contol valves

CONTROL VALVEHANDBOOK

Fourth Edition

ii

NORTH AMERICAEmerson Process ManagementMarshalltown, Iowa 50158 USAT 1 (641) 7543011F 1 (641) 7542830www.EmersonProcess.com/Fisher

LATIN AMERICAEmerson Process ManagementSorocaba, Sao Paulo 18087 BrazilT +(55)(15)2383788F +(55)(15)2283300www.EmersonProcess.com/Fisher

EUROPEEmerson Process ManagementCernay 68700 FranceT +(33) (0)3 89 37 64 00F +(33) (0)3 89 37 65 18www.EmersonProcess.com/Fisher

MIDDLE EAST & AFRICAEmerson FZEDubai, United Arab EmiratesT +971 4 883 5235F +971 4 883 5312www.EmersonProcess.com/Fisher

ASIA PACIFICEmerson Process Management Singapore 128461 SingaporeT +(65) 6777 8211F +(65) 6777 0947www.EmersonProcess.com/Fisher

Fisher is a mark owned by Fisher Controls International LLC, a member of the EmersonProcess Management business division of Emerson Electric Co. The Emerson logo is atrademark and service mark of Emerson Electric Co. All other marks are the property of theirrespective owners.

The contents of this publication are presented for informational purposes only, and whileevery effort has been made to ensure their accuracy, they are not to be construed aswarranties or guarantees, express or implied, regarding the products or services describedherein or their use or applicability. We reserve the right to modify or improve the designs orspecifications of such products at any time without notice.

Neither Emerson, Emerson Process Management nor any of their affiliated entities assumesresponsibility for the selection, use and maintenance of any product. Responsibility for theselection, use and maintenance of any product remains with the purchaser and end-user.

Printed in U.S.A.

Fisher Controls International LLC 2005

D101881X012

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Preface to Fourth Edition

Control valves are an increasingly vital component of modern manufacturing aroundthe world. Properly selected and maintained control valves increase efficiency, safe-ty, profitability, and ecology.The Control Valve Handbook has been a primary reference since its first printing in1965. This fourth edition presents vital information on control valve performance andthe latest technologies.

Chapter 1 offers an introduction to control valves including definitions forcommon control valve and instrumentation terminology.

Chapter 2 develops the vital topic of control valve performance. Chapter 3 covers valve and actuator types. Chapter 4 describes digital valve controllers, analog positioners, boosters,

and other control valve accessories. Chapter 5 is a comprehensive guide to selecting the best control valve for

an application. Chapter 6 covers the selection and use of special control valves. Chapter 7 covers desuperheaters, steam conditioning valves, and turbine

bypass systems. Chapter 8 offers typical control valve installation and maintenance proce-

dures. Chapter 9 includes information on control valve standards and approval

agencies throughout the world. Chapter 10 offers useful tables of engineering reference data. Chapter 11 includes piping reference data. Chapter 12 is a handy resource for common conversions.

The Control Valve Handbook is both a textbook and a reference on the strongest linkin the control loop: the control valve and its accessories. This book includes exten-sive and proven knowledge from leading experts in the process control field includ-ing contributions from the ISA and the Crane Company.

vTable of Contents

Chapter 1. Introduction to Control Valves 1. . . . . . . . . . . . . . . . . . . . . . . . . . . . . What Is A Control Valve? 1. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Process Control Terminology 2. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Sliding-Stem Control Valve Terminology 6. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Rotary-Shaft Control Valve Terminology 13. . . . . . . . . . . . . . . . . . . . . . . . . . . . Control Valve Functions and Characteristics Terminology 16. . . . . . . . . . . . . Other Process Control Terminology 18. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Chapter 2. Control Valve Performance 23. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Process Variability 23. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Dead Band 25. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Actuator-Positioner Design 27. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Valve Response Time 29. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Valve Type And Characterization 31. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Valve Sizing 36. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Economic Results 37. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Summary 39. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Chapter 3. Valve and Actuator Types 41. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Control Valves 41. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Globe Valves 41. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Single-Port Valve Bodies 41. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Balanced-Plug Cage-Style Valve Bodies 43. . . . . . . . . . . . . . . . . . . . . . High Capacity, Cage-Guided Valve Bodies 43. . . . . . . . . . . . . . . . . . . . Port-Guided Single-Port Valve Bodies 44. . . . . . . . . . . . . . . . . . . . . . . . . Double-Ported Valve Bodies 44. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Three-Way Valve Bodies 45. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Rotary Valves 45. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Butterfly Valve Bodies 45. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . V-Notch Ball Control Valve Bodies 46. . . . . . . . . . . . . . . . . . . . . . . . . . . . Eccentric-Disk Control Valve Bodies 46. . . . . . . . . . . . . . . . . . . . . . . . . . Eccentric-Plug Control Valve Bodies 47. . . . . . . . . . . . . . . . . . . . . . . . . .

Control Valve End Connections 47. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Screwed Pipe Threads 47. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Bolted Gasketed Flanges 48. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Welding End Connections 48. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Valve Body Bonnets 49. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Extension Bonnets 50. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Bellows Seal Bonnets 51. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Control Valve Packing 52. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . PTFE V-Ring 52. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Laminated and Filament Graphite 52. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . USA Regulatory Requirements for Fugitive Emissions 53. . . . . . . . . . . . .

Single PTFE V-Ring Packing 54. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ENVIRO-SEAL PTFE Packing 54. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ENVIRO-SEAL Duplex Packing 55. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . KALREZ Packing 55. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ENVIROSEAL Graphite ULF 55. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . HIGH-SEAL Graphite ULF 55. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ENVIRO-SEAL Graphite for Rotary Valves 57. . . . . . . . . . . . . . . . . . . . . Graphite Ribbon for Rotary Valves 57. . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Characterization of Cage-Guided Valve Bodies 58. . . . . . . . . . . . . . . . . . . . . . Characterized Valve Plugs 59. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Valve Plug Guiding 60. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Restricted-Capacity Control Valve Trim 61. . . . . . . . . . . . . . . . . . . . . . . . . . . . . Actuators 61. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Diaphragm Actuators 62. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Piston Actuators 63. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Electrohydraulic Actuators 64. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Manual Actuators 64. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Rack and Pinion Actuators 64. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Electric Actuators 64. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Chapter 4. Control Valve Accessories 67. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Positioners 67. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Other Control Valve Accessories 69. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Limit Switches 69. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Solenoid Valve Manifold 71. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Supply Pressure Regulator 71. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Pneumatic Lock-Up Systems 72. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Fail-Safe Systems for Piston Actuators 72. . . . . . . . . . . . . . . . . . . . . . . . . . Electro-Pneumatic Transducers 72. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Electro-Pneumatic Valve Positioners 72. . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Diagnostics 74. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Chapter 5. Control Valve Selection 75. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Valve Body Materials 76. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Designations for the High Nickel Alloys 77. . . . . . . . . . . . . . . . . . . . . . . . . . . . . Pressure-Temperature Ratings for Standard Class 78. . . . . . . . . . . . . . . . . . .

Cast Carbon Steel (ASTM A216 Grade WCC) 78. . . . . . . . . . . . . . . . . . . . Cast Chromium-Molybdenum Steel (ASTM A217 Grade WC9) 79. . . . . . Cast Chromium-Molybdenum Steel (ASTM A217 Grade C5) 80. . . . . . . . Cast Type 304 Stainless Steel (ASTM A351 Grade CF3) 81. . . . . . . . . . . Cast Type 316 Stainless Steel (ASTM A351 Grades CF8M and CG8M) 82

Pressure-Temperature Ratings for ASTM A216 Cast Iron Valves 84. . . . . . . Pressure-Temperature Ratings for ASTM B61 and B62 Cast Bronze Valves 85. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Face-to-Face Dimensions for Flanged Globe-Style Control Valves 86. . . . . Face-to-Face Dimensions for Buttweld-End Globe-Style Control Valves 88. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Face-to-Face Dimensions for Socket Weld-End Globe-Style Control Valves 89. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Face-to-Face Dimensions for Screwed-End Globe-Style Control Valves 90. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Face-to-Centerline Dimensions for Raised Face Globe-Style Angle Control Valves 90. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Face-to-Face Dimensions for Separable Flanged Globe-Style Control Valves 90. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Face-to-Face Dimensions for Flangeless, Partial-Ball Control Valves 91. . . Face-to-Face Dimensions for Single Flange (Lug-Type) and Flangeless (Wafer-Type) Butterfly Control Valves 91. . . . . . . . . . . . . . . . . . . . Face-to-Face Dimensions for High Pressure Butterfly Valves with Offset Design 92. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Wear & Galling Resistance Chart Of Material Combinations 92. . . . . . . . . . . Control Valve Seat Leakage Classifications 93. . . . . . . . . . . . . . . . . . . . . . . . . Class VI Maximum Seat Leakage Allowable 94. . . . . . . . . . . . . . . . . . . . . . . . Typical Valve Trim Material Temperature Limits 94. . . . . . . . . . . . . . . . . . . . . . Service Temperature Limitations for Elastomers 95. . . . . . . . . . . . . . . . . . . . . Ambient Temperature Corrosion Information 96. . . . . . . . . . . . . . . . . . . . . . . Elastomer Information 101. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Fluid Compatibility 104. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Control Valve Flow Characteristics 108. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Flow Characteristics 108. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Selection of Flow Characteristic 109. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Valve Sizing 110. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Sizing Valves for Liquids 110. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Abbreviations and Terminology 112. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Equation Constants 113. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Determining Fp, the Piping Geometry Factor 114. . . . . . . . . . . . . . . . . . . . .

