CONTROL VALVE
HANDBOOK
Fifth Edition
Emerson Automation Solutions
Flow Controls
Marshalltown, Iowa 50158 USA
Sorocaba, 18087 Brazil
Cernay, 68700 France
Dubai, United Arab Emirates
Singapore 128461 Singapore
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D101881X012/ Sept19
Preface
Control valves are an increasingly vital component of modern manufacturing around
the world. Properly selected and maintained control valves increase efciency,
safety, protability, and ecology.
The Control Valve Handbook has been a primary reference since its rst printing in
1965. This fth edition presents vital information on control valve performance and
the latest technologies.
Chapter 1 offers an introduction to control valves, including denitions for common
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 addresses the selection and use of special control valves.
Chapter 7 explains desuperheaters, steam conditioning valves, and turbine bypass
systems.
Chapter 8 details typical control valve installation and maintenance procedures.
Chapter 9 contains information on control valve standards and approval agencies
across the world.
Chapter 10 identies isolation valves and actuators.
Chapter 11 covers discrete automation.
Chapter 12 discusses various process safety instrumented systems.
Chapter 13 provides useful tables of engineering reference data.
Chapter 14 includes piping reference data.
Chapter 15 is a handy resource for common conversions.
The Control Valve Handbook is both a textbook and a reference on the strongest link
in the control loop: the control valve and its accessories. This book includes extensive
and proven knowledge from leading experts in the process control eld, including
contributions from the ISA.
Table of Contents
Control Valve Handbook | Table of Contents
Chapter 1: Introduction to Control Valves ............................................. 14
1.1 What is a Control Valve? .............................................................................. 15
1.2 Sliding-Stem Control Valve Terminology .....................................................15
1.3 Rotary Control Valve Terminology ...............................................................21
1.4 Control Valve Functions and Characteristics Terminology ............................23
1.5 Process Control Terminology .......................................................................25
Chapter 2: Control Valve Performance .................................................. 32
2.1 Process Variability ....................................................................................... 33
2.1.1 Deadband ................................................................................................................ 35
2.1.1.1 Causes of Deadband .........................................................................................................35
2.1.1.2 Effects of Deadband .........................................................................................................36
2.1.1.3 Performance Tests ............................................................................................................36
2.1.1.4 Friction ............................................................................................................................36
2.1.2 Actuator and Positioner Design ................................................................................ 37
2.1.3 Valve Response Time ................................................................................................ 38
2.1.3.1 Dead Time .......................................................................................................................38
2.1.3.2 Dynamic Time ..................................................................................................................38
2.1.3.3 Solutions ..........................................................................................................................39
2.1.3.4 Supply Pressure ................................................................................................................40
2.1.3.5 Minimizing Dead Time .....................................................................................................40
2.1.3.6 Valve Response Time ........................................................................................................41
2.1.4 Valve Type and Characterization ............................................................................... 41
2.1.4.1 Installed Gain ...................................................................................................................43
2.1.4.2 Loop Gain ........................................................................................................................43
2.1.4.3 Process Optimization .......................................................................................................44
2.1.5 Valve Sizing .............................................................................................................. 45
2.2 Economic Results ........................................................................................46
2.3 Summary .................................................................................................... 48
Chapter 3: Valve and Actuator Types ..................................................... 50
3.1 Control Valve Styles .................................................................................... 51
3.1.1 Globe Valves ............................................................................................................ 51
3.1.1.1 Single-Port Valve Bodies ...................................................................................................51
3.1.1.2 Post- and Port-Guided Valve Bodies ...................................................................................52
3.1.1.3 Cage-Style Valve Bodies ....................................................................................................52
3.1.1.4 Double-Ported Valve Bodies ..............................................................................................53
3.1.1.5 Three-Way Valve Bodies....................................................................................................53
3.1.2 Sanitary Valves ........................................................................................................ 54
3.1.3 Rotary Valves ........................................................................................................... 54
3.1.3.1 Buttery Valve Bodies .......................................................................................................54
3.1.3.2 Segmented Ball Valve Bodies ............................................................................................55
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Control Valve Handbook | Table of Contents
3.1.3.3 High-Performance Buttery Valve Bodies ..........................................................................55
3.1.3.4 Eccentric Plug Valve Bodies ...............................................................................................56
3.1.3.5 Full-Port Ball Valve Bodies .................................................................................................57
3.1.3.6 Multi-Port Flow Selector ...................................................................................................57
3.2 Control Valve End Connections ................................................................... 57
3.2.1 Screwed Pipe Threads .............................................................................................. 57
3.2.2 Bolted Gasketed Flanges ........................................................................................... 58
3.2.3 Welded End Connections .......................................................................................... 58
3.2.4 Other Valve End Connections ................................................................................... 59
3.3 Valve Body Bonnets .....................................................................................59
3.3.1 Extension Bonnets.................................................................................................... 60
3.3.2 Bellows Seal Bonnets ................................................................................................ 61
3.4 Control Valve Packing..................................................................................61
3.4.1 PTFE V-Ring .............................................................................................................. 62
3.4.2 Laminated and Filament Graphite ............................................................................ 62
3.4.3 U.S. Regulatory Requirements for Fugitive Emissions ................................................ 62
3.4.4 Global Standards for Fugitive Emissions .................................................................... 63
3.4.5 Single PTFE V-Ring Packing ....................................................................................... 65
3.4.6 ENVIRO-SEAL PTFE Packing....................................................................................... 65
3.4.7 ENVIRO-SEAL Duplex Packing ................................................................................... 67
3.4.8 ISO-Seal PTFE Packing .............................................................................................. 67
3.4.9 ENVIRO-SEAL Graphite ULF ....................................................................................... 67
3.4.10 HIGH-SEAL Graphite ULF ........................................................................................ 67
3.4.11 ISO-Seal Graphite Packing ...................................................................................... 67
3.4.12 ENVIRO-SEAL Graphite for Rotary Valves ................................................................. 67
3.4.13 Graphite Ribbon for Rotary Valves .......................................................................... 67
3.4.14 Sliding-Stem Environmental Packing Selection........................................................ 67
3.4.15 Rotary Environmental Packing Selection ................................................................. 69
3.5 Characterization of Cage-Guided Valve Bodies ............................................ 69
3.6 Valve Plug Guiding ......................................................................................70
3.7 Restricted-Capacity Control Valve Trim .......................................................70
3.8 Actuators .................................................................................................... 71
3.8.1 Diaphragm Actuators .............................................................................................. 71
3.8.2 Piston Actuators ...................................................................................................... 72
3.8.4 Rack-and-Pinion Actuators ....................................................................................... 73
3.8.5 Electric Actuators ..................................................................................................... 73
Chapter 4: Control Valve Accessories .................................................... 74
4.1 Environmental & Application Considerations ............................................... 75
4.2 Positioners ..................................................................................................75
4.2.1 Pneumatic Positioners .............................................................................................. 75
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Control Valve Handbook | Table of Contents
4.2.2 Analog I/P Positioners .............................................................................................. 76
4.2.3 Digital Valve Controllers ........................................................................................... 77
4.2.3.1 Diagnostics ......................................................................................................................77
4.2.3.2 Two-Way Digital Communication .....................................................................................78
4.3 I/P Transducers ............................................................................................ 78
4.4 Volume Boosters ......................................................................................... 78
4.5 Safety Instrumented Systems (SIS) .............................................................. 80
4.5.1 Partial Stroke Testing ............................................................................................... 80
4.6 Controllers ..................................................................................................81
4.7 Position Transmitters ..................................................................................83
4.8 Limit Switches .............................................................................................83
4.9 Solenoid Valves ........................................................................................... 83
4.10 Trip Systems .............................................................................................. 84
4.11 Handwheels ..............................................................................................84
Chapter 5: Control Valve Sizing ............................................................. 86
5.1 Control Valve Dimensions ...........................................................................88
5.1.1 Face-to-Face Dimensions for Flanged, Globe-Style Valves .......................................... 88
5.1.2 Face-to-Face Dimensions for Butt Weld-End, Globe-Style Valves ................................ 90
