control valve terminology A Flashcards
Accessory
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.
Backlash
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
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 elemen
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
Manual Control
This is typically accomplished by placing the controller in the manual operating position
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 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
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
Transmitter
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.
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
