CONTROL VALVE HANDBOOK. Fourth Edition

Transcription

1 ONTROL VLVE HNDOOK Fourth Edition

2 NORTH MERI Emerson Process Management Marshalltown, Iowa US T 1 (641) F 1 (641) EUROPE Emerson Process Management ernay France T +(33) (0) F +(33) (0) LTIN MERI Emerson Process Management Sorocaba, Sao Paulo razil T +(55)(15) F +(55)(15) MIDDLE EST & FRI Emerson FZE Dubai, United rab Emirates T F SI PIFI Emerson Process Management Singapore Singapore T +(65) F +(65) Fisher is a mark owned by Fisher ontrols International LL, a member of the Emerson Process Management business division of Emerson Electric o. The Emerson logo is a trademark and service mark of Emerson Electric o. ll other marks are the property of their respective owners. The contents of this publication are presented for informational purposes only, and while every effort has been made to ensure their accuracy, they are not to be construed as warranties or guarantees, express or implied, regarding the products or services described herein or their use or applicability. We reserve the right to modify or improve the designs or specifications of such products at any time without notice. Neither Emerson, Emerson Process Management nor any of their affiliated entities assumes responsibility for the selection, use and maintenance of any product. Responsibility for the selection, use and maintenance of any product remains with the purchaser and end-user. Printed in U.S.. Fisher ontrols International LL 2005 D101881X012 ii

3 Preface to Fourth Edition ontrol valves are an increasingly vital component of modern manufacturing around the world. Properly selected and maintained control valves increase efficiency, safety, profitability, and ecology. The ontrol Valve Handbook has been a primary reference since its first printing in This fourth edition presents vital information on control valve performance and the latest technologies. hapter 1 offers an introduction to control valves including definitions for common control valve and instrumentation terminology. hapter 2 develops the vital topic of control valve performance. hapter 3 covers valve and actuator types. hapter 4 describes digital valve controllers, analog positioners, boosters, and other control valve accessories. hapter 5 is a comprehensive guide to selecting the best control valve for an application. hapter 6 covers the selection and use of special control valves. hapter 7 covers desuperheaters, steam conditioning valves, and turbine bypass systems. hapter 8 offers typical control valve installation and maintenance procedures. hapter 9 includes information on control valve standards and approval agencies throughout the world. hapter 10 offers useful tables of engineering reference data. hapter 11 includes piping reference data. hapter 12 is a handy resource for common conversions. The ontrol 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 field including contributions from the IS and the rane ompany. iii

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5 Table of ontents hapter 1. Introduction to ontrol Valves What Is ontrol Valve? Process ontrol Terminology Sliding-Stem ontrol Valve Terminology Rotary-Shaft ontrol Valve Terminology ontrol Valve Functions and haracteristics Terminology Other Process ontrol Terminology hapter 2. ontrol Valve Performance Process Variability Dead and ctuator-positioner Design Valve Response Time Valve Type nd haracterization Valve Sizing Economic Results Summary hapter 3. Valve and ctuator Types ontrol Valves Globe Valves Single-Port Valve odies alanced-plug age-style Valve odies High apacity, age-guided Valve odies Port-Guided Single-Port Valve odies Double-Ported Valve odies Three-Way Valve odies v

6 Table of ontents Rotary Valves utterfly Valve odies V-Notch all ontrol Valve odies Eccentric-Disk ontrol Valve odies Eccentric-Plug ontrol Valve odies ontrol Valve End onnections Screwed Pipe Threads olted Gasketed Flanges Welding End onnections Valve ody onnets Extension onnets ellows Seal onnets ontrol Valve Packing PTFE V-Ring Laminated and Filament Graphite US Regulatory Requirements for Fugitive Emissions Single PTFE V-Ring Packing ENVIRO-SEL PTFE Packing ENVIRO-SEL Duplex Packing KLREZ Packing ENVIRO SEL Graphite ULF HIGH-SEL Graphite ULF ENVIRO-SEL Graphite for Rotary Valves Graphite Ribbon for Rotary Valves haracterization of age-guided Valve odies haracterized Valve Plugs Valve Plug Guiding Restricted-apacity ontrol Valve Trim ctuators Diaphragm ctuators Piston ctuators Electrohydraulic ctuators Manual ctuators Rack and Pinion ctuators Electric ctuators hapter 4. ontrol Valve ccessories Positioners Other ontrol Valve ccessories Limit Switches Solenoid Valve Manifold Supply Pressure Regulator Pneumatic Lock-Up Systems Fail-Safe Systems for Piston ctuators Electro-Pneumatic Transducers Electro-Pneumatic Valve Positioners vi