5.1.3 Face-to-Face Dimensions for Socket Weld-End, Globe-Style Valves ............................ 91
5.1.4 Face-to-Face Dimensions for Screwed-End, Globe-Style Valves .................................. 92
5.1.5 Face-to-Centerline Dimensions for Raised-Face, Globe-Style Angle Valves ..................... 92
5.1.6 Face-to-Face Dimensions for Separable Flange, Globe-Style Valves ............................ 93
5.1.7 Face-to-Face Dimensions for Flanged and Flangeless Rotary Valves ........................... 93
5.1.8 Face-to-Face Dimensions for Single Flange and Flangeless Buttery Valves ................... 94
5.1.9 Face-to-Face Dimensions for High-Pressure, Offset Buttery Valves ........................... 94
5.2 Control Valve Seat Leakage Classications ................................................... 95
5.3 Class VI Maximum Seat Leakage Allowable ..................................................96
5.4 Control Valve Flow Characteristics...............................................................96
5.4.1 Flow Characteristics ................................................................................................. 96
5.4.2 Selection of Flow Characteristics .............................................................................. 97
5.5 Valve Sizing .................................................................................................97
5.7 Equation Constants ....................................................................................99
5.8 Sizing Valves for Liquids .............................................................................100
5.8.1 Determining Piping Geometry Factor and Liquid Pressure-Recovery Factor .................. 100
5.8.2 Determining the Pressure Drop to Use for Sizing ..................................................... 101
5.8.3 Calculating the Required Flow Coefcient ............................................................... 101
5.8.4 Liquid Sizing Sample Problem ................................................................................. 102
5.9 Sizing Valves for Compressible Fluids.........................................................104
5.9.1 Determining Piping Geometry Factor and Pressure Drop Ratio Factor at Choked Flow ................. 105
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Control Valve Handbook | Table of Contents
5.9.2 Determining Pressure Drop Ratio to Use for Sizing and Expansion Factor ................... 105
5.9.3 Calculating Flow Coefcient ................................................................................... 105
5.9.4 Compressible Fluid Sizing Sample Problem No. 1 .................................................... 106
5.9.5 Compressible Fluid Sizing Sample Problem No. 2 .................................................... 107
5.10 Representative Sizing Coefcients ...........................................................109
5.10.1 Representative Sizing Coefcients for Single-Ported, Globe-Style Valves................. 109
5.10.2 Representative Sizing Coefcients for Rotary Valves ............................................. 110
5.11 Actuator Sizing........................................................................................111
5.11.1 Globe Valves ........................................................................................................ 111
5.11.1.1 Unbalance Force ..........................................................................................................111
5.11.1.2 Force to Provide Seat Load ............................................................................................112
5.11.1.3 Packing Friction ............................................................................................................112
5.11.1.4 Additional Forces ..........................................................................................................112
5.11.2 Actuator Force Calculations ................................................................................. 114
5.12 Actuator Sizing for Rotary Valves ............................................................. 114
5.12.1 Torque Equations ................................................................................................. 114
5.12.2 Breakout Torque .................................................................................................. 114
5.12.3 Dynamic Torque .................................................................................................. 114
5.13 Typical Rotary Valve Torque Factors .........................................................115
5.13.1 Torque Factors for V-Notch Ball Valve with Composition Seal ................................ 115
5.13.2 Torque Factors for Buttery Valve with Composition Seal ...................................... 115
5.13.2.1 Maximum Rotation ......................................................................................................115
5.14 Cavitation and Flashing ...........................................................................116
5.14.1 Choked Flow Causes Flashing and Cavitation ........................................................ 116
5.14.2 Valve Selection for Flashing Service ....................................................................... 117
5.14.3 Valve Selection for Cavitation Service ................................................................... 118
5.15 Noise Prediction ...................................................................................... 118
5.15.1 Aerodynamic ....................................................................................................... 118
5.15.2 Hydrodynamic ..................................................................................................... 120
5.16 Noise Control ..........................................................................................120
5.17 Noise Summary ......................................................................................123
5.18 Packing Selection ....................................................................................124
5.18.1 Packing Selection Guidelines for Sliding-Stem Valves............................................. 125
5.18.2 Packing Selection Guidelines for Rotary Valves ...................................................... 126
5.19 Valve Body Materials ...............................................................................127
5.19.1 Designations for Common Valve Body Materials ................................................... 129
5.20 Pressure-Temperature Ratings ................................................................. 130
5.20.1 Standard Class ASTM A216 Grade WCC Cast Valves ................................................ 130
5.20.2 Standard Class ASTM A217 Grade WC9 Cast Valves .............................................. 131
5.20.3 Standard Class ASTM A351 Grade CF3 Cast Valves ................................................ 132
5.20.4 Standard Class ASTM A351 Grades CF8M and CG8M Valves .................................. 133
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Control Valve Handbook | Table of Contents
5.21 Non-Metallic Material Abbreviations .......................................................135
5.22 Non-Destructive Test Procedures ............................................................ 135
5.22.1 Magnetic Particle (Surface) Examination .............................................................. 135
5.22.2 Liquid Penetrant (Surface) Examination ............................................................... 136
5.22.3 Radiographic (Volumetric) Examination ............................................................... 136
5.22.4 Ultrasonic (Volumetric) Examination .................................................................... 136
Chapte 6: Special Control Valves ......................................................... 138
6.1 High-Capacity Control Valves ....................................................................139
6.2 Low-Flow Control Valves ...........................................................................140
6.3 High-Temperature Control Valves .............................................................140
6.4 Cryogenic Service Valves ...........................................................................141
6.5 Valves Subjected to Cavitation and Fluids with Particulate .........................141
6.6 Customized Characteristics, Noise-Abatement, and Cavitation-Mitigation
Trims ..............................................................................................................142
6.7 Control Valves for Nuclear Service in the U.S. ............................................142
6.8 Valves Subjected to Sulde Stress Cracking ............................................... 143
6.8.1 Pre-2003 Revisions of NACE MR0175 ...................................................................... 143
6.8.2 NACE MR0175/ISO 15156 ........................................................................................ 144
6.8.3 NACE MR0103 .......................................................................................................... 145
Chapter 7: Steam Conditioning ........................................................... 146
7.1 Understanding Desuperheating ................................................................147
7.1.1 Technical Aspects of Desuperheating ...................................................................... 147
7.2 Typical Desuperheater Designs ..................................................................150
7.2.1 Fixed-Geometry Nozzle Design ............................................................................... 150
7.2.2 Variable-Geometry Nozzle Design .......................................................................... 151
7.2.3 Self-Contained Design ............................................................................................ 151
7.2.5 Geometry-Assisted Wafer Design ........................................................................... 152
7.3 Understanding Steam Conditioning Valves................................................153
7.4 Steam Conditioning Valves ........................................................................153
7.4.1 Steam Attemperator .............................................................................................. 155
7.4.2 Steam Sparger ....................................................................................................... 155
7.6 Turbine Bypass System Components .........................................................156
7.6.1 Turbine Bypass Valves............................................................................................. 156
7.6.2 Turbine Bypass Water Control Valves ...................................................................... 156
7.6.3 Actuation............................................................................................................... 157
Chapter 8: Installation and Maintenance ............................................. 158
8.1 Proper Storage and Protection .................................................................. 159
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Control Valve Handbook | Table of Contents
8.2 Proper Installation Techniques ..................................................................159
8.2.1 Read the Instruction Manual .................................................................................. 159
8.2.2 Be Sure the Pipeline Is Clean ................................................................................... 159
8.2.4 Use Good Piping Practices ...................................................................................... 160
8.2.5 Flushing/Hydro/Start-Up Trim ................................................................................ 161
8.3 Control Valve Maintenance........................................................................161
8.3.1 Reactive Maintenance ............................................................................................ 162
8.3.2 Preventive Maintenance ......................................................................................... 162
8.3.3 Predictive Maintenance .......................................................................................... 162
8.3.4 Using Control Valve Diagnostics ............................................................................. 162
8.3.4.1 Instrument Air Leakage ..................................................................................................163
8.3.4.2 Supply Pressure ..............................................................................................................163
8.3.4.3 Travel Deviation and Relay Adjustment ...........................................................................163
8.3.4.4 Instrument Air Quality ....................................................................................................164
8.3.4.5 In-Service Friction and Friction Trending ..........................................................................164
8.3.4.6 Other Examples ..............................................................................................................164
8.3.5 Continued Diagnostics Development ...................................................................... 164
8.4 Service and Repair Parts ............................................................................165
8.4.1 Recommended Spare Parts .................................................................................... 165
8.4.2 Using Original Equipment Manufacturers (OEM) Parts ............................................ 165
8.4.3 Consider Upgrades for the Valve Trim ..................................................................... 165
8.5 Actuator Maintenance ............................................................................... 165
