized Ball Segment Valve Despite all the benefits offered by the full-ball valve, it was basically designed for on-off control, with the flow characteristics more or less fixed. In the characterized ball segment valve (Figure 4.49), the opening between the ball and seal is modified to provide different flow characteristics. The V-notched ball segment produces an equal percentage flow characteristic and provides a shear-on-close action, making it suitable for slurry applications and for fluids that include fibrous and stringy material. Other contours include the U-notch and parabolic notch providing differing flow characteristics (Figure 4.50). In the ball segment valve shown in Figure 4.51, the shaft is centrally mounted to give constant contact between the segment and the seat during

Figure 4.49.  Characterized V-notch ball valve in which the opening between the ball and seal is modified to provide different flow characteristics (courtesy Emerson–Fisher). 100

60 40 B

ge

ca

Pa

ra

20

nd

a all

U- V-n no o bo tch tch lic -n ot ch

Percentage flow

80

20 40 60 Percentage travel

80

100

Figure 4.50.  Different flow characteristics of the characterized ball valve.

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Figure 4.51.  Ball ­segment valve with centrally mounted shaft (courtesy Somas Instrument AB).

Figure 4.52.  Ball segment valve with eccentrically mounted shaft, allowing pressure between the segment and seat to be increased by increasing torque ­(courtesy Somas Instrument AB).

opening and closing. Suitable for both clean liquids and fiber suspensions, the seat acts like a scraper, keeping the surface of the segment clean. In the version shown in Figure 4.52, the shaft is located eccentrically. This enables the pressure between the segment and the seat to be increased by increasing the torque on the shaft. Typical applications are clean, hot media such as steam and gases, where it is necessary to use a Stellite seat. Due to the eccentricity, friction between the seat and segment is avoided, resulting in increased valve life. Ball valves are available in threaded, flanged, or flangeless construction. However, in keeping the philosophy of lightweight designs in large sizes, most ball valves are of the flangeless variety.

4.28 Butterfly Valve As the larger nominal bore diameters are reached, that is, from 100 mm upward, the ball valve tends to become bulky, and if manufactured of a sophisticated heat and/or corrosion-resistant material, can become expensive. This has led to the development of the butterfly valve as a control device. The butterfly valve comprises a circular disc-shaped damper mounted on a shaft (Figure 4.53) that rotates in the flow path to regulate flow. In a conventional center-disc butterfly valve, when the disc is rotated from the closed position (Figure 4.54), it pulls away from the seal. Unlike the ball valve, which provides fully open to fully closed ­control with a 90°rotation of the valve stem, rotation of the conventional butterfly valve is limited to about 60°. This is because, as the disc is rotated nearer to the fully open position, the leading and trailing edges of the disc are shadowed by the shaft.

Valve Construction   •  83

Figure 4.53.  Butterfly valve comprises a circular disc-shaped damper mounted on a shaft.

The disc of a center-disc type valve is either a one-piece casting where the upper and lower stems are integral or welded on to the disc. Alternatively, the stem passes through the disc and is bolted or pinned to the disc. The body of the valve is lined with an elastomeric material that is held in position in the body by either bonding or mechanical means. As the disc moves into the closed position, the material of the liner is an interference fit with the disc edge, and this interference determines the pressure sealing capability of the valve and the torque required to close and open the valve into or from the full closed position. In the offset disc, or high performance butterfly valve (Figure 4.55), the disc is offset in two planes, resulting in an eccentric rotating or camming action. This results in the disc moving away from the seat after approximately 10° of rotation, and equal loading of the seat whenever the valve is closed. Seat designs vary from pressure-energized reinforced PTFE to solid-filled PTFE. Often supplied with a metal seat for Ring seal higher pressure (100 bar) and temperatures Disc (cryogenic to 600°C), advantages of the offset disc butterfly valve include: better seal perStem formance, lower dynamic torque, and higher allowable pressure drops. The improvement in seal performance is because the disc cams in and out of the seat, and thus contact is made only at closure. Apart from the conventional disc shape shown in Figure 4.56(a), which is limited to rotation over 60°, the fishtail disc (Figure 4.56 Figure 4.54. Conventional (b)) permits control through full 90° rotation. center-disc butterfly valve.

84  •   The Concise Valve Handbook Seal ring PTFE seal Eccentric disk path of rotation

Eccentric disk centre of rotation Valve body center-line

Conventional disk centre of rotation

Conventional disk centre of rotation

Figure 4.55.  Offset disc, or high performance butterfly valve.

(a)

(b)

Figure 4.56.  (a) conventional disc shape (b) the fishtail disc (courtesy Emerson–Fisher).

The flow characteristics (Figure 4.57) of a butterfly valve are essentially equal percentage, but are dependent on the shaft location and the comparative size of the shaft to the valve. These flow characteristics, together with low cost, make the butterfly particularly attractive for inline flow control. However, it should be noted that effective control in butterfly valves is over the first part of its opening, where both instability and ­serious cavitation occurs.

4.29 Plug Valve The conventional plug valve is one of the oldest valves in use and consists of either a parallel or tapered metal plug that is fitted into a metal body

Valve Construction   •  85

60

Ty pur pical pos ge e nera l

Percentage flow

80

40

6 pe 10 m rfo m rm hi an ghce

5 pe 0 m rfo m rm hi an gh ce -

100

20

20

40

60

80

100

Percentage travel

Figure 4.57.  Flow characteristics of different-­ shaped butterfly discs.

Port

Rotary tapered plug

Figure 4.58.  Basic plug valve.

(Figure 4.58). When the plug is rotated, it permits flow through the port of the plug, while a 90° turn in either direction completely blocks the flow path. The sealing mechanism between the plug and body can be either of the lubricated type, where a suitable sealant/lubricant is injected under pressure between the machined faces of plug and body or of the PTFE liner type where the liner is permanently fitted between plug and body. The lubricated type suffers from a major defect, in that each time the valve is operated, the lubricant is washed away by the media and has to be replaced by the injection of fresh lubricant through porting and pressure seals in the plug stem arrangement. Consequently, the lubricated valve does not lend itself to automation unless a very sophisticated automatic

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lubrication system is used, making the valve expensive both in initial cost and maintenance. The PTFE-lined valve has a limitation to class 300 pressures and a maximum temperature of 230°C due to the use of PTFE. Nonetheless, automation of the PTFE lined plug valve is a better proposition, but because of the large area of contact and the ‘stiction’ of PTFE, high output torques are required from the actuator in order to ‘break out’ the valve, particularly if the valve has been in one position for any length of time.

4.30 Eccentric Plug Valve The Camflex eccentric rotary plug valve (Figure 4.59) is a patented device from Masoneilan and is regarded by many as one of the most significant new products introduced to the control valve world in the last 45 years. The Camflex plug, the joining flexible arms, and the hub are formed from a single casting. This one-piece design ensures that there are no parts inside the valve that can loosen and/or become disengaged to pass downstream during operation. The seat portion of the plug has the form of a spherical segment and is rotated through a nominal angle of 50°. The center of the spherical seating surface is offset from the shaft axis. The one piece shaft, which is connected to an actuating arm linked to the actuator piston rod, rotates the plug face in an eccentric, cam-like motion, down and forward into the seat. As the plug rotates into its seated position, it makes no contact with the seat until the actual moment of seating. Once seating occurs, a positive seal between plug and seat is achieved by the elastic deformation of the

Figure 4.59.  Camflex: eccentric rotary plug valve (courtesy Masoneilan).

Valve Construction   •  87

Figure 4.60.  Once seating occurs, a positive seal between plug and seat is achieved by the elastic deformation of the plug arms (courtesy Masoneilan).

plug arms (Figure 4.60). When the plug seats, the arms ‘flex’ such that additional actuator thrust forces the plug deeper and into tighter contact with the seat. The shaft connection allows the plug to center itself along the shaft axis. The angle of contact between the plug and seat lets the plug wipe larger particles off the seating surfaces, yet permits no rubbing, once contact is established. Standard leakage classifications for Camflex valves with metal-to-metal seating surfaces conform to ANSI B16.104 Class IV. For ANSI B16.104 Class VI (bubble tight), use is made of PTFE soft seat construction. Another feature of the Camflex valve is that only one-third the amount of the force required to stroke a conventional single-seated globe valve is required of the Camflex actuator to stroke against a given pressure drop. This reduction in the required actuator force is created by the mechanical advantage used in the design in which the lever arm ratio between the actuator lever and the plug eccentricity substantially amplifies the seating force. The spherically shaped, eccentric rotating plug provides an inherent flow characteristic that is essentially linear. As the plug approaches the seat, the characteristic curve is modified as the rate of change in flow is reduced smoothly until the plug actually contacts the seat. The offset between the plug and shaft develops a mechanical advantage that leads to greater torque and dynamic stability. Integral extension bonnets make this valve acceptable on cryogenic fluids and temperatures to 400°C. It also eliminates a gasket joint at the body to bonnet interface and associated potential for leakage.