7 Table of ontents Diagnostics hapter 5. ontrol Valve Selection Valve ody Materials Designations for the High Nickel lloys Pressure-Temperature Ratings for Standard lass ast arbon Steel (STM 216 Grade W) ast hromium-molybdenum Steel (STM 217 Grade W9) ast hromium-molybdenum Steel (STM 217 Grade 5) ast Type 304 Stainless Steel (STM 351 Grade F3) ast Type 316 Stainless Steel (STM 351 Grades F8M and G8M) 82 Pressure-Temperature Ratings for STM 216 ast Iron Valves Pressure-Temperature Ratings for STM 61 and 62 ast ronze Valves Face-to-Face Dimensions for Flanged Globe-Style ontrol Valves Face-to-Face Dimensions for uttweld-end Globe-Style ontrol Valves Face-to-Face Dimensions for Socket Weld-End Globe-Style ontrol Valves Face-to-Face Dimensions for Screwed-End Globe-Style ontrol Valves Face-to-enterline Dimensions for Raised Face Globe-Style ngle ontrol Valves Face-to-Face Dimensions for Separable Flanged Globe-Style ontrol Valves Face-to-Face Dimensions for Flangeless, Partial-all ontrol Valves Face-to-Face Dimensions for Single Flange (Lug-Type) and Flangeless (Wafer-Type) utterfly ontrol Valves Face-to-Face Dimensions for High Pressure utterfly Valves with Offset Design Wear & Galling Resistance hart Of Material ombinations ontrol Valve Seat Leakage lassifications lass VI Maximum Seat Leakage llowable Typical Valve Trim Material Temperature Limits Service Temperature Limitations for Elastomers mbient Temperature orrosion Information Elastomer Information Fluid ompatibility ontrol Valve Flow haracteristics Flow haracteristics Selection of Flow haracteristic Valve Sizing Sizing Valves for Liquids bbreviations and Terminology Equation onstants Determining Fp, the Piping Geometry Factor vii

8 Table of ontents Determining qmax (the Maximum Flow Rate) or Pmax (the llowable Sizing Pressure Drop) Determining qmax (the Maximum Flow Rate) Determining Pmax (the llowable Sizing Pressure Drop) Liquid Sizing Sample Problem Sizing Valves for ompressible Fluids Determining x TP, the Pressure Drop Ratio Factor ompressible Fluid Sizing Sample Problem No ompressible Fluid Sizing Sample Problem No Representative Sizing oefficients for Single-Ported Globe-Style Valve odies Representative Sizing oefficients for Rotary-Shaft Valves ctuator Sizing Globe Valves Unbalance Force Typical Unbalance reas of ontrol Valves Force to Provide Seat Load Packing Friction Typical Packing Friction Values D. dditional Forces ctuator Force alculations Rotary ctuator Sizing Torque Equations reakout Torque Dynamic Torque Typical Rotary Shaft Valve Torque Factors V-Notch all Valve with omposition Seal High Performance utterfly Valve with omposition Seal Maximum Rotation Non-Destructive Test Procedures Magnetic Particle (Surface) Examination Liquid Penetrant (Surface) Examination Radiographic (Volumetric) Examination Ultrasonic (Volumetric) Examination avitation and Flashing hoked Flow auses Flashing and avitation Valve Selection for Flashing Service Valve Selection for avitation Service Noise Prediction erodynamic Hydrodynamic Noise ontrol Noise Summary Packing Selection Packing Selection Guidelines for Sliding Stem Valves Packing Selection Guidelines for Rotary Valves hapter 6. Special ontrol Valves viii