8.5.1 Spring-and-Diaphragm Actuators ......................................................................... 165
8.5.2 Piston Actuators .................................................................................................... 166
8.5.3 Stem Packing ......................................................................................................... 166
8.5.4 Seat Rings .............................................................................................................. 166
8.5.4.1 Replacing Seat Rings ......................................................................................................166
8.5.4.2 Connections: Plug-to-Stem, Ball-to-Shaft, and Disk-to-Shaft...........................................167
8.5.5 Bench Set ............................................................................................................... 167
8.5.6 Valve Travel ............................................................................................................ 167
Chapter 9: Standards and Approvals ................................................... 168
9.1 Control Valve Standards ............................................................................169
9.1.1 American Petroleum Institute (API) ......................................................................... 169
9.1.2 American Society of Mechanical Engineers (ASME) ................................................. 169
9.1.3 European Committee for Standardization (CEN) ..................................................... 169
9.1.3.1 European Industrial Valve Standards ...............................................................................169
9.1.3.2 European Material Standards .........................................................................................170
9.1.3.3 European Flange Standards ............................................................................................170
9.1.4 Fluid Controls Institute (FCI) ................................................................................... 170
9.1.5 Instrument Society of America (ISA) ....................................................................... 170
9.1.6 International Electrotechnical Commission (IEC) ..................................................... 171
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Control Valve Handbook | Table of Contents
9.1.7 Manufacturers Standardization Society (MSS) ........................................................ 171
9.1.8 NACE International ................................................................................................. 171
9.2 Product Approvals for Hazardous (Classied) Locations .............................172
9.2.1 Hazardous Location Approvals and Denitions ....................................................... 172
9.3 Classication Systems ...............................................................................172
9.3.1 Class/Division System ............................................................................................. 172
9.3.2 Zone System .......................................................................................................... 173
9.3.3 Equipment Groups ................................................................................................. 174
9.3.4 Equipment Subgroups ............................................................................................ 174
9.3.4.1 Group II (Commonly referred to as the “Gas Group”) .......................................................174
9.3.4.2 Group III (Commonly referred to as the “Dust Group”) ....................................................174
9.3.5 Type of Protection .................................................................................................. 175
9.3.5.1 Electrical Equipment .......................................................................................................175
9.3.5.2 Non-Electrical Equipment ...............................................................................................176
9.3.6 Level of Protection .................................................................................................. 177
9.3.7 Equipment Protection Level (EPL) ............................................................................ 177
9.4 Temperature Code ....................................................................................178
9.5 Nomenclature ...........................................................................................179
9.5.1 Class/Division System ............................................................................................. 179
9.5.2 Zone System .......................................................................................................... 179
9.5.3 Wiring Practices ..................................................................................................... 179
9.5.4 European Union (EU) – ATEX Directive 2014/34/EU ................................................ 180
9.6 Protection Techniques and Methods .........................................................181
9.6.1 Explosion-Proof or Flame-Proof Technique .............................................................. 181
9.6.2 Intrinsically-Safe Technique .................................................................................... 181
9.6.3 Non-Incendive or Type n Technique......................................................................... 182
9.6.4 Increased Safety ..................................................................................................... 182
9.6.5 Dust Ignition-Proof or Enclosure Dust-Proof............................................................ 183
9.7 Enclosure Ratings ......................................................................................183
Chapter 10: Isolation Valves ............................................................... 186
10.1 Basic Valve Types .....................................................................................187
10.1.1 Gate Valves .......................................................................................................... 187
10.1.2 Globe Valves ........................................................................................................ 188
10.1.3 Check Valves ........................................................................................................ 191
10.1.4 Bypass Valves ....................................................................................................... 192
10.1.6 Pinch Valves ......................................................................................................... 193
10.1.7 Ball Valves ............................................................................................................ 194
10.1.8 Buttery Valves .................................................................................................... 194
10.1.9 Plug Valves ........................................................................................................... 195
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Control Valve Handbook | Table of Contents
Chapter 11: Solenoid Valves ................................................................ 210
11.1 Solenoid Valves ....................................................................................... 211
Chapter 12: Safety Instrumented Systems ........................................... 214
12.1 Safety and Layers of Protection ...............................................................215
12.2 Safety Instrumented Systems (SIS) .......................................................... 216
12.3 Safety Standards .....................................................................................217
12.4 Safety Integrity Level (SIL) .......................................................................217
12.5 Probability of Failure Upon Demand ........................................................ 218
Final Elements, Proof Testing, and Partial Stroke Testing Techniques ...............219
12.6
12.7 Partial Stroke Testing ..............................................................................219
12.8 Online Testing Methods for the Final Element..........................................220
12.9 Digital Valve Controller Use for Partial Stroke Testing ..............................220
12.10 High-Integrity Pressure Protection System (HIPPS) ...............................221
12.11 Functionality of the HIPPS ....................................................................221
12.12 Testing Requirements ...........................................................................221
Chapter 13: Engineering Data ............................................................. 224
13.1 Standard Specications for Pressure-Retaining Valve Materials ................225
13.2 Valve Material Properties for Pressure-Containing Components .............. 232
13.3 Physical Constants of Hydrocarbons ........................................................234
13.4 Specic Heat Ratio ..................................................................................237
13.5 Physical Constants of Various Fluids ........................................................238
Refrigerant 717 (Ammonia) Properties of Liquid and Saturated Vapor ..............240
13.6
13.7 Properties of Water .................................................................................247
13.8 Properties of Saturated Steam .................................................................248
13.9 Properties of Superheated Steam ............................................................ 257
Chapter 14: Pipe Data ......................................................................... 266
14.1 Pipe Engagement ....................................................................................267
14.2 Carbon and Alloy Steel - Stainless Steel ....................................................267
14.3 American Pipe Flange Dimensions ...........................................................275
14.3.1 Diameter of Bolt Circles ....................................................................................... 275
14.3.2 Number of Stud Bolts and Diameter ..................................................................... 276
14.3.3 Flange Diameter .................................................................................................. 277
14.3.4 Flange Thickness for Flange Fittings ...................................................................... 278
14.4 Cast Steel Flange Standards ..................................................................... 280
14.4.1 Cast Steel Flange Standard for PN 10 .................................................................... 280
14.4.2 Cast Steel Flange Standard for PN 16 .................................................................... 281
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Control Valve Handbook | Table of Contents
14.4.3 Cast Steel Flange Standard for PN 25 .................................................................... 282
14.4.4 Cast Steel Flange Standard for PN 40 .................................................................... 283
14.4.5 Cast Steel Flange Standard for PN 63 .................................................................... 284
14.4.6 Cast Steel Flange Standard for PN 100 .................................................................. 284
14.4.7 Cast Steel Flange Standard for PN 160 .................................................................. 285
14.4.8 Cast Steel Flange Standard for PN 250 .................................................................. 285
14.4.9 Cast Steel Flange Standard for PN 320 .................................................................. 286
14.4.10 Cast Steel Flange Standard for PN 400 ................................................................ 286
Chapter 15: Conversions and Equivalents ............................................ 288
15.1 Length Equivalents ..................................................................................289
15.2 Whole Inch to Millimeter Equivalents ......................................................289
15.3 Fractional Inch to Millimeter Equivalents .................................................290
15.4 Additional Fractional Inch to Millimeter Equivalents ................................291
15.5 Area Equivalents ......................................................................................293
15.6 Volume Equivalents .................................................................................293
15.7 Volume Rate Equivalents ......................................................................... 293
15.8 Mass Conversion–Pounds to Kilograms ................................................... 294
15.9 Pressure Equivalents ...............................................................................294
15.10 Pressure Conversion–Pounds Per Square Inch to Bar .............................295
15.11 Temperature Conversion Formulas ........................................................ 296
15.12 Temperature Conversions .....................................................................296
15.13 API and Baumé Gravity Tables and Weight Factors ................................ 299
15.14 Other Useful Conversions ...................................................................... 301
15.15 Metric Prexes and Sufxes ...................................................................302
Index .................................................................................................. 304
13
Chapter 1
Introduction to Control Valves
Control Valve Handbook | Chapter 1: Introduction to Control Valves
See Additional Resources »