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4.31 Check Valves A check valve is simply a one-way valve designed to prevent fluid flow reversal. The two most common applications are at pump discharges and where a piping system joins a common header. Although available in a variety of configurations, the two most common designs are the ‘swing check valve’ and the ‘pinch check valve.’ In the hinged ‘swing check valve’ illustrated in Figure 4.61, the sealing disc is attached to a hinge, which is free to rotate around the hinge pin. The force of a fluid entering the valve from the inlet overcomes the weight of the disc and hinge and causes them to rotate around the hinge pin and swing upward, opening the valve. If the fluid stops, the weight of the disc and hinge causes the disc to swing down to its seated position. If fluid enters from the outlet end, it presses the disc against the seat ring and the valve stays closed. The pinch check valve, illustrated in Figure 4.62, comprises a onepiece rubber matrix and ply reinforcement, similar in construction to a truck tire, operates using line pressure and backpressure to open and close. With a typical 30-year operational lifespan, such valves have, a low headloss; they do not rust or corrode; they are not affected by UV; and their flexibility allows them to compress around trapped solids, thus providing a much better seal than flap gates.

4.32 Valve Sizes and Pipe Schedules Valves, like pipes, are rated according to their size. The normal pipe diameter is given according to a preferred series and referred to as their nominal pipe size (NPS) or their nominal bore (NB)—both based on inches. Cover Hinge

Hinge pin

Disc nut

Seat ring

Disc

Figure 4.61.  In the hinged ‘swing check valve,’ the sealing disc is attached to a hinge, which is free to rotate around the hinge pin.

Valve Construction   •  89

Figure 4.62.  The pinch check valve feature: a typical 30-year operational lifespan, a low headloss; they do not rust or corrode; they are not affected by UV; and their flexibility allows them to compress around trapped solids (courtesy Tideflex).

The European equivalent to NPS is the nominal diameter (DN) measured in millimetres (Table 4.1). It should be noted that, in this case, ‘nominal’ means just that—in other words: approximate or ‘not necessarily corresponding exactly to the real value.’ The question might also be raised as to whether the term ‘diameter’ refers to the ID (inner diameter) or the OD (outer diameter) of the pipe. In fact, it depends. Because pipes are designed to carry fluid under pressure, another ­critical dimension must be their strength—a function of the wall thickness expressed in ‘pipe schedules’ (sched. or sch.). The wall thickness, associated with a particular schedule, depends on the pipe size. Table 4.1.  Preferred series of ANSI and DN pipe sizes Pipe diameter in Pipe ­diameter Pipe diameter in Pipe diameter inches in mm inches in mm (ANSI - NPS) (DN) (ANSI - NPS) (DN) 0.5 15 8 200 0.75 20 10 250 1 25 12 300 1.5 40 14 350 2 50 16 400 3 80 24 600 4 100 36 900 6 150 48 1200

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Based on the NPS and the pipe schedule, the pipe OD and wall thickness can be obtained from reference tables based on and B36.19M. An example of some of the values is given in Table 4.2. For NPS 0.125to 12 inches, the NPS refers to the ID. Thus, for example, the OD of an NPS 12 pipe is actually 12.75 inches. However, for NPS 14 inches and up, the NPS is the OD and an NPS 14 pipe is actually 14 inches OD. Most confusing. Although not shown in Table 4.2, some specifications use pipe schedules called standard wall (STD), extra strong (XS), and double extra strong (XXS). Another pipe schedule is the ‘S’ designation—most often indicating stainless steel pipes.

4.33 Material Selection The valve body must contain the fluid without leaking and resist corrosion and erosion by the fluid. A major feature of the globe-style body is the smooth, streamlined, constant-area internal passages. The absence of pockets permits high capacity flow with minimum turbulence. Bodies are available in a wide variety of castable or machinable materials depending on the fluid that must flow through them. The most common cast materials are listed in Table 4.3. The main factors to be taken into account in selecting a suitable material are: corrosion and erosion.

4.34 Corrosion Corrosion charts give an indication of which materials are suitable for various fluids. However, factors such as concentrations, temperatures, contaminant substances, and pressure can substantially alter these recommendations. As an example, while dry chlorine gas can be used successfully with carbon steel, it should not be used with titanium. However, the opposite is true for wet chlorine gas. While bronze and stainless steel are widely used for salt water and most corrosive liquids, Monel (high nickel/high copper alloy) and Hastelloy B (nickel, molybdenum, iron and vanadium alloy) and C (nickel, molybdenum, chromium iron and tungsten alloy) are frequently used for very corrosive service. Generally, the best indicators come from the end user’s experience with materials used on similar applications. Line materials are also useful

5 0.065 1.651

5 0.065 1.651

5 0.083 2.108

1 inch 25 mm Wall thickness (inches) (mm)

2 inch 50 mm Wall thickness (inches) (mm)

4 inch 100 mm Wall thickness (inches) (mm)

10 0.120 3.048

10 0.109 2.769

10 0.109 2.769

20 – –

20 – –

20 – –

30 0.188 4.775

30 0.125 3.175

30 – –

OD = 1.315 inch = 33.4 mm Schedule 40 60 80 – 0.133 0.179 3.378 4.547 OD = 2.375 inch = 60.33 mm Schedule 40 60 80 – 0.154 0.218 – 3.912 5.537 OD = 4.500 inch = 114.30 mm Schedule 40 60 80 0.237 0.281 0.337 6.020 7.137 8.560 100 – –

100 – –

100 – –

Table 4.2.  Example of some pipe schedules based on ASME standards B36.10M and B36.19M

120 0.437 11.100

120 0.250 6.350

120 – –

140 – –

140 – –

140 – –

(Continued)

160 0.531 13.487

160 – –

160 – –

Valve Construction   •  91

5 0.109 2.769

5 0.134 3.404

5 0.156 3.962

5 0.188 3.962

8 inch 200 mm Wall thickness (inches) (mm)

10 inch 250 mm Wall thickness (inches) (mm)

14 inch 350 mm Wall thickness (inches) (mm)

20 inch 500 mm Wall thickness (inches) (mm)

Table 4.2. (Continued)

10 0.250 6.350

10 0.250 6.350

10 0.165 4.191

10 0.148 3.759

20 0.375 9.525

20 0.312 7.925

20 0.250 6.350

20 0.250 6.350

30 0.500 12.700

30 0.375 9.525

30 0.307 7.798

30 0.277 7.036

OD = 8.625 inch = 219.08 mm Schedule 40 60 80 100 0.322 0.406 0.500 0.593 8.719 10.312 12.700 15.062 OD = 10.75 inch = 273.05 mm Schedule 40 60 80 100 0.365 0.50 0.593 0.718 9.271 12.700 15.062 18.237 OD = 14.00 inch = 355.60.05 mm Schedule 40 60 80 100 0.437 0.593 0.750 0.937 11.100 15.062 19.050 23.800 OD = 20.00 inch = 508.00 mm Schedule 40 60 80 100 0.593 0.812 1.031 1.280 15.062 20.625 26.187 32.512 120 1.500 38.100

120 1.093 27.762

120 0.843 21.412

120 0.718 18.237

140 1.750 44.45

140 1.250 31.750

140 1.000 25.400

140 0.812 20.625

160 1.968 49.987

160 1.406 35.712

160 1.125 28.575

160 0.906 23.012

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Valve Construction   •  93

Table 4.3.  Some of the most common cast materials Material Cast carbon steel

Cast chromium– molybdenum steel

Cast stainless steel 304L

Cast stainless steel 316

Cast stainless steel 317

Cast iron

Bronze

ASTM grade A216 WCC

Temperature (°C) Comments 427 Moderate service on air, steam, and noncorrosive fluids. Can be welded without heat treatment. A217 593 Has replaced Grade WC9 C5. Provides erosion/ corrosion and creep resistance. Requires pre- and post-welding heat treatment. A351 CF3 427 Best material for nitric acid and other chemical service applications. A351 –200 to 815 Industry standard with CF8M highest temperature range. A molybdenum additive gives greater resistance to corrosion, pitting, creep, and oxidizing fluids than 304. A479 UNS –200 to 815 1% increased nickel and molybdenum affords greater resistance to pitting. Excellent resistance to digester liquor and dry chlorine dioxide. A216 232 Inexpensive nonductile for use on steam, water, gas, and non-corrosive fluids. B61/B62 288 Salt water and corrosive service as well as oxygen.

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guides, but since the velocities are higher in the valve than the line, more severe corrosion can be expected.

4.35 Erosion Normal steels do not stand up well to erosion. To combat erosion, the material must be soft like rubber so that the surface absorbs the abrasion without wearing, or it must be very hard so it can resist the abrasion. For highly erosive applications at high temperatures, use is often made of ceramic trim. Ceramics also have very good corrosion properties, but are very expensive.

4.36 End Connections In order to install the valve into the process pipeline, some form of end connection must be used. The three methods used are screwed pipe thread, bolted flanges, and welded end connections.

4.37 Screwed End Connections Screwed end valve connections are by far the most widely used, but are usually limited to valves smaller than 50 mm manufactured in brass, iron, steel, and alloy piping materials. Although suited for all pressures, this connection style is not recommended for elevated temperature service. Usually, the threads are tapered female BSPT (British Standard Pipe Thread) or NPT (National Pipe Thread) on the valve body (Figure 4.63) that form a metal-to-metal seal with the male threads of the pipeline ends.

Valve body with tapered female screw thread

Figure 4.63. Screwed end valve connections with tapered female thread.

Valve Construction   •  95

This type of end connection complicates maintenance because the valve cannot by removed without breaking a flanged joint or union connection to permit unscrewing the valve body from the pipeline.