9 Table of ontents High apacity ontrol Valves Low Flow ontrol Valves High-Temperature ontrol Valves ryogenic Service Valves ustomized haracteristics and Noise batement Trims ontrol Valves for Nuclear Service in the US Valves Subject to Sulfide Stress racking Pre-2003 Revisions of MR NE MR0175/ISO NE MR hapter 7. Steam onditioning Valves Understanding Desuperheating Technical spects of Desuperheating Typical Desuperheater Designs Fixed Geometry Nozzle Design Variable Geometry Nozzle Design Self-ontained Design Steam tomized Design Geometry-ssisted Wafer Design Understanding Steam onditioning Valves Steam onditioning Valves Steam ooler Steam Sparger Understanding Turbine ypass Systems Turbine ypass System omponents Turbine ypass Valves Turbine ypass Water ontrol Valves Electro-Hydraulic System hapter 8. Installation and Maintenance Proper Storage and Protection Proper Installation Techniques Read the Instruction Manual e Sure the Pipeline Is lean Inspect the ontrol Valve Use Good Piping Practices ontrol Valve Maintenance Reactive Maintenance Preventive Maintenance Predictive Maintenance Using ontrol Valve Diagnostics Instrument ir Leakage Supply Pressure Travel Deviation and Relay djustment ix

10 Table of ontents Instrument ir Quality In-Service Friction and Friction Trending Other Examples ontinued Diagnostics Development ctuator Diaphragm Stem Packing Seat Rings Grinding Metal Seats Replacing Seat Rings ench Set hapter 9. Standards and pprovals ontrol Valve Standards merican Petroleum Institute (PI) merican Society of Mechanical Engineers (SME) European ommittee for Standardization (EN) European Industrial Valve Standards European Material Standards European Flange Standards Fluid ontrols Institute (FI) Instrument Society of merica (IS) International Electrotechnical ommission (IE) International Standards Organization (ISO) Manufacturers Standardization Society (MSS) NE International Product pprovals for Hazardous (lassified) Locations References anadian Standards ssociation (S) Standards European ommittee for Electrotechnical Standardization (ENELE) Standards Instrument Society of merica (IS) Standards International Electrotechnical ommission (IE) Standards National Electrical Manufacturer s ssociation (NEM) Standards National Fire Protection ssociation (NFP) Standards North merican pprovals pproval gencies Types of Protection Nomenclature Hazardous Location lassification Temperature ode NEM Enclosure Rating General Locations Hazardous (lassified) Locations S Enclosure Ratings Intrinsically Safe pparatus x

11 Table of ontents Entity oncept S System Parameter oncept Loop Schematic (ontrol Drawing) omparison of Protection Techniques Explosion-proof Technique dvantages of this Technique Disadvantages of this Technique Installation Requirements Intrinsically Safe Technique dvantages of this Technique Disadvantages of this Technique Dust Ignition proof Technique Non-Incendive Technique dvantages of this Technique Disadvantages of this Technique European and sia/pacific pprovals pproval gencies ENELE pprovals Types of Protection Flameproof Increased Safety Intrinsically Safe Non-Incendive Nomenclature Hazardous Location lassification Group Zone Temperature ode IE Enclosure Rating NEM and IE Enclosure Rating omparison omparison of Protection Techniques Flameproof Technique dvantages of this Technique Disadvantages of this Technique Increased Safety Technique dvantages of this Technique Disadvantages of this Technique Intrinsically Safe Technique dvantages of this Technique Disadvantages of this Technique Type n Technique dvantages of this Technique Disadvantages of this Technique hapter 10. Engineering Data Standard Specifications For Valve Materials xi