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Determining qmax (the Maximum Flow Rate) or Pmax (the Allowable Sizing Pressure Drop) 115. . . . . . . . . . . . . . . . . . . . . . . . . . . Determining qmax (the Maximum Flow Rate) 115. . . . . . . . . . . . . . . . . . . . . Determining Pmax (the Allowable Sizing Pressure Drop) 115. . . . . . . . . . Liquid Sizing Sample Problem 117. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Sizing Valves for Compressible Fluids 119. . . . . . . . . . . . . . . . . . . . . . . . . . . Determining xTP, the Pressure Drop Ratio Factor 121. . . . . . . . . . . . . . . . . Compressible Fluid Sizing Sample Problem No. 1 121. . . . . . . . . . . . . . . . Compressible Fluid Sizing Sample Problem No. 2 123. . . . . . . . . . . . . . . . Representative Sizing Coefficients for Single-Ported Globe-Style Valve Bodies 126. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Representative Sizing Coefficients for Rotary-Shaft Valves 127. . . . . . .

Actuator Sizing 129. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Globe Valves 129. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

A. Unbalance Force 129. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Typical Unbalance Areas of Control Valves 129. . . . . . . . . . . . . . . .

B. Force to Provide Seat Load 130. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C. Packing Friction 131. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Typical Packing Friction Values 132. . . . . . . . . . . . . . . . . . . . . . . . . . D. Additional Forces 132. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Actuator Force Calculations 133. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Rotary Actuator Sizing 133. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Torque Equations 133. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Breakout Torque 133. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Dynamic Torque 133. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Typical Rotary Shaft Valve Torque Factors 134. . . . . . . . . . . . . . . . . . . . . . . . . V-Notch Ball Valve with Composition Seal 134. . . . . . . . . . . . . . . . . . . . . . . . . High Performance Butterfly Valve with Composition Seal 134. . . . . . . . . . . . .

Maximum Rotation 134. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Non-Destructive Test Procedures 134. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Magnetic Particle (Surface) Examination 135. . . . . . . . . . . . . . . . . . . . . . . . Liquid Penetrant (Surface) Examination 135. . . . . . . . . . . . . . . . . . . . . . . . . Radiographic (Volumetric) Examination 135. . . . . . . . . . . . . . . . . . . . . . . . . Ultrasonic (Volumetric) Examination 136. . . . . . . . . . . . . . . . . . . . . . . . . . . .

Cavitation and Flashing 136. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Choked Flow Causes Flashing and Cavitation 136. . . . . . . . . . . . . . . . . . . Valve Selection for Flashing Service 137. . . . . . . . . . . . . . . . . . . . . . . . . . . Valve Selection for Cavitation Service 138. . . . . . . . . . . . . . . . . . . . . . . . . .

Noise Prediction 139. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Aerodynamic 139. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Hydrodynamic 140. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Noise Control 140. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Noise Summary 143. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Packing Selection 144. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Packing Selection Guidelines for SlidingStem Valves 145. . . . . . . . . . . . . . . Packing Selection Guidelines for Rotary Valves 146. . . . . . . . . . . . . . . . . . . .

Chapter 6. Special Control Valves 147. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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High Capacity Control Valves 147. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Low Flow Control Valves 148. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . High-Temperature Control Valves 148. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Cryogenic Service Valves 149. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Customized Characteristics and Noise Abatement Trims 150. . . . . . . . . . . . . Control Valves for Nuclear Service in the USA 151. . . . . . . . . . . . . . . . . . . . . . Valves Subject to Sulfide Stress Cracking 151. . . . . . . . . . . . . . . . . . . . . . . . .

Pre-2003 Revisions of MR0175 152. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . NACE MR0175/ISO 15156 152. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . NACE MR0103 153. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Chapter 7. Steam Conditioning Valves 155. . . . . . . . . . . . . . . . . . . . . . . . . . . . . Understanding Desuperheating 155. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Technical Aspects of Desuperheating 156. . . . . . . . . . . . . . . . . . . . . . . . . . Typical Desuperheater Designs 158. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Fixed Geometry Nozzle Design 158. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Variable Geometry Nozzle Design 159. . . . . . . . . . . . . . . . . . . . . . . . . . . . . Self-Contained Design 159. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Steam Atomized Design 160. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Geometry-Assisted Wafer Design 161. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Understanding Steam Conditioning Valves 161. . . . . . . . . . . . . . . . . . . . . . . . Steam Conditioning Valves 162. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Steam Cooler 164. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Steam Sparger 164. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Understanding Turbine Bypass Systems 165. . . . . . . . . . . . . . . . . . . . . . . . . . Turbine Bypass System Components 166. . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Turbine Bypass Valves 166. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Turbine Bypass Water Control Valves 166. . . . . . . . . . . . . . . . . . . . . . . . . . Electro-Hydraulic System 166. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Chapter 8. Installation and Maintenance 167. . . . . . . . . . . . . . . . . . . . . . . . . . . Proper Storage and Protection 167. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Proper Installation Techniques 168. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Read the Instruction Manual 168. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Be Sure the Pipeline Is Clean 168. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Inspect the Control Valve 168. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Use Good Piping Practices 168. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Control Valve Maintenance 169. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Reactive Maintenance 169. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Preventive Maintenance 169. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Predictive Maintenance 170. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Using Control Valve Diagnostics 170. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Instrument Air Leakage 170. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Supply Pressure 170. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Travel Deviation and Relay Adjustment 171. . . . . . . . . . . . . . . . . . . . . . . .

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Instrument Air Quality 171. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . In-Service Friction and Friction Trending 171. . . . . . . . . . . . . . . . . . . . . . . Other Examples 172. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Continued Diagnostics Development 172. . . . . . . . . . . . . . . . . . . . . . . . . .

Actuator Diaphragm 172. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Stem Packing 172. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Seat Rings 173. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Grinding Metal Seats 173. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Replacing Seat Rings 173. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Bench Set 174. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Chapter 9. Standards and Approvals 175. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Control Valve Standards 175. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

American Petroleum Institute (API) 175. . . . . . . . . . . . . . . . . . . . . . . . . . . . . American Society of Mechanical Engineers (ASME) 175. . . . . . . . . . . . . . European Committee for Standardization (CEN) 176. . . . . . . . . . . . . . . . .

European Industrial Valve Standards 176. . . . . . . . . . . . . . . . . . . . . . . . European Material Standards 176. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . European Flange Standards 176. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Fluid Controls Institute (FCI) 176. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Instrument Society of America (ISA) 177. . . . . . . . . . . . . . . . . . . . . . . . . . . . International Electrotechnical Commission (IEC) 177. . . . . . . . . . . . . . . . . International Standards Organization (ISO) 178. . . . . . . . . . . . . . . . . . . . . . Manufacturers Standardization Society (MSS) 178. . . . . . . . . . . . . . . . . . . NACE International 178. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Product Approvals for Hazardous (Classified) Locations 178. . . . . . . . . . . . . References 178. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Canadian Standards Association (CSA) Standards 178. . . . . . . . . . . . European Committee for Electrotechnical Standardization (CENELEC) Standards 178. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Instrument Society of America (ISA) Standards 178. . . . . . . . . . . . . . . . International Electrotechnical Commission (IEC) Standards 178. . . . . National Electrical Manufacturers Association (NEMA) Standards 179. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . National Fire Protection Association (NFPA) Standards 179. . . . . . . . .