1.1 What is a Control Valve?
Modern processing plants utilize a vast
network of control loops to produce an
end product for market. These control
loops are designed to keep a process
variable (i.e. pressure, ow, level,
temperature, etc.) within a required
operating range to ensure a quality end
product is produced. Each of these loops
receives and internally creates
disturbances that detrimentally affect
the process variable (PV). Interaction
from other loops in the network also
provide disturbances that inuence the
process variable. See Figure 1.1.
Manipulated
Variable
Control
Valve
Figure 1.1 Feedback Control Loop
Process
Controller
To reduce the effect of these load
disturbances, sensors and transmitters
collect information about the process
variable (PV) and its relationship to some
desired set point. A controller processes
this information and decides what must
be done to get the process variable back
to where it should be after a load
disturbance occurs. When all the
measuring, comparing, and calculating
are done, some type of nal control
element must implement the strategy
selected by the controller.
The most common nal control element
in the process control industries is the
control valve. The control valve
manipulates a owing uid, such as gas,
steam, water, or chemical compounds to
compensate for the load disturbance and
keep the regulated process variable as
close as possible to the desired set point.
Controlled
Variable
Sensor
Transmitter
The control valve is a critical part of the
control loop. Many people who talk
about control valves are really referring
to a control valve assembly. The control
valve assembly typically consists of the
valve body, the internal trim parts, an
actuator to provide the motive power to
operate the valve, and a variety of
additional valve accessories, which can
includes, transducers, supply pressure
regulators, manual operators, snubbers,
or limit switches.
There are two main types of control
valve designs, depending on the action
of the closure member: sliding-stem or
rotary. Sliding-stem valves, as seen in
Figure 1.2 and 1.3, use linear motion to
move a closure member into and out of
a seating surface. Rotary valves, as seen
in Figure 1.13 and 1.17, use rotational
motion to turn a closure member into
and out of a seating surface.
1.2 Sliding-Stem Control Valve Terminology
The following terminology applies to the
physical and operating characteristics of
standard sliding-stem control valves
with diaphragm or piston actuators.
Some of the terms, particularly those
pertaining to actuators, are also
appropriate for rotary control valves.
Many of the denitions presented are in
accordance with ANSI/ISA-75.05.01,
Control Valve Terminology, although
other popular terms are also included.
Additional explanation is provided for
some of the more complex terms.
Additional sections in this chapter follow
that dene specic terminology for
rotary control valves, general process
control, and control valve functions and
characteristics.
15
Control Valve Handbook | Chapter 1: Introduction to Control Valves
Actuator Stem Force: The net force
from an actuator that is available for
actual positioning of the valve plug,
referred to as valve travel.
Angle Valve: A valve design in which the
inlet and outlet ports are perpendicular
to each other. See also Globe Valve.
Figure 1.2 Sliding-Stem Control Valve
1. Bonnet
2. Packing Box
3. Cage or Seat
Ring Retainer
4. Valve Stem
1
2
3
6
5. Valve Plug
6. Valve Body
7. Seat Ring
8. Port
4
5
Figure 1.4 Angle Valve
Bellows Seal Bonnet: A bonnet that
uses a bellows for sealing against
leakage around the closure member
stem. See Figure 1.5.
Bonnet: The portion of the valve that
contains the packing box and stem seal
and can provide guiding for the valve
7
8
stem. It provides the principal opening
to the body cavity for assembly of
internal parts or it can be an integral part
of the valve body. It can also provide for
the attachment of the actuator to the
Figure 1.3 Sliding-Stem Control Valve
Actuator Spring: A spring, or group of
springs, enclosed in the yoke or actuator
casing or piston cylinder that moves the
actuator stem in a direction opposite to
that created by loading pressure.
Actuator Stem: The part that connects
the actuator to the valve stem and
transmits motion (force) from the
actuator to the valve.
Actuator Stem Extension: An extension
of the piston actuator stem to provide a
means of transmitting piston motion to
the valve positioner.
valve body. Typical bonnets are bolted,
threaded, welded, pressure sealed, or
integral with the body. This term is often
used in referring to the bonnet and its
included packing parts. More properly,
this group of component parts should
be called the bonnet assembly.
Bonnet Assembly (Commonly Bonnet,
more properly Bonnet Assembly): An
assembly including the part through
which a valve stem moves and a means
for sealing against leakage along the
stem. It usually provides a means for
mounting the actuator and loading the
packing assembly, and maintains proper
16
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alignment of the plug to the rest of the
control valve assembly. See Figure 1.6.
1. Bonnet
1
2
3
4
5
Figure 1.5 Bellows Seal Bonnet
1
2
3
4
Figure 1.6 Bonnet Assembly
2. Packing
3. Packing Box
4. Bellows
5. Valve Stem
1. Bonnet
2. Packing
3. Packing Box
4. Valve Stem
Bottom Flange: A part that closes a
valve body opening opposite the bonnet
opening. It can include a guide bushing
and/or serve to allow reversal of the
valve action.
Bushing: A device that supports and/or
guides moving parts such as valve stems
and plugs.
Cage: A part of the valve trim that
surrounds the closure member and can
provide ow characterization and/or a
seating surface. It also provides stability,
guiding, balance, and alignment, and
facilitates assembly of other parts of the
valve trim. The walls of the cage contain
openings that usually determine the
ow characteristic of the control valve.
See Figure 1.7.
Closure Member: The movable part of
the valve that is positioned in the ow
path to modulate the rate of ow
through the valve.
Closure Member Guide: That portion of
a closure member that aligns its
movement in either a cage, seat ring
(port guiding), bonnet, bottom ange,
stem or any two of these.
Cylinder: The chamber of a piston
actuator in which the piston moves.
Cylinder Closure Seal: The sealing
element at the connection of the
piston actuator cylinder to the yoke.
Diaphragm: A exible, pressure
responsive element that transmits
force to the diaphragm plate and
actuator stem.
Diaphragm Actuator: A uid-powered
device in which the uid, usually
compressed air (see Loading Pressure),
acts upon a exible component, the
diaphragm to produce a force to move
the closure member.
Diaphragm Case: A housing,
consisting of top and bottom
section, used for supporting a
diaphragm and establishing one or
two pressure chambers.
Figure 1.7 Cages (left to right): Linear, Equal-Percentage, Quick-Opening
17
Control Valve Handbook | Chapter 1: Introduction to Control Valves
Diaphragm Plate: A rigid plate
concentric with the diaphragm for
transmitting force to the actuator stem.