4.38 Flanged End Connections Where the valve must be removed from the process line for inspection or maintenance, use can be made of flanged end connections. Suitable for temperatures up to 800°C, flanged end connectors are used on all valve sizes. Flanges are available in discrete pressure ratings given according to a preferred series—the ‘ANSI class’ or the ‘PN series’ (Table 4.4). It should be noted that there is not a direct relationship between the ANSI class and the PN rating. Whereas the PN series is rated directly in bar (where PN = nominal pressure), the ANSI class number relates to a pressure/temperature relationship, as illustrated in Figure 4.64. Table 4.4.  Preferred series of ANSI class and PN pressures ANSI class 150 300 600

PN (bar) 20 50 110

ANSI class 900 1500 2500

PN (bar) 150 260 420

450 400

Class 2500

Pressure (bar)

350 300 250 200 150 100 50 0

Class 1500

Class 900 Class 600 Class 300 Class 150 −50 0

50 100 150 200 250 300 350 400 Temperature (°C)

Figure 4.64.  Maximum pressure rating versus temperature for carbon steel flanges (ANSI B16.5).

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Figure 4.65.  Flat-face flanged end connection.

Flanged end connections are available in different forms, either in flat face or raised face. The flat face ­variety (Figure 4.65) allows the matching flanges to be in full-face contact with the gasket clamped between them. This construction is commonly used in low-­pressure, cast iron, and brass valves and has the advantage of ­minimized flanged stresses. The raised-face flange has a circular raised face (Figure 4.66) that is finished with concentric circular grooves for good sealing. This type of flange is suitable for temperatures up to 800°C and pressures up to 400 bar.

4.39 Hub End Body

Figure 4.66.  The raisedface flange has a circular raised face.

Also known as the separable flange body (Figure 4.67), it has been designed to accommodate loose flanges. The flange is slipped over the hub end and then two steel half rings are inserted into a groove and tack welded together for retention. The main advantage of this style is that one body can accommodate a wide variety of flange standards and ratings. Since the flanges do not come into contact with the process media, carbon steel flanges can be used on bodies fabricated from expensive materials.

4.40 Welded End Connections Welded end connections are leak-tight at all pressures and temperatures. Limited to weldable materials, welded end connections are also

Valve Construction   •  97 Half rings

Sealing face of valve body

Valve body with groove

Loose flange

Figure 4.67.  Hub end or separable flange body.

more difficult to remove from the process line. In the socket welding end (Figure 4.68), generally used on small sizes up to 50 mm diameter, the inside diameter bore of the valve body is just slightly larger than the pipe diameter. In the butt-welding ends (Figure 4.69), used above 50 mm, each end of the valve body is beveled to match a similar bevel on the pipe. Fillet weld

Socket

Valve body with socket bore

Figure 4.68.  Socket welding end. Fillet weld

Valve body with bevel

Figure 4.69.  Butt welding ends.

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The single biggest problem in the selection of valves and their associated actuators is that the valves tend to be oversized and the actuator undersized. This is due mainly to inaccurate input conditions and to the various safety margins that are added during the selection process.

4.41 Lap Joint Flange The first thing to appreciate is that a lap joint flange must be used in conjunction with a lap joint stub end that is butt-welded onto the process pipeline (Figure 4.70). The lap joint flange itself slides over the stub end fitting and is free to rotate. The radius at the flange face is carefully designed to accommodate the corresponding radius of the lap joint stub end and provide mating of the assembled system. The main benefit of the lap joint flange is that, since it is free to rotate, it avoids any issues with bolt–hole alignment. Furthermore, lap joint pipe flanges are often used for applications that require frequent dismantling for inspection or to remove build-ups. Another benefit offered by the lap joint flange is that, because it is not in direct contact with the process fluid, it will often allow inexpensive carbon steel flanges to be used in conjunction with corrosion-resistant piping. Furthermore, in systems that erode or corrode quickly, the flanges may be salvaged for re-use.

Lap joint stub end

Process pipeline Lap joint flange (free to rotate)

Lap joint stub end

Butt weld Lap joint flange (free to rotate)

Figure 4.70.  The lap joint flange is used in conjunction with a lap joint stub end that is butt-welded onto the process pipeline (courtesy Coastal Flange Inc.).

Valve Construction   •  99

However, the pressure-holding ability of lap joint flanges and their associated stub ends is little better than that of slip-on flanges. In addition, the fatigue life for the assembly is only about one-tenth that of welded neck flanges, and they are, therefore, not suitable for situations with high external or changing loads.

4.42 Flangeless Connections On larger valves, flanges can add considerably to the overall weight of the valve. Where this might be a limiting factor, use can be made of flangeless-style valve bodies (sometimes also called wafer-style). Flangeless connections are common to rotary-shaft control valves and not only reduce the weight of the valve, but are often used in applications where space constraints need to be considered. As shown in Figure 4.71, flangeless valves are held between flanges by long through-bolts using tightly fitting seals and a flat valve faces on the upstream and downstream sides of the valve. The main concern with such through-bolted is that the long bolts increase the risk of misalignment of the flange faces and consequent leakage. This risk increases as the length of the bolt increases, and therefore, is more of a problem with ball valves, than with butterfly valves, because of the appreciably larger face-to-face distance. In the event of fire, the exposed bolts are likely to expand unequally and increase the leakage Pipework and flange

Flangeless ball valve Through-bolt Pipework and flange

Through-bolt

Figure 4.71.  Flangeless valves are held between flanges by long throughbolts.

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potential. Generally, therefore, flangeless valves are proscribed for use in hydrocarbon service.

4.43 Grayloc® Connector Extensively used in the petrochemical industry the Grayloc® connector (Figure 4.72) comprises three components: • metal seal ring • clamp assembly • hubs The metal seal ring consists of a rib and two lips. During make-up, the seal ring lips deflect inward as the connector is assembled. The clamp assembly is the primary pressure retaining element of the connector—not the bolts. The two-piece configuration ensures equal loading around the entire connector and carries all of the internal pressure loads, as well as axial and bending loads transmitted by the pipe. The clamp assembly fits over the two hubs and forces them against the seal ring rib (Figure 4.73 (a)). As the hubs are drawn together, the seal ring lips deflect against the inner sealing surfaces of the hubs (Figure 4.73 (b)). This deflection elastically loads the lips of the seal ring against the inner sealing surface of the hub, forming a self-energized seal.

Figure 4.72.  The Grayloc® connector comprises three components: a metal seal ring, a two-part clamp assembly, and two hubs (courtesy Oceaneering International).

Valve Construction   •  101 Clamping force Clamp Seal ring lip Hub

Hub

Seal ring (a)

Internal force (b)

Figure 4.73.  (a) The clamp assembly fits over the two hubs and forces them against the seal ring rib. (b) As the hubs are drawn together, the seal ring lips deflect against the inner sealing surfaces of the hubs (courtesy Oceaneering International).

Major features include: • • • • •

significantly smaller and lighter than a flange diameter is less than that of the flange freedom to rotate the clamp no bolt–hole alignment constraints connectors can be assembled and disassembled in confined spaces with minimal clearance

CHAPTER 5

Valve Trim and Characterization One of the more important parameters of a valve is the relationship between the flow rate and the position of the controlling element. In a globe valve this might be the relationship between flow and plug lift ­position that, in turn, is determined by the plug shape. This relationship is called the flow characteristic, and defines the relationship between plug position and flow conditions at that point. Flow is usually plotted as a percentage of ­maximum flow and the lift as a percentage of maximum lift. There are two major forms of flow characteristic: the inherent characteristic and the installed characteristic.

5.1 Inherent Characteristics The inherent flow characteristic is the relationship between flow and plug stroke with a constant pressure drop across the valve. These characteristic curves, provided by the manufacturer, will be different from the installed characteristic where, in practice, the differential pressure across the valve varies throughout the valve position due to system characteristics. If tests are performed on the installed valve, variations of 10% or more, from the inherent characteristic, are likely to be experienced. In practice valves are generally supplied with three or four characteristic curves: linear, equal percentage, quick opening and modified equal percentage (Figure 5.1).

5.2 Linear Inherent Flow Characteristic The first and most common is the linear inherent flow characteristic, which is simply a straight line relationship between flow and stroke at constant pressure drop.

104  •   The Concise Valve Handbook

op en in g

80

Q ui ck

Percentage flow

100

60

ar ne Li

40 e l ag ua ent q E erc p

20 20

40

60

80

100

Figure 5.1.  Inherent flow characteristics.

5.3 Equal Percentage Inherent Flow Characteristic In the equal percentage inherent flow characteristic, equal increments of rated travel will give equal percentage changes of existing flow. Thus, for example, for every 10% change in stroke the flow might double, that is, a change 100%. Figure 5.1 shows the general shape of the equal percentage curve. Other types of the equal percentage flow characteristic are available. It should be noted that at a low stroke of less than 50%, the flow is low relative to the total available flow. This would provide good throttling capability due to small loop or gain of flow relative to stroke and would be a particular advantage when the control valve is oversized. The equal percentage characteristic is often designated simply as (= %).

5.4 Quick Opening Inherent Flow Characteristic The quick opening characteristic produces rapid increases in flow with small stroke increments. A typical form of the curve is shown in Figure 5.1 which shows that very close to maximum available flow is achieved at approximately 30% to 40% of stroke. This form of characteristic is most often applied in block valves or simple on-off control, where throttling is not the primary concern.