North American Approvals 179. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Approval Agencies 179. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Types of Protection 179. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Nomenclature 179. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Hazardous Location Classification 179. . . . . . . . . . . . . . . . . . . . . . . . . . . . . Temperature Code 180. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . NEMA Enclosure Rating 181. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

General Locations 181. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Hazardous (Classified) Locations 182. . . . . . . . . . . . . . . . . . . . . . . . . . .

CSA Enclosure Ratings 182. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Intrinsically Safe Apparatus 182. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Entity Concept 182. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . CSA System Parameter Concept 183. . . . . . . . . . . . . . . . . . . . . . . . . . .

Loop Schematic (Control Drawing) 183. . . . . . . . . . . . . . . . . . . . . . . . . . . . . Comparison of Protection Techniques 183. . . . . . . . . . . . . . . . . . . . . . . . . .

Explosion-proof Technique 183. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Advantages of this Technique 184. . . . . . . . . . . . . . . . . . . . . . . . . . . . Disadvantages of this Technique 184. . . . . . . . . . . . . . . . . . . . . . . . . Installation Requirements 184. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Intrinsically Safe Technique 184. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Advantages of this Technique 184. . . . . . . . . . . . . . . . . . . . . . . . . . . . Disadvantages of this Technique 184. . . . . . . . . . . . . . . . . . . . . . . . .

Dust Ignitionproof Technique 184. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Non-Incendive Technique 185. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Advantages of this Technique 185. . . . . . . . . . . . . . . . . . . . . . . . . . . . Disadvantages of this Technique 185. . . . . . . . . . . . . . . . . . . . . . . . .

European and Asia/Pacific Approvals 185. . . . . . . . . . . . . . . . . . . . . . . . . . . . . Approval Agencies 185. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . CENELEC Approvals 185. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Types of Protection 185. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Flameproof 185. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Increased Safety 186. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Intrinsically Safe 186. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Non-Incendive 186. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Nomenclature 186. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Hazardous Location Classification 186. . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Group 186. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Zone 187. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Temperature Code 187. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . IEC Enclosure Rating 187. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . NEMA and IEC Enclosure Rating Comparison 188. . . . . . . . . . . . . . . . . . . Comparison of Protection Techniques 188. . . . . . . . . . . . . . . . . . . . . . . . . .

Flameproof Technique 188. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Advantages of this Technique 188. . . . . . . . . . . . . . . . . . . . . . . . . . . . Disadvantages of this Technique 188. . . . . . . . . . . . . . . . . . . . . . . . .

Increased Safety Technique 188. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Advantages of this Technique 189. . . . . . . . . . . . . . . . . . . . . . . . . . . . Disadvantages of this Technique 189. . . . . . . . . . . . . . . . . . . . . . . . .

Intrinsically Safe Technique 189. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Advantages of this Technique 189. . . . . . . . . . . . . . . . . . . . . . . . . . . . Disadvantages of this Technique 189. . . . . . . . . . . . . . . . . . . . . . . . .

Type n Technique 189. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Advantages of this Technique 189. . . . . . . . . . . . . . . . . . . . . . . . . . . . Disadvantages of this Technique 189. . . . . . . . . . . . . . . . . . . . . . . . .

Chapter 10. Engineering Data 191. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Standard Specifications For Valve Materials 191. . . . . . . . . . . . . . . . . . . . . . .

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Valve Materials Properties for PressureContaining Components 197. . . . . Physical Constants of Hydrocarbons 200. . . . . . . . . . . . . . . . . . . . . . . . . . . . . Specific Heat Ratio (K) 202. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Physical Constants of Various Fluids 203. . . . . . . . . . . . . . . . . . . . . . . . . . . . . Refrigerant 717 (Ammonia) 206. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Properties of Water 211. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Properties of Saturated Steam 212. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Properties of Superheated Steam 219. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Velocity of Liquids in Pipe 226. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Flow of Water Through Schedule 40 Steel Pipe 228. . . . . . . . . . . . . . . . . . . Flow of Air Through Schedule 40 Steel Pipe 232. . . . . . . . . . . . . . . . . . . . . . Calculations for Pipe Other than Schedule 40 236. . . . . . . . . . . . . . . . . . . . . .

Chapter 11. Pipe Data 237. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Pipe Engagement 237. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Carbon and Alloy Steel Stainless Steel 238. . . . . . . . . . . . . . . . . . . . . . . . . . American Pipe Flange Dimensions Diameter of Bolt CircleInches 251. . American Pipe Flange Dimensions Number of Stud Bolts and Diameter in Inches 252. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . American Pipe Flange Dimensions Flange DiameterInches 253. . . . . . . . American Pipe Flange Dimensions Flange Thickness for Flange Fittings 254. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Cast Steel Flange Standard for PN 16 255. . . . . . . . . . . . . . . . . . . . . . . . . . . . Cast Steel Flange Standard for PN 25 256. . . . . . . . . . . . . . . . . . . . . . . . . . . . Cast Steel Flange Standard for PN 40 257. . . . . . . . . . . . . . . . . . . . . . . . . . . . Cast Steel Flange Standard for PN 63 258. . . . . . . . . . . . . . . . . . . . . . . . . . . . Cast Steel Flange Standard for PN 100 259. . . . . . . . . . . . . . . . . . . . . . . . . . . Cast Steel Flange Standard for PN 160 259. . . . . . . . . . . . . . . . . . . . . . . . . . . Cast Steel Flange Standard for PN 250 260. . . . . . . . . . . . . . . . . . . . . . . . . . . Cast Steel Flange Standard for PN 320 260. . . . . . . . . . . . . . . . . . . . . . . . . . . Cast Steel Flange Standard for PN 400 261. . . . . . . . . . . . . . . . . . . . . . . . . . .

Chapter 12. Conversions and Equivalents 263. . . . . . . . . . . . . . . . . . . . . . . . . Length Equivalents 263. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Whole InchMillimeter Equivalents 263. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Fractional Inches To Millimeters 264. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Additional Fractional/Decimal InchMillimeter Equivalents 264. . . . . . . . . . . . Area Equivalents 266. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Volume Equivalents 266. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Volume Rate Equivalents 266. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Mass ConversionPounds to Kilograms 267. . . . . . . . . . . . . . . . . . . . . . . . . . Pressure Equivalents 268. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Pressure ConversionPounds per Square Inch to Bar 268. . . . . . . . . . . . . . Temperature Conversion Formulas 269. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Temperature Conversions 269. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A.P.I. and Baum Gravity Tables and Weight Factors 271. . . . . . . . . . . . . . .

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Equivalent Volume and Weight Flow Rates of Compressible Fluids 273. . . . Viscosity Conversion Nomograph 274. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Other Useful Conversions 275. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Metric Prefixes and Symbols 275. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Subject Index 277. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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1Chapter 1

Introduction to Control Valves

What Is A Control Valve?Process plants consist of hundreds, oreven thousands, of control loops allnetworked together to produce a prod-uct to be offered for sale. Each ofthese control loops is designed tokeep some important process variablesuch as pressure, flow, level, temper-ature, etc. within a required operatingrange to ensure the quality of the endproduct. Each of these loops receivesand internally creates disturbancesthat detrimentally affect the processvariable, and interaction from otherloops in the network provides distur-bances that influence the processvariable.

To reduce the effect of these load dis-turbances, sensors and transmitterscollect information about the processvariable and its relationship to somedesired set point. A controller thenprocesses this information and de-

cides what must be done to get theprocess variable back to where itshould be after a load disturbance oc-curs. When all the measuring,comparing, and calculating are done,some type of final control elementmust implement the strategy selectedby the controller.The most common final control ele-ment in the process control industriesis the control valve. The control valvemanipulates a flowing fluid, such asgas, steam, water, or chemical com-pounds, to compensate for the loaddisturbance and keep the regulatedprocess variable as close as possibleto the desired set point.Many people who talk about controlvalves or valves are really referring toa control valve assembly. The controlvalve assembly typically consists ofthe valve body, the internal trim parts,an actuator to provide the motive pow-er to operate the valve, and a variety

Chapter 1. Introduction to Control Valves

2

of additional valve accessories, whichcan include positioners, transducers,supply pressure regulators, manualoperators, snubbers, or limit switches.Other chapters of this handbook sup-ply more detail about each of thesecontrol valve assembly components.

Whether it is called a valve, controlvalve or a control valve assembly isnot as important as recognizing thatthe control valve is a critical part of thecontrol loop. It is not accurate to saythat the control valve is the most im-portant part of the loop. It is useful tothink of a control loop as an instru-mentation chain. Like any other chain,the whole chain is only as good as itsweakest link. It is important to ensurethat the control valve is not the weak-est link.

