Accessory: A device that is mount-ed on the actuator to complement the actuator’s function and make it a com-plete operating unit. Examples include positioners, supply pressure regula-tors, solenoids, and limit switches.
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.
Backlash: The general name given to a form of dead band that results from a temporary discontinuity be-tween the input and output of a device when the input of the device changes direction. Slack, or looseness of a me-chanical connection is a typical exam-ple.
Capacity* (Valve): The rate of flow through a valve under stated condi-tions.
Closed 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 con-tinuous, automatic corrections to the process variable.
Controller: A device that operates automatically by use of some estab-lished algorithm to regulate a con-trolled variable. The controller input receives information about the status of the process variable and then pro-vides an appropriate output signal to the final control element.
Control Loop: (See Closed Loop.)
Control Range: The range of valve travel over which a control valve can maintain the installed valve gain be-tween the normalized values of 0.5 and 2.0.
Control Valve: (See Control Valve Assembly.)
Control Valve Assembly: Includes all components normally mounted on the valve: the valve body assembly, actuator, positioner, air sets, transduc-ers, limit switches, etc.
Dead Band: The range through which an input signal can be varied, upon reversal of direction, without ini-tiating 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 assembly, the controller output (CO) is the input to the valve assembly and the process variable (PV) is the output as shown in figure 1. When the term Dead Band 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 typi-cally expressed as a percent of the input span.
Figure 1. Process Dead Band
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 T63.)
Disk: A valve trim element used to modulate the flow rate with either lin-ear or rotary motion. Can also be re-ferred to as a valve plug or closure member.
Equal Percentage Characteristic*: An inherent flow characteristic that, for equal increments of rated travel, will ideally give equal percentage changes of the flow coefficient (Cv) (figure 2).
Final Control Element: The device that implements the control strategy determined by the output of the con-troller. While the final control element can be a damper, a variable speed drive pump, or an on-off switching de-vice, the most common final control element in the process control indus-tries is the control valve assembly. The control valve manipulates a flow-ing fluid, such as gasses, steam, wa-ter, or chemical compounds, to com-pensate for the load disturbance and keep the regulated process variable as close as possible to the desired set point.
First-Order: A term that refers to the dynamic relationship between the in-put and output of a device. A first-or-der system or device is one that has only one energy storage device and whose dynamic transient relationship between the input and output is char-acterized by an exponential behavior.
Friction: A force that tends to op-pose 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 characteris-tic nature of the two surfaces. Friction has two components: static friction and dynamic friction. Static friction is the force that must be overcome be-fore there is any relative motion be-tween the two surfaces. Once relative movement has begun, dynamic fric-tion is the force that must be over-come to maintain the relative motion. Running or sliding friction are colloqui-al terms that are sometimes used to describe dynamic friction. Stick/slip or “stiction” are colloquial terms that are sometimes used to describe static fric-tion. Static friction is one of the major causes of dead band in a valve as-sembly.
Gain: An 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 magni-tude of the input change that caused the output change. Gain has two com-ponents: static gain and dynamic gain. Static gain is the gain relation-ship 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. 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 in-put.
Figure 2. Inherent Valve Characteristics
Hysteresis*: The maximum differ-ence in output value for any single in-put value during a calibration cycle, excluding errors due to dead band.
Inherent Characteristic*: The rela-tionship between the flow 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 horizon-tal axis is labeled in percent travel and the vertical axis is labeled as percent flow (or Cv) (figure 2). Because valve flow is a function of both the valve travel and the pressure drop across the valve, conducting flow characteristic tests at a constant pres-sure drop provides a systematic way of comparing one valve characteristic design to another. Typical valve char-acteristics conducted in this manner are named Linear, Equal-Percentage, and Quick Opening (figure 2).
Inherent Valve Gain: The magni-tude ratio of the change in flow through the valve to the change in valve travel under conditions of con-stant 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 rela-tionship between the flow rate and the closure member (disk) travel as it is moved from the closed position to rat-ed travel as the pressure drop across the valve is influenced by the varying process conditions. (See Valve Type and Characterization in Chapter 2 for more details on how the installed characteristic is determined.)
Installed Valve Gain: The magni-tude ratio of the change in flow through the valve to the change in valve travel under actual process con-ditions. Installed valve gain is the valve gain relationship that occurs when the valve is installed in a specif-ic system and the pressure drop is al-lowed 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 Characterization in Chapter 2 for more details on how the installed gain is determined.)
I/P: Shorthand for current-to-pres-sure (I-to-P). Typically applied to input transducer modules.
Linearity*: The closeness to which a curve relating to two variables approx-imates a straight line. (Linearity also means that the same straight line will apply for both upscale and downscale directions. Thus, dead band as de-fined above, would typically be con-sidered a non-linearity.)
Linear Characteristic*: An inherent flow characteristic that can be repre-sented by a straight line on a rectan-gular plot of flow coefficient (Cv) ver-sus rated travel. Therefore equal increments of travel provide equal in-crements of flow coefficient, Cv (figure 2).
Loop: (See Closed 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 Control: (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 pro-vided. This is typically accomplished by placing the controller in the manual operating position.
Packing: A part of the valve assem-bly used to seal against leakage around the valve disk or stem.
Positioner*: A position controller (servomechanism) that is mechanical-ly connected to a moving part of a fi-nal control element or its actuator and that automatically adjusts its output to the actuator to maintain a desired po-sition in proportion to the input signal.
Process: All the combined elements in the control loop, except the control-ler. 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 vari-able to a corresponding change in the output of the controller.
Process Variability: A precise statis-tical measure of how tightly the pro-cess 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 Characteristic*: An inherent flow characteristic in which a maximum flow coefficient is achieved with minimal closure member travel (figure 2).
Relay: A device that acts as a power amplifier. It takes an electrical, pneu-matic, or mechanical input signal and produces an output of a large volume flow of air or hydraulic fluid to the ac-tuator. The relay can be an internal component of the positioner or a sep-arate 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 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 sec-ond-order system or device is one that has two energy storage devices that can transfer kinetic and potential ener-gy back and forth between them-selves, thus introducing the possibility of oscillatory behavior and overshoot.
Sensor: A device that senses the value of the process variable and pro-vides 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 repre-senting the desired value of the pro-cess variable being controlled.
Shaft Wind-Up: A phenomenon where one end of a valve shaft turns occurs in rotary-style valves where the actuator is connected to the valve clo-sure 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): A systematic proce-dure designed to ensure the correct valve capacity for a set of specified process conditions.
Stiction: (See Friction.)
T63 (Tee-63): A measure of device response. It is measured by applying a small (usually 1-5%) step input to the system. T63 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 (Td) and the system Time Constant (t). (See Dead Time and Time Constant.)
Time Constant: A time parameter that normally applies to a first-order element. It is the time interval mea-sured from the first detectable re-sponse of the system to a small (usu-ally 0.25% - 5%) step input until the system output reaches 63% of its final steady-state value. (See T63.) When applied to an open-loop process, the time constant is usually designated as t (Tau). When applied to a closed-loop system, the time constant is usually designated as ë (Lambda).
Transmitter: A device that senses the value of the process variable and transmits a corresponding output sig-nal to the controller for comparison with the set point.
Travel*: The movement of the closure member from the closed position to an intermediate or rated full open posi-tion.
Travel Indicator: A 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 Control Valve Assembly.)
Volume Booster: A stand-alone re-lay 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.)