Valve Trim and Characterization   •  105

5.5 Modified Percentage Inherent Flow Characteristic This flow characteristic combines the advantages of both linear and equal percentage.

5.6 Characteristic Profiling The inherent flow characteristic depends primarily upon the trim geometry and in globe valves is usually characterized by contouring the plugs such that the curve defines a change of flow area as the plug position is altered. This variance of flow area, with stroke, affects the inherent characteristic. As an example, the easiest way of achieving a quick opening flow characteristic is to provide a simple disc shaped plug (Figure 5.2). This provides the fastest change in flow area as a function of stroke. Figure 5.3 shows the plug outlines used to obtain linear and equal percentage flow characteristics. In this manner, it is possible to change the inherent flow characteristic by changing the plug. In cage-guided trim, flow characterization is determined by the shape of windows in the cylindrical cage (Figure 5.4.)

5.7 Installed Characteristics The inherent flow characteristics discussed previously, and supplied by the manufacturer, are determined at a cons most common is the linear tant pressure drop. However, in the field, few valves actually operate at a constant pressure drop since the pressure is actually distributed across both the valve and other parts of the system (Figure 5.5). Stem Valve plug Seat ring

Flow area

Port diameter

Figure 5.2.  Quick opening flow characteristic uses a simple disc shaped plug (courtesy Fisher Rosemount).

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Equal percentage

Quick opening

Linear

Figure 5.3.  Plug outlines used to obtain (from left to right) equal percentage; linear; and quick-opening flow characteristic (photographs courtesy of Mitech).

(b)

(a)

(c)

Figure 5.4.  For cage-guided trim, flow characterization is determined by the shape of windows in the cylindrical cage (a) equal percentage (b) linear and (c) quick-opening flow characteristics. Pressure drop across piping

Pressure drop across valve

Figure 5.5.  Differential pressure drop is actually distributed across both the valve and other parts of the system.

With the valve closed, the full head is available and the differential pressure drop across the valve is at a maximum. As the valve is opened, and the flow rate increases, other portions of the process piping take up some of the available system pressure drop and the pressure drop across the valve decreases.

Valve Trim and Characterization   •  107

This variation in pressure drop may be characterized by what is termed the ‘pressure drop ratio’ (PDR) or sometimes the ‘valve authority’ which may be defined as: ∆P valve ∆P System

PDR = ∆PR =

This ratio helps define how much deviation we would expect from the inherent flow characteristic. Thus, a PDR of 1.0 indicates that the entire system pressure drop is across the control valve (e.g., a relatively small valve in a large line) and the performance would conform to the valve’s inherent characteristic. As the valve opens, or its size increases relative to the line size, the pressure drop across the valve decreases and the PDR falls—indicating a deviation from the inherent characteristic. This is illustrated in Figure 5.6 which shows that a valve having a linear inherent characteristic (PDR equals 1.0) will tend toward a quick opening characteristic as the PDR decreases. Pressure drop ratios ranging 0.35 to 1.0 are a good choice for linear. In contrast, Figure 5.7 shows how an equal percentage inherent characteristic tends toward an installed linear characteristic as the PDR decreases. Typically, a pressure drop ratio greater than 0.1 and less than 0.35 is a good choice for the equal percentage inherent flow characteristic. The usual practice is to size the valve and the other piping components so that the valve will take 25% to 50% of available system pressure 100

04 0. 10

=

25

20

00

=

=

∆P

40

R

∆P

R

∆P

R

0.

=

50

0. 1.

R

0. = R

∆P

60

∆P

Percentage flow

80

PDR = ∆PR=

20 40 60 Percentage travel

Figure 5.6.  Installed linear flow characteristic.

ΔP valve ΔP system

80

100

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1. 00

R

=

= ∆P

∆P

R

R

= 0. 2 0. 5 50

=

∆P

40

R

∆P

R

60

∆P

Percentage flow

80

ΔP valve ΔP system

= 0. 04 0. 10

PDR = ∆PR=

20

20 40 60 Percentage travel

80

100

Figure 5.7.  Installed equal percentage flow characteristic.

drop when it is wide open. This corresponds to PDR of 0.25 and 0.5 respectively. A good rule of thumb is: • use Linear when the pressure drop across the valve is a large ­proportion of the total pressure drop. • use Equal Percent if in doubt. • use Linear for level control. • use Equal Percent for pressure control. Another guide is shown in Table 5.1.

5.8 Cavitation Control *A large amount of the material used for this section was gleaned from an article by Michael Sessions of Mitech entitled ‘Cavitation control in Control Valves. Table 5.1.  Valve characteristic selection guide Required service Orifice type flow Linear flow Level Gas pressure Liquid pressure

Valve ∆P < 2:1 Quick opening Linear Linear Linear Equal percentage

Valve ∆P >2:1< 5:1 Linear Equal percentage Equal percentage Equal percentage Equal percentage

Valve Trim and Characterization   •  109

The majority of flashing damage occurs at the point of highest velocity—at or near the seat line of the valve plug or seat ring. The process is two-stepped: a corrosion film forms at the surface that is subsequently ‘swept’ away by the high velocity liquid flow. This cycle is then repeated. The solution lies with proper material selection and a commonly used rule-of-thumb guide is that type 316 stainless steel may be used for clean fluid applications where pressure drops do not exceed 14 bar and temperatures are under 350° C. Figure 5.8 shows a basic trim selection guide from Mitech. This guide is based on the conditions of the inlet pressure P1 being at least 3 × P2 (the outlet pressure) and at least 2 × PV (the vapour pressure level of the medium). As an example consider a 100 mm size valve operating at a pressure drop of 4 bar. Since the intersection falls under the stainless steel (SS) curve, this indicates that the standard seat retainer, seat ring and plug can be used without any additional device because operating conditions are not severe. If, as also indicated on Figure 5.8, the pressure drop on the same valve is 10 bar, the operating point lies in the ‘Stellite’ region indicating that the plug and seat surfaces should be faced with Stellite on the seating surfaces Figure 5.9. Stellite, a cobalt alloy, resists the erosive effect of most chemical compounds. It is still possible, however, to use a standard seat retainer. Referring back to Figure 5.8, increasing the pressure drop to 20 bar brings the operating point into the ‘cavitation control’ area and calls for the use of addition devices to prevent cavitation damage.

Inlet pressure P1 (bar)

60

50

0

210

Energy dissipating seat retainer

50

CV

430

810

1000

Energy dissipating disk stack

40 30

Cavitation control 20

Stellite

10 0

SS 0

50

100

Size (mm)

150

200

250

Figure 5.8.  Trim selection guide—liquid applications (courtesy Mitech).

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Seat surface

Full contour

Lower guide area

Full contour and lower guide

Figure 5.9.  Plug hard facing variations: from left to right: seat surface; full contour; lower guide area; and full contour and lower guide (courtesy Valtek Control Products).

5.9 Reducing Cavitation In many cases cavitation results from a high differential pressure across the valve (Figure 5.10). Where the outlet discharges into an open tank, for example, one of the most cost-effective solutions is to create a back pressure on the valve— thus allowing the pressure drop across the valve to be reduced. By installing a choke, usually an orifice plate, cavitation across the valve is avoided (Figure 5.11). This action will often move the cavitation from the valve to the choke itself. Although this may cause damage to the pipework immediately downstream of the choke, this is likely to be less costly than damage to the valve. Further, where the flow discharges to an open tank or channel, the choke may be placed on the outlet where there is no downstream pipework to get damaged.

2 bar

20 bar ∆P = 18bar

Figure 5.10.  High differential pressure across the valve can lead to cavitation.

Valve Trim and Characterization   •  111

18 bar

20 bar ∆P = 2bar

Figure 5.11.  Cavitation across the valve can be avoided by installing a choke.

Because the choke is a fixed restriction, it will only be effective at creating a back pressure at the maximum flow for which it was designed. At lower flow rates, where a higher percentage of the total pressure drop will be taken across the valve, the recovery factor of the valve will be better at reducing the chances of cavitation and since the amount of damage that occurs with cavitation is energy related, the severity of the wear will be lower at lower flow rates. If the operating conditions are not too severe and the operating point of a globe valve still falls within the cavitation control region, use can be made of a special cavitation control seat retainer (Figure 5.12). This retainer features a large number of orifices in the wall of the retainer that are exposed or covered up by the movement of a flat bottomed plug With flow from the outside of the retainer toward the centre, these orifices form fluid jets that meet in the middle of the retainer and, since the pressure drop takes place across the orifices, this is where the bubbles form.

Figure 5.12.  Cavitation control seat retainer (courtesy Mitech).

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The pressure recovers where all the streams come together—away from most of the components of the valve. The bottom of the plug and the bore of the seat ring are still vulnerable to damage and these components are hard-faced with Stellite to reduce wear. Provided there is sufficient back pressure to collapse the bubbles inside the retainer the valve body will be protected. For small diameter valves and high pressures above 40 bar use can be made of a seat retaining energy dissipating device as shown in Figure 5.13. Mitech’s ZZ trim, for example, can handle very high pressure drops at low flows and with its high rangeability can be combined with cavitation control. On the negative side, it is only suitable for clean liquids and it’s not suitable for large Cv values.