Following are definitions for processcontrol, sliding-stem control valve,rotary-shaft control valve, and othercontrol valve functions and character-istics terminology.

NOTE:

Definitions with an as-terisk (*) are from theISA Control Valve Ter-minology standardS75.05, used with per-mission.

Process ControlTerminologyAccessory: A device that ismounted on the actuator to comple-ment the actuators function and makeit a complete operating unit. Examplesinclude positioners, supply pressureregulators, solenoids, and limitswitches.

Actuator*: A pneumatic, hydraulic,or electrically powered device thatsupplies force and motion to open orclose a valve.

Actuator Assembly: An actuator,including all the pertinent accessoriesthat make it a complete operating unit.Backlash: The general name givento a form of dead band that resultsfrom a temporary discontinuity be-tween the input and output of a devicewhen the input of the device changesdirection. Slack, or looseness of a me-chanical connection is a typical exam-ple.Capacity* (Valve): The rate of flowthrough a valve under stated condi-tions.

Closed Loop: The interconnectionof process control components suchthat information regarding the processvariable is continuously fed back tothe controller set point to provide con-tinuous, automatic corrections to theprocess variable.Controller: A device that operatesautomatically by use of some estab-lished algorithm to regulate a con-trolled variable. The controller inputreceives information about the statusof the process variable and then pro-vides an appropriate output signal tothe final control element.

Control Loop: (See Closed Loop.)Control Range: The range of valvetravel over which a control valve canmaintain the installed valve gain be-tween the normalized values of 0.5and 2.0.

Control Valve: (See Control ValveAssembly.)Control Valve Assembly: Includesall components normally mounted onthe valve: the valve body assembly,actuator, positioner, air sets, transduc-ers, limit switches, etc.

Dead Band: The range throughwhich an input signal can be varied,upon reversal of direction, without ini-tiating an observable change in theoutput signal. Dead band is the namegiven to a general phenomenon thatcan apply to any device. For the valve


3

Figure 1-1. Process Dead Band

A7152 / IL

assembly, the controller output (CO) isthe input to the valve assembly andthe process variable (PV) is the outputas shown in figure 1-1. When the termDead Band is used, it is essential thatboth the input and output variablesare identified, and that any tests tomeasure dead band be under fullyloaded conditions. Dead band is typi-cally expressed as a percent of theinput span.

Dead Time: The time interval (Td) inwhich no response of the system isdetected following a small (usually0.25% - 5%) step input. It is measuredfrom the time the step input is initiatedto the first detectable response of thesystem being tested. Dead Time canapply to a valve assembly or to theentire process. (See T63.)Disk: A valve trim element used tomodulate the flow rate with either lin-ear or rotary motion. Can also be re-ferred to as a valve plug or closuremember.

Equal Percentage Characteristic*:An inherent flow characteristic that, forequal increments of rated travel, willideally give equal percentage changesof the flow coefficient (Cv) (figure 1-2).Final Control Element: The devicethat implements the control strategydetermined by the output of the con-troller. While the final control elementcan be a damper, a variable speeddrive pump, or an on-off switching de-

vice, the most common final controlelement in the process control indus-tries is the control valve assembly.The control valve manipulates a flow-ing fluid, such as gasses, steam, wa-ter, or chemical compounds, to com-pensate for the load disturbance andkeep the regulated process variableas close as possible to the desired setpoint.First-Order: A term that refers to thedynamic relationship between the in-put and output of a device. A first-or-der system or device is one that hasonly one energy storage device andwhose dynamic transient relationshipbetween the input and output is char-acterized by an exponential behavior.Friction: A force that tends to op-pose the relative motion between twosurfaces that are in contact with eachother. The friction force is a function ofthe normal force holding these twosurfaces together and the characteris-tic nature of the two surfaces. Frictionhas two components: static frictionand dynamic friction. Static friction isthe force that must be overcome be-fore there is any relative motion be-tween the two surfaces. Once relativemovement has begun, dynamic fric-tion is the force that must be over-come to maintain the relative motion.Running or sliding friction are colloqui-al terms that are sometimes used todescribe dynamic friction. Stick/slip orstiction are colloquial terms that aresometimes used to describe static fric-tion. Static friction is one of the majorcauses of dead band in a valve as-sembly.Gain: An all-purpose term that canbe used in many situations. In its mostgeneral sense, gain is the ratio of themagnitude of the output change of agiven system or device to the magni-tude of the input change that causedthe output change. Gain has two com-ponents: static gain and dynamicgain. Static gain is the gain relation-ship between the input and output andis an indicator of the ease with whichthe input can initiate a change in the


4

Figure 1-2. Inherent ValveCharacteristics

A3449/IL

output when the system or device is ina steady-state condition. Sensitivity issometimes used to mean static gain.Dynamic gain is the gain relationshipbetween the input and output whenthe system is in a state of movementor flux. Dynamic gain is a function offrequency or rate of change of the in-put.

Hysteresis*: The maximum differ-ence in output value for any single in-put value during a calibration cycle,excluding errors due to dead band.

Inherent Characteristic*: The rela-tionship between the flow coefficientand the closure member (disk) travelas it is moved from the closed positionto rated travel with constant pressuredrop across the valve.

Typically these characteristics areplotted on a curve where the horizon-tal axis is labeled in percent travel andthe vertical axis is labeled as percentflow (or Cv) (figure 1-2). Becausevalve flow is a function of both thevalve travel and the pressure dropacross the valve, conducting flowcharacteristic tests at a constant pres-sure drop provides a systematic wayof comparing one valve characteristicdesign to another. Typical valve char-acteristics conducted in this manner

are named Linear, Equal-Percentage,and Quick Opening (figure 1-2).Inherent Valve Gain: The magni-tude ratio of the change in flowthrough the valve to the change invalve travel under conditions ofconstant pressure drop. Inherentvalve gain is an inherent function ofthe valve design. It is equal to theslope of the inherent characteristiccurve at any travel point and is a func-tion of valve travel.

Installed Characteristic*: The rela-tionship between the flow rate and theclosure member (disk) travel as it ismoved from the closed position torated travel as the pressure dropacross the valve is influenced by thevarying process conditions. (SeeValve Type and Characterization inChapter 2 for more details on how theinstalled characteristic is determined.)Installed Valve Gain: The magni-tude ratio of the change in flowthrough the valve to the change invalve travel under actual process con-ditions. Installed valve gain is thevalve gain relationship that occurswhen the valve is installed in a specif-ic system and the pressure drop is al-lowed to change naturally accordingto the dictates of the overall system.The installed valve gain is equal to theslope of the installed characteristiccurve, and is a function of valve travel.(See Valve Type and Characterizationin Chapter 2 for more details on howthe installed gain is determined.)I/P: Shorthand for current-to-pres-sure (I-to-P). Typically applied to inputtransducer modules.

Linearity*: The closeness to which acurve relating to two variables approx-imates a straight line. (Linearity alsomeans that the same straight line willapply for both upscale and downscaledirections. Thus, dead band as de-fined above, would typically be con-sidered a non-linearity.)Linear Characteristic*: An inherentflow characteristic that can be repre-


5

sented by a straight line on a rectan-gular plot of flow coefficient (Cv) ver-sus rated travel. Therefore equalincrements of travel provide equal in-crements of flow coefficient, Cv (figure1-2).Loop: (See Closed Loop.)Loop Gain: The combined gain of allthe components in the loop whenviewed in series around the loop.Sometimes referred to as open-loopgain. It must be clearly specifiedwhether referring to the static loopgain or the dynamic loop gain at somefrequency.

Manual Control: (See Open Loop.)Open Loop: The condition wherethe interconnection of process controlcomponents is interrupted such thatinformation from the process variableis no longer fed back to the controllerset point so that corrections to theprocess variable are no longer pro-vided. This is typically accomplishedby placing the controller in the manualoperating position.

Packing: A part of the valve assem-bly used to seal against leakagearound the valve disk or stem.

Positioner*: A position controller(servomechanism) that is mechanical-ly connected to a moving part of a fi-nal control element or its actuator andthat automatically adjusts its output tothe actuator to maintain a desiredposition in proportion to the input sig-nal.

Process: All the combined elementsin the control loop, except the control-ler. The process typically includes thecontrol valve assembly, the pressurevessel or heat exchanger that is beingcontrolled, as well as sensors, pumps,and transmitters.

Process Gain: The ratio of thechange in the controlled process vari-able to a corresponding change in theoutput of the controller.