Direct-Acting Actuator: An actuator,
in which the actuator stem extends
with increasing loading pressure. See
Figure 1.9.
Extension Bonnet: A bonnet with
greater dimension between the
packing box and bonnet ange for hot
or cold service.
Figure 1.8 Three-Way Globe Valve
1
3
Globe Valve: A valve with a linear
motion closure member, one or more
ports, and a body distinguished by a
globular shaped cavity around the port
region. Globe valves can be further
classied as: two-way single-ported
(Figure 1.3); two-way double-ported;
angle-style, or three-way (Figure 1.8).
Loading Pressure: Fluid, usually
compressed air, applied to the diaphragm
or piston in a pneumatic actuator.
Offset Valve: A valve construction
having inlet and outlet line connections
on different planes, but 180 degrees
opposite each other.
Packing Box (Assembly): The part of
the bonnet assembly used to seal
against leakage around the closure
member stem. Included in the complete
packing box assembly are various
combinations of some or all of the
following component parts: packing,
2
4
5
6
7
8
10
Figure 1.9 Direct-Acting Actuator
18
1. Loading Pressure Connection
2. Diaphragm Case
3. Diaphragm
4. Diaphragm Plate
5. Actuator Spring
6. Actuator Stem
7. Spring Seat
8. Spring Adjuster
9
11
9. Stem Connector
10. Valve Stem
11. Yoke
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packing follower, packing nut, lantern
ring, packing spring, packing ange,
packing ange studs or bolts, packing
ange nuts, packing ring, packing wiper
ring, felt wiper ring, Belleville springs,
anti-extrusion ring. See Figure 1.11.
Piston: A rigid movable pressure
responsive element that transmits force
to the piston actuator stem.
1
2
3
4
6
Figure 1.10 Piston-Type Actuator
5
1. Loading Pressure
Connection
2. Piston
3. Piston Seal
7
4. Cylinder
5. Cylinder Closure
Seal
6. Seal Bushing
7. Stem Connector
Piston-Type Actuator: A uid powered
device in which the uid, usually
compressed air, acts upon a movable
piston to provide motion of the actuator
stem and provide seating force upon
closure. Piston-type actuators are
classied as either double-acting, so that
full power can be developed in either
direction, or as spring-fail so that upon
loss of supply power, the actuator moves
the valve in the required direction of
travel. See Figure 1.10.
Port: The ow control orice of a
control valve.
Retaining Ring: A split ring that is used to
retain a separable ange on a valve body.
Reverse-Acting Actuator: An actuator
in which the actuator stem retracts with
increasing loading pressure. Reverse
actuators have a seal bushing installed in
the upper end of the yoke to prevent
leakage of the loading pressure along
the actuator stem. See Figure 1.12.
Rubber Boot: A protective device to
prevent entrance of damaging foreign
material into the piston actuator
seal bushing.
Seal Bushing: Top and bottom bushings
that provide a means of sealing the
piston actuator cylinder against leakage.
Synthetic rubber O-rings are used in the
bushings to seal the cylinder, the actuator
stem, and the actuator stem extension.
Seat: The area of contact between the
closure member and its mating surface
that establishes valve shutoff.
Seat Load: The net contact force
between the closure member and seat
with stated static conditions. In practice,
7
8
Figure 1.11 Packing
1
2
3
4
5
6
3
4
5
9
PTFE Packing
1. Upper Wiper
2. Packing Follower
3. Female Adaptor
4. V-Ring
5. Male Adaptor
6. Lantern Ring
7. Washer
8. Spring
9. Box Ring/Lower
Wiper
1
2
4
1
3
1
2
1
Graphite Packing
1. Filament Ring
2. Laminated Ring
3. Lantern Ring
4. Zinc Washer
4
4
19
Control Valve Handbook | Chapter 1: Introduction to Control Valves
the selection of an actuator for a given
control valve will be based on how much
force is required to overcome static,
stem, and dynamic unbalance with an
allowance made for adequate seat load.
Seat Ring: A part of the valve body
assembly that provides a seating surface
for the closure member and can provide
part of the ow control orice.
Separable Flange: A ange that ts over
a valve body ow connection. It is
generally held in place by means of a
retaining ring.
Spring Adjuster: A tting, usually
threaded on the actuator stem or into
the yoke, to adjust the spring
compression (see bench set in Control
Valve Functions and Characteristics
Terminology).
Spring Seat: A plate to hold the spring
in position and to provide a at surface
for the spring adjuster to contact.
Static Unbalance: The net force produced
on the valve stem by the process uid
pressure acting on the closure member
and stem with the uid at rest and with
stated pressure conditions.
Stem Connector: The device that
connects the actuator stem to the
valve stem.
Trim: The internal components of a valve
that modulate the ow of the controlled
uid. In a globe valve body, trim would
typically include closure member, seat
ring, cage, stem, and stem pin.
Trim, Soft-Seated: Valve trim with an
elastomeric, plastic, or other readily
deformable material used either in the
closure component or seat ring to
provide tight shutoff with minimal
actuator forces.
Valve Body: The main pressure
boundary of the valve that also provides
the pipe connecting ends, the uid ow
passageway, and supports the seating
surfaces and the valve closure member.
Among the most common valve body
constructions are: single-ported valve
bodies having one port and one valve
plug; double-ported valve bodies having
3
4
1
6
7
8
10
11
Figure 1.12 Reverse-Acting Actuator
20
1. Loading Pressure Connection
2. Diaphragm Case
3. Diaphragm
2
4. Diaphragm Plate
5
9
12
5. Seal Bushing
6. Actuator Spring
7. Actuator Stem
8. Spring Seat
9. Spring Adjuster
10. Stem Connector
11. Valve Stem
12. Yoke
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two ports and one valve plug; two-way
valve bodies having two ow
connections, one inlet and one outlet;
three-way valve bodies having three ow
connections, two of which can be inlets
with one outlet (for converging or mixing
ows), or one inlet and two outlets (for
diverging or diverting ows). The term
“valve body”, or even just “body”, is
frequently used in referring to the valve
body together with its bonnet assembly
and included trim parts. More properly,
this group of components should be
called the valve body assembly.
Valve Body Assembly (Commonly Valve
Body or Valve, more properly Valve Body
Assembly): An assembly of a valve body,
bonnet assembly, bottom ange (if
used), and trim elements. The trim
includes the closure member, which
opens, closes, or partially obstructs one
or more ports.
Valve Plug (Plug): A term frequently
used to reference the valve closure
member in a sliding-stem valve.
Valve Stem: In a linear motion valve, the
part that connects the actuator stem
with the closure member.
Yoke: The structure that rigidly connects
the actuator power unit to the valve.
1.3 Rotary Control Valve Terminology
The following terminology applies to the
physical and operating characteristics of
rotary control valves with diaphragm or
piston actuators. The closure members
(i.e. balls, disks, eccentric plugs, etc.) in
rotary designs perform a function
comparable to the valve plug in a
sliding-stem control valve. That is, as
they rotate they vary the size and shape
of the ow stream by opening more or
less of the seal area to the owing uid.
Many of the denitions presented are in
accordance with ISA S75.05, Control
Valve Terminology, although other
popular terms are also included. Terms
pertaining to actuators are also
appropriate for rotary control valves.
Additional explanation is provided for
some of the more complex terms.
Additional sections in this chapter follow
that dene specic terminology for
general process control, and control
valve functions and characteristics.
Figure 1.13 Rotary Control Valve
Actuator Lever: Arm attached to rotary
valve shaft to convert linear actuator
stem motion to rotary force (torque) to
position a disk or ball of a rotary valve.
The lever normally is positively
connected to the rotary by close
tolerance splines or other means to
minimize play and lost motion.
Ball, Full: The ow closure member of
rotary control valves using a complete
sphere with a cylindrical ow passage
through it. The ow passage equals or
matches the pipe diameter.