5.10 Eliminating Cavitation If the pressure drop and the flow rate are sufficiently high then neither of these methods will be effective and rapid wear will result. In these cases it is better to use a method that eliminates cavitation. To do this it is necessary to dissipate the high potential energy present in the liquid upstream of the valve directly into heat—without passing through the intermediate phase of high velocity and low pressure. By maintaining the internal ­pressure above the vapour pressure at all points in the valve, the bubbles do not form. The mechanisms used to convert pressure energy into heat can involve: • splitting of total flow into many small flows • wall friction

Figure 5.13.  Mitech’s ZZ seat retaining energy dissipating device for small diameter valves and pressures above 40 bar (courtesy Mitech).

Valve Trim and Characterization   •  113

• changes of direction plus swirling • series of restrictions and expansions These mechanisms are incorporated into, for example, the Mitech disk stack globe valve trim (Figure 5.14). Each passageway is a set of restrictions in series with each other. Each passageway in a disk is a set of restrictions in parallel to each other. And, each disk is a set of restrictions in parallel to each other. Thus, increasing the area over which the flow must pass increases the amount of energy that is converted into heat. In addition the liquid is forced to change direction as many times as possible. The passageways in any disk are designed to handle the full pressure drop of the application but with only a small part of the total flow. The number of disks will determine the total flow capability and if the pressure drop across the valve varies with flow then the disks will require a different design at the bottom and at the top, and maybe in the middle. Typical applications for this type of valve would be boiler feedwater pump leak-off applications where the pressure must be dropped from in the order of 80 bar to atmospheric pressure.

5.11 Noise Sources Valves often produce noise—sometimes referred to as sound without value. Noise is unwanted, often annoying, and sometimes damaging. The point at which the noise level moves from irritating, too annoying, to actual damaging is determined by the Sound Pressure Level (SPL) measured in dBA*. Figure 5.15 illustrates some relative noise levels for common sounds and activities. *See appendices for further information on sound level measurement and standards.

Figure 5.14.  Disk stack design of globe valve trim (courtesy Mitech).

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Threshold of pain Hammering on the steel plate

Shouting (1 m) Normal speech (1 m) Average residence Very soft music

130 120 110 100 90 80 70 60 50 40 30 20 10 0

Jet take-off (50 m) Thunder Loud rock music Heavy street traffic Noisy office Quiet auditorium

Minimum street noise Quiet whisper Threshold of hearing

Figure 5.15.  Some relative noise levels for common sounds and activities.

Of course, damage can refer to physical damage to plant or machinery as a result of excessive vibration, or to hearing damage to personnel. The permissible personnel noise level exposure there is varies from country to country. For example in the United States the exposures are determined by OSHA as shown in Table 5.2. Table 5.2.  Permissible personnel noise level exposure and time levels according to OSHA Duration per day (hrs) 8 6 4 3 2 1½ 1 ½ ¼ or less

SPL (dBA slow response) 90 92 95 97 100 102 105 110 115

Valve Trim and Characterization   •  115

The values according to the UK HSE are: Lower exposure action values: • daily or weekly exposure of 80 dB; • peak sound pressure of 135 dB; Upper exposure action values: • • • • •

daily or weekly exposure of 85 dB; peak sound pressure of 137 dB. There are also levels of noise exposure which must not be exceeded: daily or weekly exposure of 87 dB; peak sound pressure of 140 dB.

There are two major causes of valve-produced noise. The one is, simply, mechanical vibration of the valve components. The other is noise produced due to the flow of the fluid: either hydrodynamic or aerodynamic.

5.12 Mechanical Noise Mechanical noise arises as a result of vibration of a movable valve component. This might be due to either random pressure variations within the valve body or due to fluid impinging on the part. Typically, mechanical noise is fairly moderate (under 90 dBA) at a frequency of less than 1500 Hz and sounds like what it is—mechanical rattling. Generally, the main concern here is the mechanical damage caused by the vibrating component, rather than noise generation. The solution lies in good design, Large diameter post guiding (see Chapter 4), for example, provides better support, reduces clearances, and consequently reduces vibration due to lateral movement. Better still, in cage guiding (Chapter 4) the outside is diameter of the valve plug is in close proximity to the inside wall surface of the cylindrical cage throughout the travel range. Since the cage and seat ring are self-aligning and the plug is guided along the inside diameter of the cage, vibration is reduced and the problem of side loads is reduced—once again improving plug stability. Fluid impingement on a valve component can often give rise to resonant vibration—a narrow-band, often single pitched, whistle in the range from 3 to 7 kHz. Resonant vibration sometimes occurs at a specific opening of the valve and might be cured by moving away from that point.

116  •   The Concise Valve Handbook

A more successful cure, in up to 60% of cases, is by the simple expedient of reversing the flow through the valve—in other words by removing the valve and reinstalling it for reverse flow.

5.13 Hydrodynamic Noise In liquids, the main source of noise is flashing and cavitation. The noise associated with flashing is a high pitched hissing sound— similar to that of sand passing through the valve. The noise associated with cavitation is usually accompanied by vibration and varies considerably: from a rattling sound—similar to that of gravel passing through the valve—to a high-frequency squeal. Whilst the noise levels produced are rarely a problem to the surrounding environment they are a good indication of the severity of the cavitation. A typical liquid noise characteristic is shown in Figure 5.16 which shows the variation of the Sound Pressure Level (SPL) as a function of the ratio of the differential pressure (∆P) to the inlet pressure (P1 absolute) minus the vapour pressure (VP absolute). 110

100

SPL dBA

90

80

70

Full cavitation Incipient cavitation

60

50 0.02

0.04

0.06

0.1

0.2 ΔP P1 − P 2

0.3

0.4

Flashing

0.6 0.8 1.0

Figure 5.16.  Typical liquid noise characteristic (courtesy Fluids handling: Principles Practice).

Valve Trim and Characterization   •  117

5.14 Aerodynamic Noise Aerodynamic noise generated by gas, steam or vapour flow through a valve is the chief source of noise and is a perennial problem. When extreme, it not only threatens employees’ safety and well-being but causes “bad-neighbor” situations. Noise is a result of the vast amounts of stored potential energy, contained in pressurized gases, being converted to other forms as a consequence of sheer forces generated in the flow stream due to deceleration, expansion, or impingement. The rate of this potential energy conversion is known as stream power and the fraction of the stream power that is converted into sound power is known as ‘acoustical conversion efficiency’— typically varying from about 10−7 to 10−3. The energy source behind the generation of noise and vibration is turbulence—the seemingly random motion of a fluid—which exhibits a structure of swirling eddies ranging in size from large to small. The size of the eddies and the fluid velocity within them determine the predominate frequency of the pressure fluctuations.

5.15 Noise Prediction According to many authorities, noise generation and mechanisms are neither fully understood or predictable. Indeed, noise prediction methods may vary as much as a 30 dBA from manufacturer to manufacture. To date, the prediction method, claimed by many to be the most reliable, is the computer prediction model according to ANSI/ISA S75.17. The single greatest cause of noise is choked flow and the main area of noise generation is the area immediately downstream of the vena contracta. Although the valve itself is the noise generator, its thick-walled construction militates against the transmission of noise and consequently most noise is transmitted through the relatively thin walls of the downstream piping. Typical transmission losses, therefore, are a function of the pipe size and schedule.

5.16 Noise control As illustrated in Figure 5.17, there are two main elements to noise treatment: path treatment and source treatment. The choice of which treatment to use is determined by the level of noise produced. As a general rule of thumb, levels in the range of 75 to

118  •   The Concise Valve Handbook AERODYNAMIC NOISE CONTROL PATH TREATMENT

SOURCE TREATMENT

Increase distance

Velocity control

Increase pipe schedule

Acoustic control

Insulation

Location control

Silencers

Diffusers attenuators

Figure 5.17.  There are two main elements to noise treatment: path treatment and source treatment.

90 dBA (generally subsonic) would entail path treatment whilst noise ­levels of 105 dBA and upwards would entail source treatment. Noise levels lying between these two levels might make use of either path or source treatment.

5.17 Path Treatment 5.17.1 Increase Distance By definition, the Sound Pressure Level (SPL) is a measure of the sound pressure at a distance of 1 m—halving (6 dBA) with each doubling of the distance. Consequently, ensuring that plant personnel are located at a sufficient distance away from the noise source is a possible control mechanism. 5.17.2 Increase Pipe Schedule Increasing the wall thickness of downstream piping can be an effective means to reduce control valve noise. Table 5.3 shows some typical pipe wall attenuations (dBA) for a carbon steel pipe according to different pipe schedules and pipe sizes, when referenced to a 10 NPS Schedule 40 pipe. As an example, noise levels downstream of a control valve are 96 dBA at an operator station with the noise emanating from a 10 inch schedule

Valve Trim and Characterization   •  119

Table 5.3.  Typical pipe wall attenuation (dBA) for a carbon steel pipe according to different pipe schedules and pipe sizes Nominal Pipe Size (NPS) 4 6 8 10 12 16 20

40 0 0 0 0 −2 −5 −6

Typical pipe wall attenuation (dBA) according to pipe schedule 80 120 160 −5 −9 −12 −7 −10 −14 −8 −13 −15 −8 −12 −16 −9 −15 −18 −11 −16 −20 −12 −18 −22

40 carbon steel pipe. What pipe schedule replacement would reduce the noise to 85 dBA? The minimum noise reduction required is: 96–85 = 11 dBA and, according to Table 5.3, a schedule 120 pipe would meet this need. Unfortunately, noise, once generated, does not dissipate rapidly with downstream pipe length (typically less than 3 dBA/300 m distance). Consequently, the heavier schedule piping must be continued for the whole of the downstream piping. This can prove very expensive.