Process Variability: A precise statis-tical measure of how tightly the pro-cess is being controlled about the setpoint. Process variability is defined inpercent as typically (2s/m), where m isthe set point or mean value of themeasured process variable and s isthe standard deviation of the processvariable.

Quick Opening Characteristic*: Aninherent flow characteristic in which amaximum flow coefficient is achievedwith minimal closure member travel(figure 1-2).Relay: A device that acts as a poweramplifier. It takes an electrical, pneu-matic, or mechanical input signal andproduces an output of a large volumeflow of air or hydraulic fluid to the ac-tuator. The relay can be an internalcomponent of the positioner or a sep-arate valve accessory.

Resolution: The minimum possiblechange in input required to produce adetectable change in the output whenno reversal of the input takes place.Resolution is typically expressed as apercent of the input span.

Response Time: Usually measuredby a parameter that includes bothdead time and time constant. (SeeT63, Dead Time, and Time Constant.)When applied to the valve, it includesthe entire valve assembly.

Second-Order: A term that refers tothe dynamic relationship between theinput and output of a device. A sec-ond-order system or device is one thathas two energy storage devices thatcan transfer kinetic and potential ener-gy back and forth between them-selves, thus introducing the possibilityof oscillatory behavior and overshoot.

Sensor: A device that senses thevalue of the process variable and pro-vides a corresponding output signal toa transmitter. The sensor can be anintegral part of the transmitter, or itmay be a separate component.


6

Set Point: A reference value repre-senting the desired value of the pro-cess variable being controlled.

Shaft Wind-Up: A phenomenonwhere one end of a valve shaft turnsand the other does not. This typicallyoccurs in rotary-style valves where theactuator is connected to the valve clo-sure member by a relatively longshaft. While seal friction in the valveholds one end of the shaft in place,rotation of the shaft at the actuatorend is absorbed by twisting of theshaft until the actuator input transmitsenough force to overcome the friction.

Sizing (Valve): A systematic proce-dure designed to ensure the correctvalve capacity for a set of specifiedprocess conditions.

Stiction: (See Friction.)T63 (Tee-63): A measure of deviceresponse. It is measured by applyinga small (usually 1-5%) step input tothe system. T63 is measured from thetime the step input is initiated to thetime when the system output reaches63% of the final steady-state value. Itis the combined total of the systemDead Time (Td) and the system TimeConstant (t). (See Dead Time andTime Constant.)Time Constant: A time parameterthat normally applies to a first-orderelement. It is the time interval mea-sured from the first detectable re-sponse of the system to a small (usu-ally 0.25% - 5%) step input until thesystem output reaches 63% of its finalsteady-state value. (See T63.) Whenapplied to an open-loop process, thetime constant is usually designated as (Tau). When applied to a closed-loopsystem, the time constant is usuallydesignated as (Lambda).Transmitter: A device that sensesthe value of the process variable andtransmits a corresponding output sig-nal to the controller for comparisonwith the set point.

Travel*: The movement of the closuremember from the closed position to anintermediate or rated full open posi-tion.

Travel Indicator: A pointer and scaleused to externally show the position ofthe closure member typically withunits of opening percent of travel ordegrees of rotation.

Trim*: The internal components of avalve that modulate the flow of thecontrolled fluid.

Valve: (See Control Valve Assembly.)Volume Booster: A stand-alonerelay is often referred to as a volumebooster or simply booster because itboosts, or amplifies, the volume of airsupplied to the actuator. (See Relay.)

Sliding-Stem ControlValve TerminologyThe following terminology applies tothe physical and operating character-istics of standard sliding-stem controlvalves with diaphragm or piston ac-tuators. Some of the terms, particular-ly those pertaining to actuators, arealso appropriate for rotary-shaft con-trol valves. Many of the definitionspresented are in accordance with ISAS75.05, Control Valve Terminology,although other popular terms are alsoincluded. Additional explanation isprovided for some of the more com-plex terms. Component part namesare called out on accompanying fig-ures 1-3 through 1-6. Separate sec-tions follow that define specificrotary-shaft control valve terminology,control valve functions and character-istics terminology, and other processcontrol terminology.

Actuator Spring: A spring, or groupof springs, enclosed in the yoke or ac-tuator casing that moves the actuatorstem in a direction opposite to thatcreated by diaphragm pressure.

Actuator Stem: The part that con-nects the actuator to the valve stem


7

Figure 1-3. Major Components of Typical Sliding-Stem Control Valve AssembliesW0989

LOADING PRESSURE CONNECTION

DIAPHRAGM CASING

DIAPHRAGM AND STEMSHOWN IN UP POSITION

DIAPHRAGM PLATE

ACTUATOR SPRINGACTUATOR STEM

SPRING SEAT

SPRING ADJUSTORSTEM CONNECTOR

YOKE

TRAVEL INDICATOR

INDICATOR SCALE

W0363-1

BONNETGASKET

SPIRAL WOUND GASKET

CAGEGASKET

VALVE BODY

SEATRING

VALVE PLUG STEMPACKING FLANGE

ACTUATOR YOKE LOCKNUTPACKINGPACKING BOXBONNET

VALVE PLUG

CAGE

SEAT RING GASKET

PUSH-DOWN-

TO-CLOSE

VALVE BODY

ASSEMBLY

DIRECT-ACTING

ACTUATOR

W8486-3

COMPACT FIELD-REVERSIBLEMULTI-SPRING ACTUATOR

INTEGRATED POSITIONERMOUNTINGNAMUR POSITIONERMOUNTING CAPABILITY

ONE-PIECE SCREWEDPACKING FOLLOWER

CLAMPED BONNET DESIGN

STANDARD LIVE-LOADEDPACKING

INTEGRAL PNEUMATICPASSAGEWAYS

AIR-TO-OPEN

VALVE ASSEMBLY


8

Figure 1-4. Typical Reverse-Acting Diaphragm Actuator

DIAPHRAGM CASINGS

DIAPHRAGM ANDSTEM SHOWN INDOWN POSITION

DIAPHRAGMPLATE

LOADING PRESSURECONNECTION

ACTUATOR SPRING

ACTUATOR STEM

SPRING SEAT

SPRING ADJUSTOR

STEM CONNECTOR

YOKE

TRAVEL INDICATOR

INDICATOR SCALEW0364-1/IL

Figure 1-5. Extension Bonnet

W0667/IL

and transmits motion (force) from theactuator to the valve.

Actuator Stem Extension: An ex-tension of the piston actuator stem toprovide a means of transmitting piston

Figure 1-6. Bellows Seal BonnetW6434/IL

motion to the valve positioner (figure1-7).Actuator Stem Force: The net forcefrom an actuator that is available foractual positioning of the valve plug.


9

Figure 1-7. Typical Double-Acting Piston Actuator with Bias SpringW7447-1/IIL

PISTON SEAL

CYLINDERCLOSURE SEAL

ACTUATOR STEM

STEM CONNECTOR

YOKE

TRAVEL INDICATOR

TRAVELINDICATOR SCALE SEAL BUSHING

CYLINDER SEAL

ACTUATORSTEM SEAL

PISTON

CYLINDER

Angle Valve: A valve design in whichone port is co-linear with the valvestem or actuator, and the other port isat a right angle to the valve stem.(See also Globe Valve.)

Bellows Seal Bonnet: A bonnet thatuses a bellows for sealing againstleakage around the closure memberstem (figure 1-6).

Bonnet: The portion of the valve thatcontains the packing box and stemseal and can guide the stem. It pro-vides the principal opening to thebody cavity for assembly of internalparts or it can be an integral part ofthe valve body. It can also provide forthe attachment of the actuator to thevalve body. Typical bonnets arebolted, threaded, welded,pressure-seals, or integral with thebody. (This term is often used in refer-ring to the bonnet and its includedpacking parts. More properly, thisgroup of component parts should becalled the bonnet assembly.)

Bonnet Assembly: (Commonly Bon-net, more properly Bonnet Assembly):An assembly including the partthrough which a valve stem movesand a means for sealing against leak-age along the stem. It usually pro-vides a means for mounting the actua-tor and loading the packing assembly.

Bottom Flange: A part that closes avalve body opening opposite the bon-net opening. It can include a guidebushing and/or serve to allow reversalof the valve action.

Bushing: A device that supports and/or guides moving parts such as valvestems.