Ball, Segmented: The ow closure
member of rotary control valves using a
partial sphere with a ow passage
through it.
21
Control Valve Handbook | Chapter 1: Introduction to Control Valves
opening. This allows the disk to be swung
out of contact with the seal as soon as it
is opened, reducing friction and wear.
Figure 1.14 Segmented Ball
Ball, V-Notch: The most common type
of segmented ball control valve. The
Figure 1.16 Eccentric Disk Valve
V-notch ball includes a polished or
plated partial sphere surface that rotates
against the seal ring throughout the
travel range. The V-shaped notch in the
ball permits wide rangeability and
produces an equal-percentage ow
characteristic.
Flangeless Valve: Valve style common
to rotary control valves. Flangeless
valves are held between ANSI/ASME-
class anges by long through-bolts
(sometimes also called wafer-style
valve bodies).
Plug, Eccentric: Style of rotary control
valve with an eccentrically-rotating plug
which cams into and out of the seat,
which reduces friction and wear. This
style of valve is well suited for erosive
applications.
Reverse Flow: Flow from the shaft/hub
Figure 1.15 V-Notch Ball
Disk, Conventional: The symmetrical
ow closure member used in the most
common varieties of buttery rotary
valves. Highly-dynamic torques normally
limit conventional disks to 60 degrees
maximum rotation in throttling service.
Disk, Dynamically-Designed: A
buttery valve disk contoured to reduce
dynamic torque at large increments of
rotation, thereby making it suitable for
throttling service with up to 90 degrees
of disk rotation.
Disk, Eccentric: Common name for valve
design in which the off-centered
positioning of the valve shaft/disk
connections causes the disk to take a
slightly eccentric (cammed) path on
side over the back of the disk, ball, or
plug. Some rotary control valves are
capable of handling ow equally well in
either direction. Other rotary designs
might require modication of actuator
linkage to handle reverse ow.
Rod End Bearing: The connection often
used between actuator stem and
actuator lever to facilitate conversion of
linear actuator thrust to rotary force
(torque) with minimum of lost motion.
Use of a standard reciprocating actuator
on a rotary valve body commonly
requires linkage with two rod end
bearings. However, selection of an
actuator specically designed for rotary
valve service requires only one such
bearing and thereby reduces lost motion.
Rotary Control Valve: A valve style in
which the ow closure member (full ball,
22
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partial ball, disk or plug) is rotated in the
ow stream to control the capacity of
the valve. See Figure 1.17.
Seal Ring: The portion of a rotary
control valve assembly corresponding to
the seat ring of a globe valve. Positioning
of the disk or ball relative to the seal ring
determines the ow area and capacity of
the unit at that particular increment of
rotational travel.
Shaft: The portion of a rotary control
valve assembly corresponding to the
valve stem of a globe valve. Rotation of
the shaft positions the disk or ball in the
ow stream and controls ow through
the valve.
Sliding Seal: The lower cylinder seal in a
pneumatic piston-style actuator
designed for rotary valve service. This seal
permits the actuator stem to move both
vertically and laterally without leakage of
lower cylinder loading pressure, allowing
for a single rod end bearing.
Standard Flow: For those rotary control
valves having a separate seal ring or ow
ring, the ow direction in which uid
enters the valve body through the
pipeline adjacent to the seal ring and
exits from the side opposite the seal
ring. Sometimes called forward ow or
ow into the face of the closure
member. See also Reverse Flow.
Trunnion Mounting: A style of
mounting the disk or ball on the valve
shaft or stub shaft with two
diametrically opposed bearings.
1.4 Control Valve Functions and Characteristics Terminology
Bench Set: The calibration procedure of
an actuator spring so that it can use a
pressure range to fully stroke a valve to
its rated travel (see Inherent Diaphragm
Pressure Range).
Capacity: Amount of ow through a
valve (C
Clearance Flow: Flow that occurs below
the minimum controllable ow with the
closure member not fully seated.
Diaphragm Pressure Span: Difference
between the high and low values of the
diaphragm loading pressure range.
or Kv), under stated conditions.
v
13
14
Figure 1.17 Rotary Control Valve
1
3
5
11
8
10
12
9
2
4
6
7
1. Loading Pressure
Connection
2. Diaphragm Case
3. Diaphragm
4. Diaphragm Plate
5. Spring
6. Actuator Stem
7. Lever
8. Shaft
9. Tra vel Stop
10. Packing
11. Disk
12. Body
13. Seal
14. Seal Retainer
23
Control Valve Handbook | Chapter 1: Introduction to Control Valves
Double-Acting Actuator: An actuator in
which pneumatic, hydraulic, or electric
power is supplied in either direction.
Dynamic Unbalance: The net force
produced on the valve plug in any stated
open position by the uid process
pressure acting upon it.
Effective Area: In an actuator, the part
of the diaphragm or piston area that
produces a stem force. The effective
area of a diaphragm might change as it
is stroked, usually being a maximum at
the start and a minimum at the end of
the travel range. Molded diaphragms
have less change in effective area than
at sheet diaphragms; thus, molded
diaphragms are recommended.
Fail-Closed: A condition wherein the
valve closure member moves to a closed
position when the actuating energy
source fails.
Fail-Open: A condition wherein the
valve closure member moves to an open
position when the actuating energy
source fails.
Fail-Safe: A characteristic of a valve and
its actuator, which upon loss of actuating
energy supply, will cause a valve closure
member to be fully closed, fully open, or
remain in the last position, whichever
position is dened as necessary to
protect the process and equipment.
action can involve the use of auxiliary
controls connected to the actuator.
Flow Characteristic: Relationship
between ow through the valve and
percent rated travel as the latter is varied
from 0 to 100%. This term should always
be designated as either inherent ow
characteristic or installed ow
characteristic (See denitions in Process
Control Terminology Section).
Flow Coefcient (C
): A constant related
v
to the geometry of a valve, for a given
travel, that can be used to establish ow
capacity. It is the number of U.S. gallons
per minute of 16°C (60°F) water that will
ow through a valve with a one pound
per square inch pressure drop.
High-Recovery Valve: A valve design
that dissipates relatively little ow
stream energy due to streamlined
internal contours and minimal ow
turbulence. Therefore, pressure
downstream of the valve vena contracta
recovers to a high percentage of its inlet
value. Straight-through ow valves, such
as rotary ball valves, are typically
high-recovery valves.
Inherent Diaphragm Pressure Range:
The high and low values of pressure
applied to the diaphragm to produce
rated valve plug travel with atmospheric
pressure in the valve body. This range is
often referred to as a bench set range
because it will be the range over which
the valve will stroke when it is set on
the work bench.
Inherent Flow Characteristic: The
relationship between the ow rate and
the closure member travel as it is
moved from the closed position to
rated travel with constant pressure drop
across the valve.
Installed Diaphragm Pressure Range:
The high and low values of pressure
applied to the diaphragm to produce
rated travel with stated conditions in the
valve body. It is because of the forces
acting on the closure member that the
inherent diaphragm pressure range can
differ from the installed diaphragm
pressure range.
Installed Flow Characteristic: The
relationship between the ow rate and
the closure member travel as it is
moved from the closed position to
rated travel as the pressure drop across
the valve is inuenced by the varying
process conditions.
Low-Recovery Valve: A valve design
that dissipates a considerable amount of
ow stream energy due to turbulence
24
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created by the contours of the ow path.
Consequently, pressure downstream of
the valve vena contracta recovers to a
lesser percentage of its inlet value than is
the case with a valve having a more
streamlined ow path. Although
individual designs vary, conventional
globe-style valves generally have low
pressure recovery capability.
Modied Parabolic Flow
Characteristic: An inherent ow
characteristic that provides equalpercentage characteristic at low closure
member travel and approximately a
linear characteristic for upper portions
of closure member travel.
