A feedback arm or a non-contact sensor reads stem travel, and the positioner adds or vents actuator air until the valve sits exactly where the 4 to 20 mA or digital signal asks. It overcomes packing friction and process forces that would otherwise leave the valve off position. Digital positioners also run calibration routines and diagnostics such as valve signatures and partial stroke tests, while a pneumatic positioner takes its signal from a separate I/P transducer.
Positioner's out of cal, controller says 50 percent and the travel indicator is at 42, run an auto-cal on it.
Before blaming the positioner, check the supply air regulator; a starved positioner looks exactly like a broken one.
Also heard as
- valve positioner
- digital valve controller
- DVC
Related terms
Actuator
The powered device that opens, closes or positions a valve using air, hydraulic oil or an electric motor.
Control valve
A power-operated valve that modulates flow on a controller signal to hold pressure, level, flow or temperature at setpoint.
Stiction
Static friction that makes a control valve stick, then jump, instead of moving smoothly with its signal.
Partial stroke test
Moving an emergency shutdown valve a small part of its travel while online, to prove it is not stuck.
Bench set
The air pressure range over which a spring-and-diaphragm actuator strokes the valve with no process pressure acting.