5.18 Insulation Acoustic insulation may be used to reduce noise levels up to 4 dBA per 10 mm of insulation thickness up to a maximum of about 20 dBA. Extreme care must be taken to ensure that the material is correctly installed to prevent any ‘voids’ in the material that could seriously reduce its effectiveness. Furthermore, in a similar manner to the treatment in which the pipe schedule is increased, the insulation must be continued for the full downstream run of the pipe. Unfortunately, such materials are often not acceptable at high ­temperature, since their binders may burn outradically changing their acoustical and thermal qualities. Use could be made of thermal insulation, as for example used on steam pipes, but such materials can only provide noise level reductions of typically 1 to 2 dBA per 10 mm of thickness. In practice, noise reduction using acoustical insulation reaches a practical limit of 11 to 12 dBA due to acoustical ‘leaks’ from the valve bonnet and top works.

120  •   The Concise Valve Handbook

5.19 Silencers Silencers or mufflers (Figure 5.18), packed with sound absorbing material, and installed directly downstream of the valve, can be quite effective— providing 15 dBA or even more of noise reduction. However, there are several potential problems that may preclude their use. Firstly, they are only effective with sub-sonic flow velocities. Secondly, it may be difficult to find sound absorbent packing materials that are compatible with the process gases or whose acoustic damping properties do not deteriorate too rapidly with time. Unfortunately, materials that might prove most suitable for the task, are often not suitable at high temperatures since their binders may burn out and change both their acoustic and thermal qualities. Lastly, they are expensive to purchase and install—often requiring support steel and guying.

5.20 Source Treatment Source treatment is generally used for noise levels in excess of 105 dBA and makes use of several solutions including: velocity control; acoustic control; location control; and diffusers or Attenuator plates.

5.21 Velocity Control Simple low noise retainer devices (Figure 5.19) are relatively inexpensive and, in conjunction with diffuser plates can reduce noise by between 5 to 15 dBA depending on number of stages and pressure drop ratio. A diffuser plate (Figure 5.20) is a plate with a large number of small holes that is installed in the downstream pipework.

Figure 5.18.  Cut-away section of an in-line silencer making use of an inlet diffuser to break up turbulence. (courtesy Valtek Control Valves, Flowserve Corporation).

Valve Trim and Characterization   •  121

Figure 5.19.  Simple low noise retainer device (courtesy Mitech).

Figure 5.20.  Simple diffuser plate (courtesy Mitech).

Noise reduction trims minimize the conversion of turbulent kinetic energy into piping vibration and associated air-borne noise by reducing the pressure in a fashion that intentionally controls the frequency, location, intensity, and nature of the resulting turbulence. Generally such trim seeks to combine a number of key noise control techniques: Passage shape in which the trim devices make use of a series of tortuous paths—either in a two-dimensional design, with flow paths that lie in a single plane or, as in Fisher Rosemount’s WhisperFlo, a three dimensional manipulation of the fluid (Figure 5.21). As shown the fluid undergoes a series of contractions, expansions, direction changes and splits in axial, radial and circumferential directions. Multistage pressure reduction in which the pressure drop is divided over two stages—with the first stage greater than that of the second stage.

122  •   The Concise Valve Handbook

Figure 5.21.  Tortuous path in which the fluid undergoes a series of contractions, expansions, direction changes and splits in axial, radial and circumferential directions (courtesy Emerson-Fisher).

Frequency spectrum shift. This involves limiting the turbulent eddy size so that the low frequency content, which might vibrate the piping structure, is reduced. High frequency vibration is not as efficiently induced in the pipe wall which means pipe stress is reduced and pipe fatigue is less likely. Exit jet independence. An important corollary to passageway size is the independence of fluid jets as they exit the cage. If jets coalesce due to inadequate passage spacing, the frequency shift effect is defeated. Aligning the exit jets to be parallel, minimizes interaction between them. Velocity management. By splitting the pressure reduction, noise generated by the upstream stage is attenuated before it reaches the downstream body cavity. Accordingly, the sound power from the unattenuated last stage is much less than that of a single-stage device. When combined, these noise control techniques can provide noise reduction of 45 dBA or more.

CHAPTER 6

Valve Selection Both the selection and sizing of a control valve require a good basic knowledge of valve technology and the process itself. But, even before the technical aspects are looked at, it is first necessary to define the ­decision  criteria: • • • •

lowest initial purchase price lowest life-time cost longest time between maintenance smallest size and weight Only then can we look at specifying the technical requirements:

• • • • • • • •

maximum pressure and temperature face-to-face restrictions allowable pressure drop leak rate requirement size and weight restrictions end connections materials, if known type of operator Firstly, let us have a look at price: Small valve: e.g., 50mm No significant difference between valve types Large valve: e.g., 250mm Butterfly 1 Diaphragm 2

124  •   The Concise Valve Handbook

Eccentric disc 3 Pinch 4 Ball: soft seat 5 Rotary plug 6 Plug: non-lubricated 7 Globe 8 Ball: metal-seated 9 On the technical side, there are many guidelines laid down that can simplify this selection. Tables 6.1 and 6.2, for example, offer a rough set of selection criteria. A problem with many such guides such as this, however, is that, because they are proprietary, they are necessarily biased. Crane Process Flow Technologies are, for example, manufacturers of diaphragm valves.

Table 6.1.  Comparison table: rating from 1 down to 5 (1 = highest rating 5 = poorest rating) (courtesy Crane Process Flow Technologies Ltd.) Valve Diaphragm Ball Butterfly Globe Gate Wide choice of 1 2 1 3 4 materials to match service Non-turbulent 1 1 2 5 1 flow path Low fluid1 1 1 4 1 friction loss Fire-safe 3 1 1 1 1 design Weight/size 2 3 1 3 5 ratio 1 2 2 4 5 Ability to shutoff against gasses, liquids, and solids Resistance to 1 3 2 5 4 abrasion and erosion Resistance to 1 1 1 5 4 corrosion

Lubricated plug 2

1 1 2 4 3

3

3

Valve Selection   •  125

Valve Diaphragm Ball Butterfly Globe Gate Compact 2 1 1 3 5 overall height Cost 2 3 2 3 3 Pressure range 4 1 2 1 2 Vacuum 1 2 2 3 5 capability Maintenance: 1 2 2 3 4 inline servicing, low-cost spares High purity 1 3 1 5 5 Control 3 2 3 1 5 applications On/off 2 1 1 3 5 applications Temperature 4 1 3 1 2 range

Lubricated plug 1 4 1 4 5

5 2 4 1

Table 6.2.  Comparison of different control valves where: 1 = good; 2 = average; 3 = poor; and x = not suitable (courtesy Mitech) Pinch/ Disk/rotary Application Globe diaphragm Butterfly plug Ball Controllability/ 1 3 1 1 1 turndown High pressure 1 x x 2 1 > 30 bar High pressure 1 x x 3 3 drop ∆P> 0.5P1; P1 > 10 bar Slurry service 3 1 2 3 3 Cost < 100 mm 2 1 3 3 3 Cost > 100 mm 3 2 1 1 2 Anti-corrosion 2 1 1 2 2 (Continued)

126  •   The Concise Valve Handbook

Table 6.2.  (Continued) Pinch/ Disk/rotary Globe diaphragm Butterfly plug Ball 2 2 2 2 2

Application Size and weight < 100 mm Size and weight > 100 mm Temperature > 100°C

3

3

1

1

3

1

x

3

1

2

Another simple control valve selection guide that will meet many applications is shown in Figure 6.1. The flowcharts shown in Figures 6.2, 6.3, and 6.4 have been prepared by CSD Controls and enable users to come to grips with the selection of the valve body, trim, and actuator.

NO

Is Line >= 150mm ?

YES

NO

Globe NO

Butterfly

Is T1 > 100oC?

Is P1 > 10 bar? YES

NO

Rotary plug

NO

Is Full Bore OK?

YES

Is P1/P2 > 3?

YES

YES

Globe

Disk

Figure 6.1.  Control valve selection guide for gases and liquids (courtesy Mitech).

Valve Selection   •  127 PROCESS APPLICATION INFORMATION

CALCULATE CV VALUES AND CRITICAL CONDITIONS USER PREFERENCE

CORROSION CONSIDERATION SELECT BODY-STYLE AND MATERIALS

OTHER CONSIDERATIONS

CRITICAL PARAMETERS ENVIRONMENTAL COMPATIBILITY

CALCULATE FOR NOISE AND VELOCITY

PROCESS TEMPERATURE AND PRESSURE SELECT BODY AND BODYRATING

SPECIAL CERTIFICATION

SELECT SOFT PARTS AND BODY SIZE

COST OF OWNERSHIP LINE SIZE AND CONNECTIONS CHOICE ACCEPTABLE?

