(valve) Process control terminology Flashcards
acessory
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
Actuator
A pneumatic, hydraulic, or electrically powered device that supplies force and
motion to open or close a valve.
Capacity
The rate of flow through a valve under stated conditions
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 continuous, automatic corrections to the process variable
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 final control element
Control
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
Control Valve
Assembly
Includes all components normally mounted on the valve: the valve body
assembly, actuator, positioner, air sets, transducers, limit switches, etc.
Dead Band
The range through which an input signal can be varied, upon reversal of
direction, without initiating an observable change in the output signal
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
disk
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
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)
Final Control
Element
The device that implements the control strategy determined by the output of
the controller
Friction
A force that tends to oppose the relative motion between two surfaces that are
in contact with each other
Gain
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
Hysteresis
The maximum difference in output value for any single input value during a
calibration cycle, excluding errors due to dead band
Inherent
Characteristic
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
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
Installed
Characteristic
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
Installed
Valve Gain
The magnitude ratio of the change in flow through the valve to the change in
valve travel under actual process conditions.
I/P
Shorthand for current-to-pressure transducer
Linearity
The closeness to which a curve relating to two variables approximates a
straight line
Linear
Characteristic
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
Loop Gain
The combined gain of all the components in the loop when viewed in series
around the 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
Packing
A part of the valve assembly used to seal against leakage around the valve
disk or stem.
Positioner
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
Process
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
Process Gain
The ratio of the change in the controlled process variable to a corresponding
change in the output of the controller
Quick
Opening
Characteristic
An inherent flow characteristic in which a maximum flow coefficient is
achieved with minimal closure member travel
Relay
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
Resolution
The minimum possible change in input required to produce a detectable
change in the output when no reversal of the input takes place.
Response
Time
Usually measured by a parameter that includes both dead time and time
constant
Sensor
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.
Set Point
A reference value representing the desired value of the process variable
being controlled
Stiction
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
Travel
The movement of the closure member from the closed position to an
intermediate or rated full open position
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 Sizing
A systematic procedure designed to ensure the correct valve capacity for a set
of specified process conditions
Volume
Booster
A stand-alone relay that boosts, or amplifies, the volume of air supplied to the
actuator
