(valve) Process control terminology

Question 1

acessory

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

Capacity

Answer 3

The rate of flow through a valve under stated conditions

Question 4

Closed Loop

Answer 4

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 5

Controller

Answer 5

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 element

Question 6

Control
Range

Answer 6

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 7

Control Valve
Assembly

Answer 7

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

Question 8

Dead Band

Answer 8

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

Question 9

Dead Time

Answer 9

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 10

disk

Answer 10

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 11

Equal
Percentage
Characteristic

Answer 11

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

Question 12

Final Control
Element

Answer 12

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

Question 13

Friction

Answer 13

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

Question 14

Gain

Answer 14

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 15

Hysteresis

Answer 15

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

Question 15

Inherent
Characteristic

Answer 15

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 16

Inherent
Valve Gain

Answer 16

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

Question 17

Installed
Characteristic

Answer 17

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 18

Installed
Valve Gain

Answer 18

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

Question 19

I/P

Answer 19

Shorthand for current-to-pressure transducer

Question 20

Linearity

Answer 20

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

Question 21

Linear
Characteristic

Answer 21

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 22

Loop Gain

Answer 22

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

Question 23

Open Loop

Answer 23

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 24

Packing

Answer 24

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

Question 25

Positioner

Answer 25

A device that is mechanically connected to a moving part of a final control
element or its actuator , that automatically adjusts its output to the actuator to
maintain a desired position in proportion to the input signal

Question 26

Process

Answer 26

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 27

Process Gain

Answer 27

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

Question 28

Quick
Opening
Characteristic

Answer 28

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

Question 29

Relay

Answer 29

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 30

Resolution

Answer 30

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

Question 31

Response
Time

Answer 31

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

Question 32

Sensor

Answer 32

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 33

Set Point

Answer 33

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

Question 34

Stiction

Answer 34

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 35

Travel

Answer 35

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

Question 36

Travel
Indicator

Answer 36

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 37

Trim

Answer 37

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

Question 38

Valve Sizing

Answer 38

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

Question 39

Volume
Booster

Answer 39

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