12 Table of ontents Valve Materials Properties for Pressure ontaining omponents Physical onstants of Hydrocarbons Specific Heat Ratio (K) Physical onstants of Various Fluids Refrigerant 717 (mmonia) Properties of Water Properties of Saturated Steam Properties of Superheated Steam Velocity of Liquids in Pipe Flow of Water Through Schedule 40 Steel Pipe Flow of ir Through Schedule 40 Steel Pipe alculations for Pipe Other than Schedule hapter 11. Pipe Data Pipe Engagement arbon and lloy Steel Stainless Steel merican Pipe Flange Dimensions Diameter of olt ircle Inches merican Pipe Flange Dimensions Number of Stud olts and Diameter in Inches merican Pipe Flange Dimensions Flange Diameter Inches merican Pipe Flange Dimensions Flange Thickness for Flange Fittings ast Steel Flange Standard for PN ast Steel Flange Standard for PN ast Steel Flange Standard for PN ast Steel Flange Standard for PN ast Steel Flange Standard for PN ast Steel Flange Standard for PN ast Steel Flange Standard for PN ast Steel Flange Standard for PN ast Steel Flange Standard for PN hapter 12. onversions and Equivalents Length Equivalents Whole Inch Millimeter Equivalents Fractional Inches To Millimeters dditional Fractional/Decimal Inch Millimeter Equivalents rea Equivalents Volume Equivalents Volume Rate Equivalents Mass onversion Pounds to Kilograms Pressure Equivalents Pressure onversion Pounds per Square Inch to ar Temperature onversion Formulas Temperature onversions P.I. and aumé Gravity Tables and Weight Factors xii

13 Table of ontents Equivalent Volume and Weight Flow Rates of ompressible Fluids Viscosity onversion Nomograph Other Useful onversions Metric Prefixes and Symbols Subject Index xiii

14 Table of ontents xiv

15 hapter 1 Introduction to ontrol Valves What Is ontrol Valve? Process plants consist of hundreds, or even thousands, of control loops all networked together to produce a product to be offered for sale. Each of these control loops is designed to keep some important process variable such as pressure, flow, level, temperature, etc. within a required operating range to ensure the quality of the end product. Each of these loops receives and internally creates disturbances that detrimentally affect the process variable, and interaction from other loops in the network provides disturbances that influence the process variable. To reduce the effect of these load disturbances, sensors and transmitters collect information about the process variable and its relationship to some desired set point. controller then 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 final control element must implement the strategy selected by the controller. The most common final control element in the process control industries is the control valve. The control valve manipulates a flowing fluid, 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. Many people who talk about control valves or 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 1

16 hapter 1. Introduction to ontrol Valves of additional valve accessories, which can include positioners, transducers, supply pressure regulators, manual operators, snubbers, or limit switches. Other chapters of this handbook supply more detail about each of these control valve assembly components. Whether it is called a valve, control valve or a control valve assembly is not as important as recognizing that the control valve is a critical part of the control loop. It is not accurate to say that the control valve is the most important part of the loop. It is useful to think of a control loop as an instrumentation chain. Like any other chain, the whole chain is only as good as its weakest link. It is important to ensure that the control valve is not the weakest link. Following are definitions for process control, sliding-stem control valve, rotary-shaft control valve, and other control valve functions and characteristics terminology. NOTE: Definitions with an asterisk (*) are from the IS ontrol Valve Terminology standard S75.05, used with permission. Process ontrol Terminology ccessory: device that is mounted on the actuator to complement the actuator s function and make it a complete operating unit. Examples include positioners, supply pressure regulators, solenoids, and limit switches. ctuator * : pneumatic, hydraulic, or electrically powered device that supplies force and motion to open or close a valve. 2 ctuator ssembly: n actuator, including all the pertinent accessories that make it a complete operating unit. acklash: The general name given to a form of dead band that results from a temporary discontinuity between the input and output of a device when the input of the device changes direction. Slack, or looseness of a mechanical connection is a typical example. apacity * (Valve): The rate of flow through a valve under stated conditions. losed Loop: The interconnection of process control components such that information regarding the process variable is continuously fed back to the controller set point to provide continuous, automatic corrections to the process variable. ontroller: 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 final control element. ontrol Loop: (See losed Loop.) ontrol Range: The range of valve travel over which a control valve can maintain the installed valve gain between the normalized values of 0.5 and 2.0. ontrol Valve: (See ontrol Valve ssembly.) ontrol Valve ssembly: Includes all components normally mounted on the valve: the valve body assembly, actuator, positioner, air sets, transducers, limit switches, etc. Dead and: The range through which an input signal can be varied, upon reversal of direction, without initiating an observable change in the output signal. Dead band is the name given to a general phenomenon that can apply to any device. For the valve