Cage: A part of a valve trim that sur-rounds the closure member and canprovide flow characterization and/or aseating surface. It also provides stabil-ity, guiding, balance, and alignment,and facilitates assembly of other partsof the valve trim. The walls of thecage contain openings that usuallydetermine the flow characteristic of


10

Figure 1-8. Characterized Cages for Globe-Style Valve Bodies

W0958/IL W0959/ILW0957/IL

QUICK OPENING EQUAL PERCENTAGELINEAR

the control valve. Various cage stylesare shown in figure 1-8.

Closure Member: The movable partof the valve that is positioned in theflow path to modify the rate of flowthrough the valve.

Closure Member Guide: That por-tion of a closure member that alignsits movement in either a cage, seatring, bonnet, bottom flange, or anytwo of these.

Cylinder: The chamber of a pistonactuator in which the piston moves(figure 1-7).

Cylinder Closure Seal: The sealingelement at the connection of the pis-ton actuator cylinder to the yoke.

Diaphragm: A flexible, pressure re-sponsive element that transmits forceto the diaphragm plate and actuatorstem.

Diaphragm Actuator: A fluid pow-ered device in which the fluid actsupon a flexible component, the dia-phragm.

Diaphragm Case: A housing, con-sisting of top and bottom section,used for supporting a diaphragm andestablishing one or two pressurechambers.

Diaphragm Plate: A plate concentricwith the diaphragm for transmittingforce to the actuator stem.

Direct Actuator: A diaphragm actua-tor in which the actuator stem extendswith increasing diaphragm pressure.Extension Bonnet: A bonnet withgreater dimension between the pack-ing box and bonnet flange for hot orcold service.

Globe Valve: A valve with a linearmotion closure member, one or moreports, and a body distinguished by aglobular shaped cavity around the portregion. Globe valves can be furtherclassified as: two-way single-ported;two-way double-ported (figure 1-9);angle-style (figure 1-10); three-way(figure 1-11); unbalanced cage-guided(figure 1-3); and balance cage-guided(figure 1-12).Lower Valve Body: A half housingfor internal valve parts having oneflow connection. The seat ring is nor-mally clamped between the uppervalve body and the lower valve bodyin split valve constructions.

Offset Valve: A valve constructionhaving inlet and outlet line connec-tions on different planes but 180 de-grees opposite each other.Packing Box (Assembly): The partof the bonnet assembly used to sealagainst leakage around the closure


11

Figure 1-9. Reverse Double-PortedGlobe-Style Valve Body

W0467/IL

Figure 1-10. Flanged Angle-Style Con-trol Valve Body

W0971/IL

member stem. Included in the com-plete packing box assembly are vari-ous combinations of some or all of thefollowing component parts: packing,packing follower, packing nut, lanternring, packing spring, packing flange,packing flange studs or bolts, packingflange nuts, packing ring, packing wip-er ring, felt wiper ring, bellevillesprings, anti-extrusion ring. Individual

Figure 1-11. Three-Way Valve withBalanced Valve Plug

W0665/IL

Figure 1-12. Valve Body withCage-Style Trim, Balanced Valve

Plug, and Soft Seat

W0992/IL

packing parts are shown in figure1-13.Piston: A movable pressure respon-sive element that transmits force tothe piston actuator stem (figure 1-7).Piston Type Actuator: A fluid pow-ered device in which the fluid actsupon a movable piston to provide mo-tion to the actuator stem. Piston typeactuators (figure 1-7) are classified aseither double-acting, so that full power


12

Figure 1-13. Comprehensive Packing Material Arrangementsfor Globe-Style Valve Bodies

B2565 / IL LOCATION OF SACRIFICIAL ZINC WASHER,IF USED.

GRAPHITE PACKING ARRANGEMENTS

14A1849-E

1

12A7837-A

STANDARD

TFE V-RING

13A9775-E

can be developed in either direction,or as spring-fail so that upon loss ofsupply power, the actuator moves thevalve in the required direction of trav-el.

Plug: A term frequently used to referto the closure member.

Port: The flow control orifice of acontrol valve.

Retaining Ring: A split ring that isused to retain a separable flange on avalve body.

Reverse Actuator: A diaphragm ac-tuator in which the actuator stem re-tracts with increasing diaphragm pres-sure. Reverse actuators have a sealbushing (figure 1-4) installed in theupper end of the yoke to prevent leak-age of the diaphragm pressure alongthe actuator stem.

Rubber Boot: A protective device toprevent entrance of damaging foreignmaterial into the piston actuator sealbushing.

Seal Bushing: Top and bottom bush-ings that provide a means of sealing

the piston actuator cylinder againstleakage. Synthetic rubber O-rings areused in the bushings to seal the cylin-der, the actuator stem, and the actua-tor stem extension (figure 1-7).Seat: The area of contact betweenthe closure member and its matingsurface that establishes valve shut-off.

Seat Load: The net contact force be-tween the closure member and seatwith stated static conditions. In prac-tice, the selection of an actuator for agiven control valve will be based onhow much force is required to over-come static, stem, and dynamic un-balance with an allowance made forseat load.

Seat Ring: A part of the valve bodyassembly that provides a seating sur-face for the closure member and canprovide part of the flow control orifice.

Separable Flange: A flange that fitsover a valve body flow connection. Itis generally held in place by means ofa retaining ring.

Spring Adjustor: A fitting, usuallythreaded on the actuator stem or into


13

the yoke, to adjust the spring com-pression.

Spring Seat: A plate to hold thespring in position and to provide a flatsurface for the spring adjustor to con-tact.

Static Unbalance: The net force pro-duced on the valve stem by the fluidpressure acting on the closure mem-ber and stem with the fluid at rest andwith stated pressure conditions.

Stem Connector: The device thatconnects the actuator stem to thevalve stem.

Trim: The internal components of avalve that modulate the flow of thecontrolled fluid. In a globe valve body,trim would typically include closuremember, seat ring, cage, stem, andstem pin.

Trim, Soft-Seated: Valve trim with anelastomeric, plastic or other readilydeformable material used either in theclosure component or seat ring to pro-vide tight shutoff with minimal actuatorforces.

Upper Valve Body: A half housingfor internal valve parts and having oneflow connection. It usually includes ameans for sealing against leakagealong the stem and provides a meansfor mounting the actuator on the splitvalve body.

Valve Body: The main pressureboundary of the valve that also pro-vides the pipe connecting ends, thefluid flow passageway, and supportsthe seating surfaces and the valveclosure member. Among the mostcommon valve body constructionsare: a) single-ported valve bodieshaving one port and one valve plug; b)double-ported valve bodies havingtwo ports and one valve plug; c)two-way valve bodies having two flowconnections, one inlet and one outlet;d) three-way valve bodies havingthree flow connections, two of whichcan be inlets with one outlet (for con-

verging or mixing flows), or one inletand two outlets (for diverging or di-verting flows). The term valve body, oreven just body, frequently is used inreferring to the valve body togetherwith its bonnet assembly and includedtrim parts. More properly, this group ofcomponents should be called thevalve body assembly.

Valve Body Assembly (CommonlyValve Body or Valve, more properlyValve Body Assembly): An assemblyof a valve, bonnet assembly, bottomflange (if used), and trim elements.The trim includes the closure member,which opens, closes, or partially ob-structs one or more ports.

Valve Plug: A term frequently inter-changed with plug in reference to theclosure member.

Valve Stem: In a linear motion valve,the part that connects the actuatorstem with the closure member.

Yoke: The structure that rigidly con-nects the actuator power unit to thevalve.

Rotary-Shaft Control ValveTerminologyThe definitions that follow apply spe-cifically to rotary-shaft control valves.

Actuator Lever: Arm attached torotary valve shaft to convert linear ac-tuator stem motion to rotary force toposition disk or ball of rotary-shaftvalve. The lever normally is positivelyconnected to the rotary shaft by closetolerance splines or other means tominimize play and lost motion.

Ball, Full: The flow-controlling mem-ber of rotary-shaft control valves usinga complete sphere with a flow pas-sage through it. The flow passageequals or matches the pipe diameter.


14

W6213/ILECCENTRIC DISK VALVE

Figure 1-14. Typical Rotary-Shaft Control Valve Constructions

W5477/IL

W4920/IL

W4641

CONVENTIONAL DISK

BUTTERFLY VALVE

SEGMENTED BALL VALVE

CONTOURED DISK BUTTERFLY VALVE


15

Ball, Segmented: The flow-control-ling member of rotary shaft controlvalves using a partial sphere with aflow passage through it.

Ball, V-notch: The most commontype of segmented ball control valve.The V-notch ball includes a polishedor plated partial-sphere surface thatrotates against the seal ring through-out the travel range. The V-shapednotch in the ball permits wide range-ability and produces an equal percent-age flow characteristic.