NO

YES BODY SIZE, STYLE, MATERIALS AND RATING SELECTED

CHECK AND RECONSIDER ALERNATIVES

Figure 6.2.  Control valve body selection guide (courtesy CSD Controls).

BODY SELECTION DATA

CALCULATE CVVALUES

USER PREFERENCE

SELECT TRIM SIZE

SHUT-OFF REQUIREMENT

CRITICAL AND NOISE CONSIDERATIONS

RANGEABILITY AND LIFE SPAN

SELECT TRIM TYPE AND MATERIALS

CORROSION CONSIDERATIONS

COST OF OWNERSHIP

FUNCTION CHARACTERISTIC

CHOICE ACCEPTABLE?

NO

YES BODY SIZE, STYLE, MATERIALS AND RATING SELECTED

CHECK AND RECONSIDER ALERNATIVES

Figure 6.3.  Control valve trim selection guide (courtesy CSD Controls).

128  •   The Concise Valve Handbook BODY AND TRIM SELECTION DATA USER PREFERENCE

FAIL SAFE AND HANDWHEEL CHECK OR CALCULATE ACTUATOR FORCE

CONTROL SIGNAL AND POWER SUPPLY POSITIONING ACCURACY

RELIABILITY AND AND REPEATABILITY AMBIENT CONDITIONS

SELECT ACTUATOR TYPE AND SIZE

OTHER CONSIDERATIONS

SPEED AND FREQUENCY POSITIONER ACCESSORIES

ENVIRONMENTAL COMPATIBILITY

SELECT POSITIONER AND ACCESSORIES

FUNCTION CHARACTERISTIC

COST OF OWNERSHIP CHOICE ACCEPTABLE?

NO

YES ACTUATOR, POSITIONER AND ACCESSORIES SELECTED

CHECK AND RECONSIDER ALERNATIVES

Figure 6.4.  Actuator and accessory selection guide (courtesy CSD Controls).

Glossary ∆P AChI ANSI API ASME ASTM AWG BSI CO CV DIN DN E/P FCI FL I/P IEC IEEE ISA ISO



J-T KV MAWP MOV MV NAMUR



Differential pressure American Chemical Institute American National Standards Institute American Petroleum Institute American Society of Mechanical Engineers American Society for Testing and Materials American Wire Gauge British Standards Institute Controller output Valve flow coefficient Deutsches Institit für Normung Nominal diameter Voltage to pneumatic converter Fluid Controls Institute Pressure recovery coefficient Current to pneumatic converter International Electrotechnical Commission Institute of Electrical and Electronic Engineers International Society for Automation International Organization for Standardization Note: ISO is not an acronym, but is based on the Greek word isos meaning equal. Joule–Thomson (effect) Valve flow coefficient (SI alternative = 0.865 × CV) Maximum allowable working pressure Motor-operated valve Manipulated variable Normen Arbeitsgen Mess Und Regeltechnik (loosely interpreted as Standards Work Group for Instruments and Controls.)

130  •  Glossary

NEMA OP PD PV PDR PN Q Qm Re SG SGf SGg SPL x XT Y Z







National Electrical Manufacturers Association Output Process demand Process variable Pressure drop ratio Nominal pressure Volumetric flow rate Mass flow rate Reynolds number Specific gravity Specific gravity of fluid Specific gravity of gas Sound pressure level Pressure drop ratio Choked value of pressure drop ratio Gas expansion factor Compressibility factor

Bibliography “Control Valve Trims and Devices to Control Cavitation Damage and Excessive Noise,” Mitech, Technical Product Bulletin No 1. “Introduction to Safety Valves.” Spirax Sarco, at: http://spiraxsarco.com/resources/ steam-engineering-tutorials/safety-valves/introduction-to-safety-valves.as “Pressure Relief Valve Engineering Handbook” Technical Document No. TP-V300, Crosby Valve Inc. “The Mitech Globe Control Valve Body,” Mitech, Technical Product Bulletin No 2. “Valve Signature Analysis” at: http://www2.emersonprocess.com/enUS/brands/ fisher/DigitalValveControllers/FIELDVUESolutions/ValveDiagnostics/ Pages/ValveSignatureBasics.aspx Bell, L.H., and D.H. Bell. 1994. Industrial Noise Control: Fundamentals and Applications. Marcel Dekker Inc. Boger, H., and L.Mazot. Why Most Control Valves Today are Throttling Around 60% Opening. Masoneilan-Dresser. Borden Jr., G. 1998. Control Valves. ISA. Campbell, J.M. 2004. Gas Conditioning and Processing, Vol. 1: Basic Principles, 8th ed. Chris, W. 1999. A User’s Guide, Understanding Valve Actuators. Rotork Controls Inc. Chris, W. 2000. “New generation of Valve Actuators can Provide Important MOV Predictive Maintenance Data.” Rotork Controls Inc., Valve Magazine. Comparison of Different Valve Types. Crane Process Flow Technologies Ltd. Control Valve Noise Reduction. Fisher Rosemount. Dave, H. Understanding Control Valve Bench Set. Control Engineering. Elonka, S., and A.R. Parsons. 1962. Standard Instrumentation Questions and Answers For Production-Processes Control, Vol. 1. McGraw-Hill. Emerson, G. 2005. Control Valve Handbook, 4th ed. Emerson Process Management. Herrmann, U.F. 1974. “Sound Reinforcement.” N.V. Philips’ Gloeilampenfabrieken, Eindhoven. Husu, M., I. Niemelä, J. Pyötsiä, and M. Simula. 1992. Flow Control Manual. Neles-Jamesbury. Hutchison, J.W. 1976. ISA Handbook of Control Valves. ISA.

132  •  Bibliography John, E. Positioner Guidelines. Emerson-Fisher-Rosemount. Mike Sessions, Cavitation Control in Control Valves. Practical Industrial Process Measurement for Engineers and Technicians. IDC Technologies. Richard, R. Designing a Positioner for the South African Market. Mitech. Sam, L. Control Valve Manual. Masoneilan. Stojkov, B.T. 1997. The Valve Primer. Industrial Press Inc.

About the Author Michael (Mick) Crabtree, Joining the Royal Air Force as an apprentice, Mick Crabtree trained in aircraft instrumentation and guided missiles. Completing his service career seconded to the Ministry of Defense as a technical writer, he emigrated to South Africa in 1966 where he worked, for many years, for a local manufacturing and systems integration company involved in industrial process control, SCADA, and PLC-based systems. Later, as an editor and managing editor of a leading monthly engineering journal, Mick wrote and published hundreds of articles, as well as eight technical resource handbooks on industrial process control: ‘Flow Measurement,’ ‘Temperature Measurement,’ ‘Analytical On-line Measurement,’ ‘Pressure and Level Measurement,’ ‘Valves,’ ‘Industrial Communications,’ and ‘The Complete Profibus Handbook.’ He subsequently founded his own PR and advertising company and was retained by a number of leading companies involved in the process control industry, including: Honeywell, Fisher-Rosemount, Krohne, Milltronics, and AEG. Apart from producing all their press releases and articles, he also undertook the conceptualization and production of a wide range of advertisements and data sheets, as well as newsletters. For the last 16 years, he has been involved in technical training and consultancy and has run workshops on industrial instrumentation and networking throughout the world (United States, Canada, United Kingdom, France, Southern Africa, Trinidad, Middle East, Australia, and New Zealand). During this period, he has led more than 6,000 engineers, ­technicians, and scientists on a variety of practical training workshops covering the fields of process control (loop tuning), process instrumentation, data communications, fieldbus, safety instrumentation systems (according to both ISA S84 and IEC 61508/61511), project management, online liquid analysis, and technical writing and communications.

134  •   About the Author

Completing his studies in Electrical, Electronic, and Instrumentation engineering, he holds an MSc in Industrial Flow Measurement from Huddersfield University. His hobbies and pastime include: cycling, rambling, history, and reading. After nearly 35 years spent in South Africa, he now lives in Wales, just outside Cardiff, having relocated to Britain some 18 years ago.

