It is the final control element of a loop: a body and trim, an actuator, and usually a positioner that turns a 4 to 20 mA or digital signal into an exact stem or shaft position. Globe-style sliding-stem valves are the classic form, with rotary types such as segmented ball and eccentric plug for larger or dirtier services. Sizing by Cv, characteristic, noise and cavitation decides whether it controls well.
Level control valve on the separator is slamming between 10 and 90 percent, the loop needs tuning or the valve's oversized.
An oversized control valve that lives at 10 percent open is a bigger headache than a slightly undersized one.
Also heard as
- CV
- modulating valve
Related terms
Positioner
A device on a control valve that compares the control signal with actual valve position and adjusts air to match.
Actuator
The powered device that opens, closes or positions a valve using air, hydraulic oil or an electric motor.
Cv
Flow coefficient: US gallons per minute of 60 degree F water a valve passes with a 1 psi pressure drop.
Trim
The replaceable internal parts of a valve that touch the process fluid, such as seat, disc or plug, stem and cage.
Hunting
A control valve or loop swinging back and forth around setpoint instead of settling.