control valve terminology A

Question 1

Accessory

Answer 1

A device that is mounted on the actuator to complement the actuator’s function and makes it a complete operating unit. Examples include positioners, supply pressure regulators, solenoids, and limit switches

Question 2

Actuator

Answer 2

A pneumatic, hydraulic, or electrically powered device that supplies force and motion to open or close a valve.

Question 3

Backlash

Answer 3

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

Question 4

Capacity

Answer 4

The rate of flow through a valve under stated conditions

Question 5

Closed Loop

Answer 5

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

Question 6

Controller

Answer 6

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 final control elemen

Question 7

Control Range

Answer 7

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

Question 8

Control Valve Assembly

Answer 8

Includes all components normally mounted on the valve: the valve body assembly, actuator, positioner, air sets, transducers, limit switches, etc

Question 9

Dead Band

Answer 9

The range through which an input signal can be varied, upon reversal of direction, without initiating an observable change in the output signal

Question 10

Dead Time

Answer 10

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

Question 11

Disk

Answer 11

A valve trim element used to modulate the flow rate with either linear or rotary motion. Can also be referred to as a valve plug or closure member

Question 12

Equal Percentage Characteristic

Answer 12

An inherent flow characteristic that, for equal increments of rated travel, will ideally give equal percentage changes of the flow coefficient (Cv)

Question 13

Final Control Element

Answer 13

The device that implements the control strategy determined by the output of the controller

Question 14

Friction

Answer 14

A force that tends to oppose the relative motion between two surfaces that are in contact with each other

Question 15

Gain

Answer 15

In its most general sense, it 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

Question 16

Hysteresis

Answer 16

The maximum difference in output value for any single input value during a calibration cycle, excluding errors due to dead band

Question 17

Inherent Characteristic

Answer 17

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

Question 18

Inherent Valve Gain

Answer 18

The magnitude ratio of the change in flow through the valve to the change in valve travel under conditions of constant pressure drop

Question 19

Installed Characteristic

Answer 19

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

Question 20

Installed Valve Gain

Answer 20

The magnitude ratio of the change in flow through the valve to the change in valve travel under actual process conditions

Question 21

I/P

Answer 21

Shorthand for current-to-pressure transducer

Question 22

Linearity

Answer 22

The closeness to which a curve relating to two variables approximates a straight line

Question 23

Linear Characteristic

Answer 23

An inherent flow characteristic that can be represented by a straight line on a rectangular plot of flow coefficient (Cv) versus rated travel. Therefore equal increments of travel provide equal increments of flow coefficient, Cv

Question 24

Loop Gain

Answer 24

The combined gain of all the components in the loop when viewed in series around the loop

Question 25

Manual Control

Answer 25

This is typically accomplished by placing the controller in the manual operating position

Question 26

Open Loop

Answer 26

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

Question 27

Packing

Answer 27

A part of the valve assembly used to seal against leakage around the valve disk or stem

Question 28

Positioner

Answer 28

A 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

Question 29

Process

Answer 29

All the combined elements in the control loop, except the controller. Typically includes the control valve assembly, the pressure vessel or heat exchanger that is being controlled, as well as sensors, pumps, and transmitters

Question 30

Process Gain

Answer 30

The ratio of the change in the controlled process variable to a corresponding change in the output of the controller

Question 31

Quick Opening Characteristic

Answer 31

An inherent flow characteristic in which a maximum flow coefficient is achieved with minimal closure member travel

Question 32

Relay

Answer 32

A 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. It can be an internal component of the positioner or a separate valve accessory

Question 33

Resolution

Answer 33

The minimum possible change in input required to produce a detectable change in the output when no reversal of the input takes place.

Question 34

Response Time

Answer 34

Usually measured by a parameter that includes both dead time and time constant

Question 35

Sensor

Answer 35

A device that senses the value of the process variable and provides a corresponding output signal to a transmitter. It can be an integral part of the transmitter, or it may be a separate component.

Question 36

Set Point

Answer 36

A reference value representing the desired value of the process variable being controlled

Question 37

Stiction

Answer 37

Stick/slip and this are colloquial terms that are sometimes used to describe static friction which is a major cause of dead band in a valve assembly

Question 38

Transmitter

Answer 38

A device that senses the value of the process variable and transmits a corresponding output signal to the controller for comparison with the set point.

Question 39

Travel

Answer 39

The movement of the closure member from the closed position to an intermediate or rated full open position

Question 40

Travel Indicator

Answer 40

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

Question 41

Trim

Answer 41

The internal components of a valve that modulate the flow of the controlled fluid

Question 42

Valve Sizing

Answer 42

A systematic procedure designed to ensure the correct valve capacity for a set of specified process conditions

Question 43

Volume Booster

Answer 43

A stand-alone relay that boosts, or amplifies, the volume of air supplied to the actuator