Normally-Closed Valve: See Fail-Closed.
Normally-Open Valve: See Fail-Open.
Push-Down-to-Close (PDTC)
Construction: A globe-style valve
construction in which the closure
member is located between the
actuator and the seat ring, such that
extension of the actuator stem moves
the closure member toward the seat
ring, nally closing the valve. The term
can also be applied to rotary valve
constructions where linear extension of
the actuator stem moves the ball or disk
toward the closed position. Also called
direct-acting.
Push-Down-to-Open (PDTO)
Construction: A globe-style valve
construction in which the seat ring is
located between the actuator and the
closure member, so that extension of
the actuator stem moves the closure
member from the seat ring, opening the
valve. The term can also be applied to
rotary valve constructions where linear
extension of the actuator stem moves
the ball or disk toward the open
position. Also called reverse-acting.
Rangeability: The ratio of the largest
ow coefcient (C
ow coefcient (C
or Kv) to the smallest
v
or Kv) within which
v
the deviation from the specied ow
characteristic does not exceed the
stated limits. A control valve that still
does a good job of controlling when ow
increases to 100 times the minimum
controllable ow has a rangeability
of 100 to 1. Rangeability can also be
expressed as the ratio of the maximum
to minimum controllable ow rates.
Rated Flow Coefcient (C
coefcient (C
) of the valve at rated travel.
v
): The ow
v
Rated Travel: The distance of
movement of the closure member from
the closed position to the rated full-open
position. The rated full-open position is
the maximum opening recommended
by the manufacturers.
Relative Flow Coefcient (C
ratio of the ow coefcient (C
stated travel to the ow coefcient (C
): The
v
) at a
v
)
v
at rated travel.
Seat Leakage: The quantity of uid
passing through a valve when the valve
is in the fully closed position and
maximum available seat load is applied
with pressure differential and
temperature as specied.
Spring Rate (K
): The force change per
s
unit change in length of a spring. In
diaphragm actuators, the spring rate is
usually stated in pounds force per inch
compression.
Vena Contracta: The portion of a ow
stream where uid velocity is at its
maximum and uid static pressure and
the cross-sectional area are at their
minimum. In a control valve, the vena
contracta normally occurs just
downstream of the actual physical
restriction.
1.5 Process Control Terminology
The following terms and denitions not
previously dened are frequently
encountered by people associated with
25
Control Valve Handbook | Chapter 1: Introduction to Control Valves
control valves, instrumentation, and
accessories. Some of the terms,
indicated with an asterisk (*), are
derived from the ISA standard, Process
Instrumentation Terminology, ISA 51.1.
Other popular terminology used
throughout the control valve industry is
also included.
Accessory: A device mounted to a
control valve assembly to complement
various functions or produce desired
actions, particularly actuation. (i.e.
positioners, supply pressure regulators,
solenoids, limit switches, etc.).
Actuator*: A pneumatic, hydraulic, or
electrically powered device that supplies
force and motion to open or close a valve.
Actuator Assembly: An actuator,
including all the pertinent accessories
that make it a complete operating unit.
ANSI: Abbreviation for American
National Standards Institute.
API: Abbreviation for American
Petroleum Institute.
ASME: Abbreviation for American
Society of Mechanical Engineers.
ASTM: Used to stand for American
Society for Testing and Materials. As
the scope of the organization became
international, the name was changed to
ASTM International. ASTM is no longer
an abbreviation.
Automatic Control System*: A control
system that operates without human
intervention.
Backlash: A form of deadband that
results from a temporary discontinuity
between the input and output of a
device when the input of the device
changes direction. (i.e. slack, or
looseness, of a mechanical connection).
Bode Diagram*: A plot of log amplitude
ratio and phase angle values on a log
frequency base for a transfer function. It
is the most common form of graphically
presenting frequency response data.
Calibration Curve*: A graphical
representation of the calibration report.
Steady state output of a device plotted
as a function of its steady state input.
The curve is usually shown as percent
output span versus percent input span.
Calibration Cycle*: The application of
known values of the measured variable
and the recording of corresponding
values of output readings, over the
range of the instrument, in ascending
and descending directions. A calibration
curve obtained by varying the input of a
device in both increasing and
decreasing directions. It is usually
shown as percent output span versus
percent input span and provides a
measurement of hysteresis.
Capacity*(Valve): The amount of ow
through a valve (C
) under stated
v
conditions.
Closed Loop: The interconnection of
process control components such that
information regarding the process
variable is continuously fed back to a
controller set point to provide
continuous, automatic corrections to
the process variable.
Closure Member: A valve trim element
(also known as a plug, disk, segmented
ball, or full-port ball) used to modulate
the ow rate within a control valve.
Controller: A device that operates
automatically, by use of some
established algorithm, to regulate a
controlled variable. The controller input
receives information about the status of
the process variable and then provides
an appropriate output signal to the nal
control element.
Control Loop: See Closed Loop or
Open Loop.
Control Range: The range of valve travel
over which a control valve can maintain
the installed valve gain between the
26
Control Valve Handbook | Chapter 1: Introduction to Control Valves
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normalized values of 0.5 and 2.0.
Control Valve Assembly: A device used
to modulate uid ow by varying the
size of the ow passage as directed by a
signal from a controller.
Deadband: A general phenomenon,
that can apply to any device, where the
range through which an input signal can
be varied, upon reversal of direction,
without initiating an observable change
in output signal. For control valves, the
controller output (CO) is the input to the
valve assembly and the process variable
(PV) is the output, as shown in Figure
1.18. Whenever discussing deadband, it
is essential that both the input and
output variables are identied, and that
any quantiable tests be conducted
under fully-loaded conditions.
Deadband is typically expressed as a
percent of the input span.
100%
Process Variable
100%
Controller Output
Figure 1.18 Deadband
Dead Time: The time interval (Td) in
which no response of the system is
detected following a small (usually
0.25% - 5%) step input. This time is
derived from the moment the step input
is initiated to the rst detectable
response of the system. Dead time can
apply to a valve assembly or to the entire
process. See T63.
Enthalpy: A thermodynamic quantity
that is the sum of the internal energy of
a body and the product of its volume
multiplied by the pressure: H = U + pV.
Also called the heat content.
Entropy: The theoretical measure of
energy that cannot be transformed
into mechanical work in a
thermodynamic system.
Equal-Percentage Characteristic*: An
inherent ow characteristic that, for
equal increments of rated travel, will
ideally give equal-percentage changes of
the ow coefcient (C
existing C
.
v
) from the
v
Feedback Signal*: The return signal
that results from a measurement of the
directly controlled variable. For a control
valve with a positioner, the return signal
is usually a mechanical indication of
closure member stem position that is
fed back into the positioner.
FCI: Abbreviation for Fluid Controls
Institute. Provides standards and
educational materials to assist purchasers
and users in understanding and using
uid control and conditioning equipment.
Final Control Element: A device that
implements the control strategy
determined by the output of a
controller. While this nal control
element can take many forms (dampers,
on/off switching devices, etc.) the most
common nal control element in
industry today is the control valve
assembly. Control valves modulate
owing uid (i.e. gas, steam, water,
chemical compounds, etc.) to
compensate for load disturbances and
keep the regulated process variable as
close to the desired set point as possible.
First-Order: A term referring to the
dynamic relationship between the input
and output of a device. First-order
systems, or devices, have only one
energy storage device and the dynamic
transient relationship between the input
and output is characterized by an
exponential behavior.
Frequency Response Characteristic*:
The frequency-dependent relation, in
both amplitude and phase, between
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Control Valve Handbook | Chapter 1: Introduction to Control Valves
steady-state sinusoidal inputs and the
resulting fundamental sinusoidal
outputs. Output amplitude and phase
shift are observed as functions of the
input test frequency and used to
describe the dynamic behavior of the
control device.