17 7152 / IL Figure 1-1. Process Dead and assembly, the controller output (O) is the input to the valve assembly and the process variable (PV) is the output as shown in figure 1-1. When the term Dead and is used, it is essential that both the input and output variables are identified, and that any tests to measure dead band be under fully loaded conditions. Dead band is typically expressed as a percent of the input span. Dead Time: The time interval (Td) in which no response of the system is detected following a small (usually 0.25% - 5%) step input. It is measured from the time the step input is initiated to the first detectable response of the system being tested. Dead Time can apply to a valve assembly or to the entire process. (See T 63. ) Disk: valve trim element used to modulate the flow rate with either linear or rotary motion. an also be referred to as a valve plug or closure member. Equal Percentage haracteristic * : n inherent flow characteristic that, for equal increments of rated travel, will ideally give equal percentage changes of the flow coefficient ( v ) (figure 1-2). hapter 1. Introduction to ontrol Valves Final ontrol Element: The device that implements the control strategy determined by the output of the controller. While the final control element can be a damper, a variable speed drive pump, or an on-off switching device, the most common final control element in the process control industries is the control valve assembly. The control valve manipulates a flowing fluid, such as gasses, 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. First-Order: term that refers to the dynamic relationship between the input and output of a device. first-order system or device is one that has only one energy storage device and whose dynamic transient relationship between the input and output is characterized by an exponential behavior. Friction: force that tends to oppose the relative motion between two surfaces that are in contact with each other. The friction 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 is the force that must be overcome before there is any relative motion between the two surfaces. Once relative movement has begun, dynamic friction is the force that must be overcome to maintain the relative motion. Running or sliding friction are colloquial terms that are sometimes used to describe dynamic friction. Stick/slip or stiction are colloquial terms that are sometimes used to describe static friction. Static friction is one of the major causes of dead band in a valve assembly. Gain: n all-purpose term that can be used in many situations. In its most general sense, gain is the ratio of the magnitude of the output change of a given system or device to the magnitude of the input change that caused the output change. Gain has two components: static gain and dynamic gain. Static gain 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 3

18 hapter 1. Introduction to ontrol Valves 3449/IL Figure 1-2. Inherent Valve haracteristics output when the system or device is in a steady-state condition. Sensitivity is sometimes used to mean static gain. Dynamic gain is the gain relationship between the input and output when the system is in a state of movement or flux. Dynamic gain is a function of frequency or rate of change of the input. Hysteresis * : The maximum difference in output value for any single input value during a calibration cycle, excluding errors due to dead band. Inherent haracteristic * : The relationship between the flow coefficient and the closure member (disk) 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 flow (or v ) (figure 1-2). ecause valve flow is a function of both the valve travel and the pressure drop across the valve, conducting flow characteristic tests at a constant pressure drop provides a systematic way of comparing one valve characteristic design to another. Typical valve characteristics conducted in this manner 4 are named Linear, Equal-Percentage, and Quick Opening (figure 1-2). Inherent Valve Gain: The magnitude ratio of the change in flow 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 haracteristic * : The relationship between the flow 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 influenced by the varying process conditions. (See Valve Type and haracterization in hapter 2 for more details on how the installed characteristic is determined.) Installed Valve Gain: The magnitude ratio of the change in flow 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 specific 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. (See Valve Type and haracterization in hapter 2 for more details on how the installed gain is determined.) I/P: Shorthand for current-to-pressure (I-to-P). Typically applied to input transducer modules. 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, dead band as defined above, would typically be considered a non-linearity.) Linear haracteristic * : n inherent flow characteristic that can be repre-