Note:The balls mentionedabove, and the diskswhich follow, perform afunction comparable tothe valve plug in aglobe-style controlvalve. That is, as theyrotate they vary the sizeand shape of the flow-stream by opening moreor less of the seal areato the flowing fluid.

Disk, Conventional: The symmetri-cal flow-controlling member used inthe most common varieties of butterflyrotary valves. High dynamic torquesnormally limit conventional disks to 60degrees maximum rotation in throttlingservice.

Disk, Dynamically Designed: A but-terfly valve disk contoured to reducedynamic torque at large increments ofrotation, thereby making it suitable forthrottling service with up to 90 de-grees of disk rotation.

Disk, Eccentric: Common name forvalve design in which the positioningof the valve shaft/disk connectionscauses the disk to take a slightly ec-centric path on opening. This allowsthe disk to be swung out of contactwith the seal as soon as it is opened,thereby reducing friction and wear.

Flangeless Valve: Valve style com-mon to rotary-shaft control valves.

Flangeless valves are held betweenANSI-class flanges by longthrough-bolts (sometimes also calledwafer-style valve bodies).Plug, Eccentric: Style of rotary con-trol valve with an eccentrically rotatingplug which cams into and out of theseat, which reduces friction and wear.This style of valve has been wellsuited for erosive applications.

Reverse Flow: Flow from the shaftside over the back of the disk, ball, orplug. Some rotary-shaft control valvesare capable of handling flow equallywell in either direction. Other rotarydesigns might require modification ofactuator linkage to handle reverseflow.

Rod End Bearing: The connectionoften used between actuator stem andactuator lever to facilitate conversionof linear actuator thrust to rotary forcewith minimum of lost motion. Use of astandard reciprocating actuator on arotary-shaft valve body commonly re-quires linkage with two rod end bear-ings. However, selection of an actua-tor specifically designed forrotary-shaft valve service requiresonly one such bearing and thereby re-duces lost motion.

Rotary-Shaft Control Valve: A valvestyle in which the flow closure mem-ber (full ball, partial ball, disk or plug)is rotated in the flowstream to controlthe capacity of the valve (figure 1-14).Seal Ring: The portion of arotary-shaft control valve assemblycorresponding to the seat ring of aglobe valve. Positioning of the disk orball relative to the seal ring deter-mines the flow area and capacity ofthe unit at that particular increment ofrotational travel. As indicated above,some seal ring designs permit bi-di-rectional flow.

Shaft: The portion of a rotary-shaftcontrol valve assembly correspondingto the valve stem of a globe valve.Rotation of the shaft positions the disk


16

or ball in the flowstream and therebycontrols capacity of the valve.

Sliding Seal: The lower cylinder sealin a pneumatic piston-style actuatordesigned for rotary valve service. Thisseal permits the actuator stem tomove both vertically and laterally with-out leakage of lower cylinder pres-sure.

Standard Flow: For thoserotary-shaft control valves having aseparate seal ring or flow ring, theflow direction in which fluid enters thevalve body through the pipeline adja-cent to the seal ring and exits from theside opposite the seal ring. Some-times called forward flow. (See alsoReverse Flow.)Trunnion Mounting: A style ofmounting the disk or ball on the valveshaft or stub shaft with two bearingsdiametrically opposed.

Control Valve Functionsand CharacteristicsTerminologyBench Set: The calibration of the ac-tuator spring range of a control valveto account for the in-service processforces.

Capacity: Rate of flow through avalve under stated conditions.

Clearance Flow: That flow below theminimum controllable flow with theclosure member not seated.

Diaphragm Pressure Span: Differ-ence between the high and low valuesof the diaphragm pressure range. Thiscan be stated as an inherent orinstalled characteristic.

Double-Acting Actuator: An actua-tor in which power is supplied in eitherdirection.

Dynamic Unbalance: The net forceproduced on the valve plug in anystated open position by the fluid pres-sure acting upon it.

Effective Area: In a diaphragm ac-tuator, the effective area is that part ofthe diaphragm area that is effective inproducing a stem force. The effectivearea of a diaphragm might change asit is stroked, usually being a maximumat the start and a minimum at the endof the travel range. Molded dia-phragms have less change in effectivearea than flat sheet diaphragms; thus,molded diaphragms are recom-mended.Equal Percentage Flow Character-istic: (See Process Control Terminol-ogy: Equal Percentage Flow Charac-teristic.)Fail-Closed: A condition wherein thevalve closure member moves to aclosed position when the actuating en-ergy source fails.Fail-Open: A condition wherein thevalve closure member moves to anopen position when the actuating en-ergy source fails.Fail-Safe: A characteristic of a valveand its actuator, which upon loss ofactuating energy supply, will cause avalve closure member to be fullyclosed, fully open, or remain in thelast position, whichever position is de-fined as necessary to protect the pro-cess. Fail-safe action can involve theuse of auxiliary controls connected tothe actuator.Flow Characteristic: Relationshipbetween flow through the valve andpercent rated travel as the latter isvaried from 0 to 100 percent. Thisterm should always be designated aseither inherent flow characteristic orinstalled flow characteristic.Flow Coefficient (Cv): A constant(Cv) related to the geometry of avalve, for a given travel, that can beused to establish flow capacity. It isthe number of U.S. gallons per minuteof 60F water that will flow through avalve with a one pound per squareinch pressure drop.High-Recovery Valve: A valve de-sign that dissipates relatively little


17

flow-stream energy due to streamlinedinternal contours and minimal flow tur-bulence. Therefore, pressure down-stream of the valve vena contracta re-covers to a high percentage of its inletvalue. Straight-through flow valves,such as rotary-shaft ball valves, aretypically high-recovery valves.

Inherent Diaphragm PressureRange: The high and low values ofpressure applied to the diaphragm toproduce rated valve plug travel withatmospheric pressure in the valvebody. This range is often referred toas a bench set range because it willbe the range over which the valve willstroke when it is set on the workbench.

Inherent Flow Characteristic: Therelationship between the flow rate andthe closure member travel as it ismoved from the closed position torated travel with constant pressuredrop across the valve.

Installed Diaphragm PressureRange: The high and low values ofpressure applied to the diaphragm toproduce rated travel with stated condi-tions in the valve body. It is becauseof the forces acting on the closuremember that the inherent diaphragmpressure range can differ from theinstalled diaphragm pressure range.

Installed Flow Characteristic: Therelationship between the flow rate andthe closure member travel as it ismoved from the closed position torated travel as the pressure dropacross the valve is influenced by thevarying process conditions.

Leakage: (See Seat Leakage.)Linear Flow Characteristic: (SeeProcess Control Terminology: LinearCharacteristic.)Low-Recovery Valve: A valve de-sign that dissipates a considerableamount of flowstream energy due toturbulence created by the contours ofthe flowpath. Consequently, pressure

downstream of the valve vena con-tracta recovers to a lesser percentageof its inlet value than is the case witha valve having a more streamlinedflowpath. Although individual designsvary, conventional globe-style valvesgenerally have low pressure recoverycapability.

Modified Parabolic Flow Character-istic: An inherent flow characteristicthat provides equal percent character-istic at low closure member travel andapproximately a linear characteristicfor upper portions of closure membertravel.

Normally Closed Valve: (SeeFail-Closed.)Normally Open Valve: (SeeFail-Open.)Push-Down-to-Close Construction:A globe-style valve construction inwhich the closure member is locatedbetween the actuator and the seatring, such that extension of the actua-tor stem moves the closure membertoward the seat ring, finally closing thevalve (figure 1-3). The term can alsobe applied to rotary-shaft valveconstructions where linear extensionof the actuator stem moves the ball ordisk toward the closed position. (Alsocalled direct acting.)Push-Down-to-Open Construction:A globe-style valve construction inwhich the seat ring is located betweenthe actuator and the closure member,so that extension of the actuator stemmoves the closure member from theseat ring, opening the valve. The termcan also be applied to rotary-shaftvalve constructions where linear ex-tension of the actuator stem movesthe ball or disk toward the open posi-tion. (Also called reverse acting.)Quick Opening Flow Characteristic:(See Process Control Terminology:Quick Opening Characteristic.)Rangeability: The ratio of the largestflow coefficient (Cv) to the smallestflow coefficient (Cv) within which the


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deviation from the specified flow char-acteristic does not exceed the statedlimits. A control valve that still does agood job of controlling when flow in-creases to 100 times the minimumcontrollable flow has a rangeability of100 to 1. Rangeability can also be ex-pressed as the ratio of the maximumto minimum controllable flow rates.