Friction: A force that tends to oppose
the relative motion between two
surfaces that are in contact with each
other. The associated force is a function
of the normal force holding these two
surfaces together and the characteristic
nature of the two surfaces. Friction has
two components: static friction and
dynamic friction. Static friction (also
known as stick/slip, or stiction) is the
force that must be overcome before
there is any relative motion between the
two surfaces. Static friction is also one of
the major causes of deadband in a valve
assembly. Once relative movement has
begun, dynamic friction (also known as
running friction, or sliding friction) is the
force that must be overcome to
maintain the relative motion.
Gain: Term used to describe the ratio of
the magnitude of an output change of a
given system or device to the magnitude
of an input change that caused the
output change. Gain has two
components: static gain and dynamic
gain. Static gain (also known as
sensitivity) is the gain relationship
between the input and output and is an
indicator of the ease with which the
input can initiate a change in the output
when the system or device is in a
steady-state condition. Dynamic gain is
the gain relationship between the input
and output when the system is in a state
of movement or ux. Dynamic gain is a
function of frequency or rate of change
of the input.
Hardness: Resistance of metal to
plastic deformation, usually by
indentation. Resistance of plastics and
rubber to penetration of an indentor
point into its surface.
Hunting*: An undesirable oscillation of
appreciable magnitude, prolonged after
external stimuli disappear. Sometimes
called cycling or limit cycle, hunting is
evidence of operation at or near the
stability limit. In control valve
applications, hunting would appear as
an oscillation in the loading pressure to
the actuator caused by instability in the
or the valve positioner.
Hysteresis*: The maximum difference
in output value for any single input value
during a calibration cycle, excluding
errors due to deadband. A retardation of
an effect when the forces acting upon a
body are changed (as if from viscosity or
internal friction).
100
Quick-Opening
Linear
Rated Flow Coefficient (%)
0
Figure 1.19 Inherent Valve Characteristics
Equal-Percentage
100
Rated Travel (%)
Inherent Characteristic*: The
relationship between the ow coefcient
and the closure member travel as it is
moved from the closed position to rated
travel with constant pressure drop across
the valve. Typically, these characteristics
are plotted on a curve where the
horizontal axis is labeled in percent travel
and the vertical axis is labeled as percent
ow (or C
). Because valve ow is a
v
function of both the valve travel and the
pressure drop across the valve,
conducting ow characteristic tests at a
constant pressure drop provides a
systematic way of comparing one valve
28
Control Valve Handbook | Chapter 1: Introduction to Control Valves
See Additional Resources »
characteristic design to another. Typical
valve characteristics conducted in this
manner are named linear, equalpercentage, and quick opening.
Inherent Valve Gain: The magnitude
ratio of the change in ow through the
valve to the change in valve travel under
conditions of constant pressure drop.
Inherent valve gain is an inherent
function of the valve design. It is equal
to the slope of the inherent
characteristic curve at any travel point
and is a function of valve travel.
Installed Characteristic*: The
relationship between the ow rate and
the closure member (disk) travel as it is
moved from the closed position to rated
travel as the pressure drop across the
valve is inuenced by the varying
process conditions.
Installed Valve Gain: The magnitude
ratio of the change in ow through the
valve to the change in valve travel under
actual process conditions. Installed valve
gain is the valve gain relationship that
occurs when the valve is installed in a
specic system and the pressure drop is
allowed to change naturally according to
the dictates of the overall system. The
installed valve gain is equal to the slope
of the installed characteristic curve, and
is a function of valve travel.
Instrument Pressure: The output
pressure from an automatic controller
that is used to operate a control valve.
I/P: Shorthand for current-to-pressure
(I-to-P). Typically applied to input
transducer modules.
ISA: Abbreviation for the International
Society for Automation.
Linearity*: The closeness to which a
curve relating to two variables
approximates a straight line. Linearity
also means that the same straight line
will apply for both upscale and
downscale directions. Thus, deadband
as dened above, would typically be
considered a non-linearity.
Linear Characteristic*: An inherent
ow characteristic that can be
represented by a straight line on a
rectangular plot of ow coefcient (C
)
v
versus rated travel. Therefore equal
increments of travel provide equal
increments of ow coefcient, C
.
v
Loading Pressure: The pressure
employed to position a pneumatic
actuator. This is the pressure that actually
works on the actuator diaphragm or
piston and it can be the instrument
pressure if a valve positioner is not used.
Loop: See Closed Loop or Open Loop.
Loop Gain: The combined gain of all the
components in the loop when viewed in
series around the loop. Sometimes
referred to as open loop gain. It must be
clearly specied whether referring to the
static loop gain or the dynamic loop gain
at some frequency.
Manual Control: See Open Loop.
NACE: Used to stand for National
Association of Corrosion Engineers. As
the scope of the organization became
international, the name was changed to
NACE International. NACE is no longer an
abbreviation.
Open Loop: The condition where the
interconnection of process control
components is interrupted such that
information from the process variable
is no longer fed back to the controller
set point so that corrections to the
process variable are no longer
provided. This is typically accomplished
by placing the controller in the manual
operating position.
Operating Medium: This is the uid,
generally air or gas, used to supply the
power for operation of valve positioner
or automatic controller.
Operative Limits*: The range of
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Control Valve Handbook | Chapter 1: Introduction to Control Valves
operating conditions to which a device
can be subjected without permanent
impairment of operating characteristics.
OSHA: Abbreviation for Occupational
Safety and Health Administration. (U.S.)
Packing: A part of the valve assembly
used to seal against leakage around the
valve shaft or stem.
Positioner*: A position controller
(servomechanism) that is mechanically
connected to a moving part of a nal
control element or its actuator and that
automatically adjusts its output to the
actuator to maintain a desired position
in proportion to the input signal.
Process: All the combined elements in
the control loop, except the controller.
Sometimes refers to the uid that passes
through the loop.
Process Gain: The ratio of the change in
the controlled process variable to a
corresponding change in the output of
the controller.
Process Variability: A precise statistical
measure of how tightly the process is
being controlled about the set point.
Process variability is dened in percent
as typically (2s/m), where m is the set
point or mean value of the measured
process variable and s is the standard
deviation of the process variable.
Quick-Opening (QO) Characteristic*:
An inherent ow characteristic in which
a maximum ow coefcient is achieved
with minimal closure member travel.
Range: The region between the limits
within which a quantity is measured,
received, or transmitted, expressed by
stating the lower and upper range
values. For example: 3 to 15 psi; -40 to
100°C (-40 to 212°F).
Relay: A device that acts as a power
amplier. It takes an electrical,
pneumatic, or mechanical input signal
and produces an output of a large
volume ow of air or hydraulic uid to
the actuator. The relay can be an internal
component of the positioner or a
separate valve accessory.
Repeatability*: The closeness of
agreement among a number of
consecutive measurements of the
output for the same value of the input
under the same operating conditions,
approaching from the same direction,
for full-range traverses. It is usually
measured as a non-repeatability and
expressed as repeatability in percent of
span. It does not include hysteresis.
Resolution: The minimum possible
change in input required to produce a
detectable change in the output when
no reversal of the input takes place.
Resolution is typically expressed as a
percent of the input span.
Response Time: Usually measured by a
parameter that includes both dead time
and time constant. (See T63, Dead
Time, and Time Constant.) When
applied to the valve, it includes the
entire valve assembly.
Second-Order: A term that refers to the
dynamic relationship between the input
and output of a device. A second-order
system or device is one that has two
energy storage devices that can transfer
kinetic and potential energy back and
forth between themselves, thus
introducing the possibility of oscillatory
behavior and overshoot.
Sensitivity*: The ratio of the change in
output magnitude to the change of the
input that causes it after the steadystate has been reached.
Sensor: A device that senses the value of
the process variable and provides a
corresponding output signal to a
transmitter. The sensor can be an
integral part of the transmitter, or it may
be a separate component.
Set Point: A reference value
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