19 sented by a straight line on a rectangular plot of flow coefficient ( v ) versus rated travel. Therefore equal increments of travel provide equal increments of flow coefficient, v (figure 1-2). Loop: (See losed 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 specified whether referring to the static loop gain or the dynamic loop gain at some frequency. Manual ontrol: (See Open Loop.) 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. Packing: part of the valve assembly used to seal against leakage around the valve disk or stem. Positioner * : position controller (servomechanism) that is mechanically connected to a moving part of a final 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: ll the combined elements in the control loop, except the controller. The process typically includes the control valve assembly, the pressure vessel or heat exchanger that is being controlled, as well as sensors, pumps, and transmitters. Process Gain: The ratio of the change in the controlled process variable to a corresponding change in the output of the controller. hapter 1. Introduction to ontrol Valves Process Variability: precise statistical measure of how tightly the process is being controlled about the set point. Process variability is defined 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 haracteristic * : n inherent flow characteristic in which a maximum flow coefficient is achieved with minimal closure member travel (figure 1-2). Relay: device that acts as a power amplifier. It takes an electrical, pneumatic, or mechanical input signal and produces an output of a large volume flow of air or hydraulic fluid to the actuator. The relay can be an internal component of the positioner or a separate valve accessory. 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 T 63, Dead Time, and Time onstant.) When applied to the valve, it includes the entire valve assembly. Second-Order: term that refers to the dynamic relationship between the input and output of a device. 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. Sensor: 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. 5

20 hapter 1. Introduction to ontrol Valves Set Point: reference value representing the desired value of the process variable being controlled. Shaft Wind-Up: phenomenon where one end of a valve shaft turns and the other does not. This typically occurs in rotary-style valves where the actuator is connected to the valve closure member by a relatively long shaft. While seal friction in the valve holds one end of the shaft in place, rotation of the shaft at the actuator end is absorbed by twisting of the shaft until the actuator input transmits enough force to overcome the friction. Sizing (Valve): systematic procedure designed to ensure the correct valve capacity for a set of specified process conditions. Travel * : The movement of the closure member from the closed position to an intermediate or rated full open position. Travel Indicator: pointer and scale used to externally show the position of the closure member typically with units of opening percent of travel or degrees of rotation. Trim * : The internal components of a valve that modulate the flow of the controlled fluid. Valve: (See ontrol Valve ssembly.) Volume ooster: stand-alone relay is often referred to as a volume booster or simply booster because it boosts, or amplifies, the volume of air supplied to the actuator. (See Relay.) Stiction: (See Friction.) T 63 (Tee-63): measure of device response. It is measured by applying a small (usually 1-5%) step input to the system. T 63 is measured from the time the step input is initiated to the time when the system output reaches 63% of the final steady-state value. It is the combined total of the system Dead Time (T d ) and the system Time onstant (t). (See Dead Time and Time onstant.) Time onstant: time parameter that normally applies to a first-order element. It is the time interval measured from the first detectable response of the system to a small (usually 0.25% - 5%) step input until the system output reaches 63% of its final steady-state value. (See T 63. ) When applied to an open-loop process, the time constant is usually designated as (Tau). When applied to a closed-loop system, the time constant is usually designated as λ (Lambda). Transmitter: device that senses the value of the process variable and transmits a corresponding output signal to the controller for comparison with the set point. 6 Sliding-Stem ontrol 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-shaft control valves. Many of the definitions presented are in accordance with IS S75.05, ontrol Valve Terminology, although other popular terms are also included. dditional explanation is provided for some of the more complex terms. omponent part names are called out on accompanying figures 1-3 through 1-6. Separate sections follow that define specific rotary-shaft control valve terminology, control valve functions and characteristics terminology, and other process control terminology. ctuator Spring: spring, or group of springs, enclosed in the yoke or actuator casing that moves the actuator stem in a direction opposite to that created by diaphragm pressure. ctuator Stem: The part that connects the actuator to the valve stem