Control Valve Terminology (Process Control) Flashcards
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
Accessory
A pneumatic, hydraulic, or electrically powered device that supplies force and motion to open or close a valve.
Actuator
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
Backlash
The rate of flow through a valve under stated conditions
Capacity
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
Closed Loop
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
Controller
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 Range
Includes all components normally mounted on the valve: the valve body assembly, actuator, positioner, air sets, transducers, limit switches, etc
Control Valve Assembly
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
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
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
Disk
An inherent flow characteristic that, for equal increments of rated travel, will ideally give equal percentage changes of the flow coefficient (Cv)
Equal Percentage Characteristic
The device that implements the control strategy determined by the output of the controller
Final Control Element
A force that tends to oppose the relative motion between two surfaces that are in contact with each other
Friction
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
Gain
The maximum difference in output value for any single input value during a calibration cycle, excluding errors due to dead band
Hysteresis
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 Characteristic
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
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 Characteristic
The magnitude ratio of the change in flow through the valve to the change in valve travel under actual process conditions.
Installed Valve Gain
Shorthand for current-to-pressure transducer
I/P
The closeness to which a curve relating to two variables approximates a straight line
Linearity
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
Linear Characteristic
The combined gain of all the components in the loop when viewed in series around the loop
Loop Gain
This is typically accomplished by placing the controller in the manual operating position
Manual Control
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
Open Loop
A part of the valve assembly used to seal against leakage around the valve disk or stem.
Packing
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
Positioner
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
The ratio of the change in the controlled process variable to a corresponding change in the output of the controller
Process Gain
An inherent flow characteristic in which a maximum flow coefficient is achieved with minimal closure member travel
Quick Opening Characteristic
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
Relay
The minimum possible change in input required to produce a detectable change in the output when no reversal of the input takes place.
Resolution
Usually measured by a parameter that includes both dead time and time constant
Response Time
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.
Sensor
A reference value representing the desired value of the process variable being controlled
Set Point
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
Stiction
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.
Transmitter
The movement of the closure member from the closed position to an intermediate or rated full open position
Travel
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.
Travel Indicator
The internal components of a valve that modulate the flow of the controlled fluid
Trim
A systematic procedure designed to ensure the correct valve capacity for a set of specified process conditions
Valve Sizing
A stand-alone relay that boosts, or amplifies, the volume of air supplied to the actuator
Volume Booster