Rated Flow Coefficient (Cv): Theflow coefficient (Cv) of the valve atrated travel.

Rated Travel: The distance of move-ment of the closure member from theclosed position to the rated full-openposition. The rated full-open positionis the maximum opening recom-mended by the manufacturers.

Relative Flow Coefficient: The ratioof the flow coefficient (Cv) at a statedtravel to the flow coefficient (Cv) atrated travel.

Seat Leakage: The quantity of fluidpassing through a valve when thevalve is in the fully closed positionwith pressure differential and tempera-ture as specified. (ANSI leakage clas-sifications are outlined in Chapter 5.)

Spring Rate: The force change perunit change in length of a spring. Indiaphragm control valves, the springrate is usually stated in pounds forceper inch compression.

Stem Unbalance: The net force pro-duced on the valve stem in any posi-tion by the fluid pressure acting uponit.

Vena Contracta: The portion of aflow stream where fluid velocity is atits maximum and fluid static pressureand the cross-sectional area are attheir minimum. In a control valve, thevena contracta normally occurs justdownstream of the actual physical re-striction.

Other Process ControlTerminologyThe following terms and definitionsnot previously defined are frequentlyencountered by people associatedwith control valves, instrumentation,and accessories. Some of the terms(indicated with an asterisk) are quotedfrom the ISA standard, Process Instru-mentation Terminology, ISA 51.1. Oth-ers included are also popularly usedthroughout the control valve industry.ANSI: Abbreviation for American Na-tional Standards Institute.API: Abbreviation for American Pe-troleum Institute.ASME: Abbreviation for AmericanSociety of Mechanical Engineers.ASTM: Abbreviation for AmericanSociety for Testing and Materials.Automatic Control System*: A con-trol system that operates without hu-man intervention.Bode Diagram*: A plot of log ampli-tude ratio and phase angle values ona log frequency base for a transferfunction (figure 1-15). It is the mostcommon form of graphically present-ing frequency response data.Calibration Curve*: A graphical rep-resentation of the calibration report(figure 1-15). Steady state output of adevice plotted as a function of itssteady state input. The curve is usual-ly shown as percent output span ver-sus percent input span.Calibration Cycle*: The applicationof known values of the measured vari-able and the recording of correspond-ing values of output readings, over therange of the instrument, in ascendingand descending directions (figure1-15). A calibration curve obtained byvarying the input of a device in bothincreasing and decreasing directions.It is usually shown as percent outputspan versus percent input span andprovides a measurement of hystere-sis.


19

Figure 1-15. Graphic Representation of Various Control Terms


20

Clearance Flow: That flow below theminimum controllable flow with theclosure general member not seated.

Controller*: A device that operatesautomatically to regulate a controlledvariable.

Enthalpy: A thermodynamic quantitythat is the sum of the internal energyof a body and the product of its vol-ume multiplied by the pressure: H = U+ pV. (Also called the heat content.)Entropy: The theoretical measure ofenergy that cannot be transformedinto mechanical work in a thermody-namic system.

Feedback Signal*: The return signalthat results from a measurement ofthe directly controlled variable. For acontrol valve with a positioner, the re-turn signal is usually a mechanical in-dication of closure member stem posi-tion that is fed back into the positioner.

FCI: Abbreviation for Fluid ControlsInstitute.

Frequency Response Characteris-tic*: The frequency-dependent rela-tion, in both amplitude and phase, be-tween steady-state sinusoidal inputsand the resulting fundamental sinusoi-dal outputs. Output amplitude andphase shift are observed as functionsof the input test frequency and used todescribe the dynamic behavior of thecontrol device.

Hardness: Resistance of metal toplastic deformation, usually by in-dentation. Resistance of plastics andrubber to penetration of an indentorpoint into its surface.

Hunting*: An undesirable oscillationof appreciable magnitude, prolongedafter external stimuli disappear.Sometimes called cycling or limitcycle, hunting is evidence of operationat or near the stability limit. In controlvalve applications, hunting would ap-pear as an oscillation in the loadingpressure to the actuator caused by

instability in the control system or thevalve positioner.

Hysteresis: A retardation of an effectwhen the forces acting upon a bodyare changed (as if from viscosity orinternal friction).ISA: Abbreviation for the InstrumentSociety of America. Now recognizedas the International Society for Mea-surement and Control.Instrument Pressure: The outputpressure from an automatic controllerthat is used to operate a control valve.

Loading Pressure: The pressureemployed to position a pneumatic ac-tuator. This is the pressure that actual-ly works on the actuator diaphragm orpiston and it can be the instrumentpressure if a valve positioner is notused.

NACE: Used to stand for NationalAssociation of Corrosion Engineers.As the scope of the organization be-came international, the name waschanged to NACE International.NACE is no longer an abbreviation.OSHA: Abbreviation for OccupationalSafety and Health Act. (U.S.A.)Operating Medium: This is the fluid,generally air or gas, used to supplythe power for operation of valve posi-tioner or automatic controller.

Operative Limits*: The range of op-erating conditions to which a devicecan be subjected without permanentimpairment of operating characteris-tics.

Range: The region between the limitswithin which a quantity is measured,received, or transmitted, expressed bystating the lower and upper range val-ues (for example: 3 to 15 psi; -40 to+212F; -40 to +100C).Repeatability*: The closeness ofagreement among a number of con-secutive measurements of the outputfor the same value of the input underthe same operating conditions, ap-


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proaching from the same direction, forfull range traverses. It is usually mea-sured as a non-repeatability and ex-pressed as repeatability in percent ofspan. It does not include hyesteresis(figure 1-15).

Sensitivity*: The ratio of the changein output magnitude to the change ofthe input that causes it after thesteady-state has been reached.

Signal*: A physical variable, one ormore parameters of which carry infor-mation about another variable the sig-nal represents.

Signal Amplitude Sequencing (SplitRanging)*: Action in which two ormore signals are generated or two ormore final controlling elements are ac-tuated by and input signal, each oneresponding consecutively, with or

without overlap, to the magnitude ofthat input signal (figure 1-15).Span*: The algebraic difference be-tween the upper and lower range val-ues (for example: Range = 0 to150F; Span = 150F; Range = 3 to15 psig, Span = 12 psig).Stiction: the force required to causeone body in contact with another tobegin to move.

Supply Pressure*: The pressure atthe supply port of a device. Commonvalues of control valve supply pres-sure are 20 psig for a 3 to 15 psigrange and 35 psig for a 6 to 30 psigrange.

Zero Error*: Error of a device operat-ing under specified conditions of usewhen the input is at the lower rangevalue. It is usually expressed as per-cent of ideal span.


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Chapter 2

Control Valve Performance

In todays dynamic business environ-ment, manufacturers are under ex-treme economic pressures. Marketglobalization is resulting in intensepressures to reduce manufacturingcosts to compete with lower wagesand raw material costs of emergingcountries. Competition exists betweeninternational companies to provide thehighest quality products and to maxi-mize plant throughputs with fewer re-sources, although meeting everchanging customer needs. Thesemarketing challenges must be met al-though fully complying with public andregulatory policies.

Process VariabilityTo deliver acceptable returns to theirshareholders, international industryleaders are realizing they must reduceraw material and scrap costs while in-creasing productivity. Reducing pro-

cess variability in the manufacturingprocesses through the application ofprocess control technology is recog-nized as an effective method to im-prove financial returns and meet glob-al competitive pressures.The basic objective of a company is tomake a profit through the productionof a quality product. A quality productconforms to a set of specifications.Any deviation from the establishedspecification means lost profit due toexcessive material use, reprocessingcosts, or wasted product. Thus, alarge financial impact is obtainedthrough improving process control.Reducing process variability throughbetter process control allows optimiza-tion of the process and the productionof products right the first time.The non-uniformity inherent in the rawmaterials and processes of productionare common causes of variation thatproduce a variation of the process

Chapter 2. Control Valve Performance

24

A7153 / IL

Figure 2-1. Process Variability

2-Sigma 2-Sigma

variable both above and below the setpoint. A process that is in control, withonly the common causes of variationpresent, typically follows abell-shaped normal distribution (figure2-1).

A statistically derived band of valueson this distribution, called the +/-2 sig-ma band, describes the spread of pro-cess variable deviations from the setpoint. This band is the variability of theprocess. It is a measure of how tightlythe process is being controlled. Pro-cess Variability (see definition inChapter 1) is a precise measure oftightness of control and is expressedas a percentage of the set point.

If a product must meet a certain low-er-limit specification, for example, theset point needs to be established at a2 sigma value above thi

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