Index A Acoustic insulation, 119 Actual pressure drop, 27 Aerodynamic noise, 117 Air-operated pinch valve, 71, 72 Angle valve, 60 B Ball segment valve, 81–82 Ball valve end-entry design, 76 floating ball valve, 75 ‘run’ torque, 76–77 split-body design, 75, 76 top-entry design, 76 trunnion (see Trunnion ball valve) Bar stock body valve, 61, 62 Bellows seal bonnet accordion-like tube, 52 formed-type bellows, 53 leakage detection, 54 lifespan, 54 welded bellows, 53 Bernoulli’s equation, 5–8 Bonnet assembly, 49 Butterfly valve center-disc butterfly valve, 82, 83 circular disc-shaped damper, 82, 83 conventional disc shape, 84, 85 fishtail disc, 84, 85 flow characteristics, 84, 85 offset disc, 84

C Cage-guided control valve, 55, 58–59 Camflex eccentric rotary plug valve, 86 Cavitation damage, 16–17 elimination, 112–113 flashing damage, 109 liquid noise characteristics, 17, 18 pressure dropping, 16 reducing, 110–112 Stellite region, 109 trim selection guide, 109 Center-disc butterfly valve, 82, 83 Check valve, 88, 89 Choked flow, 8–9 Choked pressure drop, 27 Compressibility factor, 43 Control valve basic theory, 3 Bernoulli’s equation, 5–8 cavitation, 16–18 choked flow, 8–9 equation of continuity, 3–4 final control element, 1–3 flashing and cavitation, 14–15 fluid properties, 1 isolation valve leakage classification, 21 leakage classification, 18–21 pressure recovery, 9–11 purpose of, 1

136  •   Index

rangeability, 11 Reynolds number, 13–14 turndown, 11 velocity and pressure distribution, 3, 4 velocity profiles, 12–13 D Diaphragm bellows, 53 Diaphragm valve advantage, 72 maintenance, 73 operating temperature, 72–73 pressure rating, 73 Saunders patent valve, 73 straight-through diaphragm valves, 74 throttle flow, 72 Discharge coefficient, 7 Double seat bottom-guided globe valve, 57 E Eccentric plug valve Camflex eccentric rotary plug valve, 86 integral extension bonnets, 87 seat portion, 86, 87 spherically shaped, 87 End connections flanged end connections, 95–96 hub end body, 96, 97 screwed end valve connections, 94–95 welded end connections, 96–98 Equal percentage inherent flow characteristics, 104 Equation of continuity, 3–4 Expanding gate valve centralizing mechanism, 66 segmented assemblies, 65–67 F Final control element, 1–3 Flanged end connections, 95–96

Flangeless connections, 99–100 Flat-face flanged end connection, 96 Flat ‘ideal’ velocity profile, 12–13 Flexible wedge gate, 63–64 Flow characteristics inherent (see Inherent flow characteristics) installed (see Installed flow characteristics) Formed-type bellows, 53 G Gas expansion factor, 40–41 Gas valve sizing compressibility factor, 43 Fisher Emerson easy-e® ES cageguided globe valve, 43–46 gas expansion factor, 40–41 mass flow rates, 42 pressure drop mechanism, 33–38 specific heat ratio factor, 38–40 volumetric flow rates, 41 Gate valve expanding gate valve, 65–67 full and no flow conditions, 62 isolation applications, 61 knife edge, 67–71 wedge shape, 62, 63 Globe valve advantages, 48–49 disadvantage, 49 globular-shaped cavity, 48 size, 49 Grayloc® connector, 100–101 H Hub end body, 96, 97 Hydrodynamic noise, 116 I Inherent flow characteristics cage-guided trim, 105, 106 curves, 103–104 equal percentage, 104

Index   •   137

linear, 103 modified percentage, 105 plug outlines, 105, 106 quick opening, 104 Installed flow characteristics differential pressure drop, 105, 106 equal percentage, 108 linear, 107 pressure drop ratio, 107 valve selection guide, 108 Isolation valve leakage classification, 21 K Knife edge gate valve advantages, 67 disc alignment, 68, 70 flow/travel characteristics, 68, 69 sliding gate regulator valve, 68, 70, 71 V-insert gate valve, 68, 69 L Laminar ‘parabolic’ velocity profile, 12 Laminated graphite packing benefits, 51 double graphite packing arrangement, 51 electrochemical reaction, 51, 52 sacrificial zinc washers, 51 temperature, 50 Lap joint flange, 98–99 Linear inherent flow characteristics, 103 Liquid valve sizing Emerson Fisher easy-e®, 27–29 identical inlet and outlet fittings, 31–32 oversized valves, 23 PC-based software package, 23–24 piping geometry factor, 29–31 valve sizing programs, 24

M Manipulated variable (MV), 2–3 Mechanical noise, 115–116 Modified percentage inherent flow characteristics, 105 Molecular weight of gas, 41, 42 Mufflers, 120 N Needle valve, 61 Noise acoustic insulation, 119 aerodynamic noise, 117 control, 117–118 hydrodynamic noise, 116 mechanical noise, 115–116 path treatment, 118–119 permissible personnel noise level exposure, 114 prediction, 117 silencers, 120 sounds and activities, 113, 114 source treatment, 120–122 Nominal bore (NB), 88–90 Nominal pipe size (NPS), 88–90 O Offset disc butterfly valve, 84 P Packing box, 49, 50 Pinch check valve, 88, 89 Pinch valve air-operated pinch valve, 71, 72 applications, 71 double vise mechanism, 71, 72 rubber hose/sleeve, 69, 71 turbulence, 70 Piping geometry factor, 29–31 Plug valve conventional, 84–85 lubricated valve, 85 PTFE-lined valve, 86 sealing mechanism, 85 Pressure drop mechanism

138  •   Index

choked flow, 37 different valve styles, 38, 39 flow rate, 36 gas mass flow, 33–34 globe valve set, 37, 38 terminal pressure drop ratio, 34, 35 Pressure drop ratio (PDR), 107 Pressure recovery coefficient, 10–11 Process demand (PD), 2–3 Q Quick opening inherent flow characteristics, 104 R Raised-face flanged end connection, 96 Rangeability, 11 Reynolds number, 13–14 Rotary control valves, 74 S Sacrificial anode, 51 Saunders patent valve, 73 Screwed end valve connections, 94–95 Silencers, 120 Single-ported balance globe valve, 57–58 Single-seat angle valve, 60 Single seat top-guided contact valve, 56 Slab valve, 64–65 Sliding gate regulator valve, 68, 70, 71 Solid wedge gate, 62–63 Specific gravity, 41 Specific heat ratio factor, 38–40 air, 38–40 natural gas, 40 Split body globe valve, 59–60 Straight-through diaphragm valves, 74

Streamlined valves, 10 Stuffing box, 49 Swing check valve, 88 T Terminal pressure drop ratio, 34 Top and bottom-guided double seat, 56–57 Trunnion ball valve double block and bleed system, 78, 79 full port design, 78–79 fully open position, 79, 80 hydraulic load, 77 leakage integrity, 77, 78 line pressure, 77, 78 pathway, 79–80 Turbulent velocity profile, 13 Turndown, 11 V Valve authority, 107 Valve construction angle valve, 60 ball segment valve, 81–82 ball valve, 75–77 bar stock body valve, 61 bellows seal bonnet, 52–54 bonnet assembly, 49 butterfly valve, 82–85 cage-guided control valve, 58–59 cavitation control, 108–110 check valve, 88, 89 control assembly, 47 corrosion, 90, 94 diaphragm valve, 72–74 eccentric plug valve, 86–87 end connections (see End connections) erosion, 94 extended bonnet, 52 flangeless connections, 99–100 gate valve (see Gate valve) globe valve, 48–49 Grayloc® connector, 100–101

Index   •   139

guiding system, 55 laminated graphite, 50–52 lap joint flange, 98–99 material selection, 90, 93 needle valve, 61 pinch valve, 69–72 plug valve, 84–86 post-guiding, 55–56 PTFE packing, 49–50 rotary control valves, 74 single-ported balance globe valve, 57–58 slab valve, 64–65 sliding stem valves, 47 split body globe valve, 59–60 top and bottom-guided double seat, 56–57 trunnion ball valve, 77–80 valve sizes and pipe schedules, 88–90 valve trim, 54–55 valve types, 48 wedge gate, 62–64 Valve flow coefficient, 7, 8 Valve selection actuator and accessory selection guide, 128 body selection guide, 127 control valve selection guide, 126 decision criteria, 123 technical requirements, 123–126 valve trim selection guide, 127 Valve trim cage-guided control valve, 55

cavitation, 109–113 flow characteristics inherent (see Inherent flow characteristics) installed (see Installed flow characteristics) noise (see Noise) noise sources, 113–118 plug, 54 seat retainer, 54–55 seat ring, 54 Velocity control exit jet independence, 122 frequency spectrum shift, 122 low noise retainer device, 120, 121 multistage pressure reduction, 121 passage shape, 121 velocity management, 122 Velocity of Approach Factor (EV), 7 Velocity profiles, 12–13 flat ‘ideal’ velocity profile, 12 laminar ‘parabolic’ velocity profile, 12 turbulent, 13 Venturi seat rings, 60 V-notched ball segment, 81 W Wedge gate, 62–64 Welded bellows, 53 Welded end connections, 96–98

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Research studies within the process industry routinely indicate

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functioning control systems. Furthermore, valves in general are consistently wrongly selected, regularly misapplied, and often incorrectly installed. This two-volume book comprises a comprehensive up-to-date body of knowledge that provides a total in-depth insight into valve and actuator technology—looking not just at control valves, but a whole host of other types including: check valves, shut-off valves, solenoid valves, and pressure relief valves. A methodology is presented to ensure the optimum selection of size, choice of body and trim materials, components, and ancillaries. Whilst studying the correct procedures for sizing, readers will also learn the correct procedures for calculating the spring ‘wind-up’ or ‘bench set’. Maintenance issues also include: testing for deadband/ hysteresis, stick-slip and non-linearity; on-line diagnostics; and signature analysis. Written in a detailed but understandable language, the two volumes are presented in a form suitable for both the beginner, with no prior knowledge of the subject, and the more advanced specialist.

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For the last sixteen years, ‘Mick’ Crabtree, who holds an MSc in industrial flow measurement, has been involved in technical training and consultancy—running workshops on industrial instrumentation and networking throughout the world covering the fields of process control (loop tuning), process instrumentation, data communications, fieldbus, safety instrumentation systems (according to both ISA S84

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and IEC 61508/61511), project management, on-line analysis, and technical writing and communications. This book represents some thirty years wealth of experiential

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