Basics of Control Valve Positioners

A valve positioner is a device used to increase or decrease the air load pressure driving the actuator of a control valve until the valve’s stem reaches a position balanced to the output signal from the process variable instrument controller.

Positioners are generally mounted on the side-yoke or top casing of the pneumatic actuator for linear-sliding-stem control valves and at or near the end-of-shaft for rotary control valves. For either basic design type, the valve positioner is connected mechanically to the valve stem or valve shaft so that their position can be compared with the position dictated by the controller. This mechanical feedback linkage work in such a way that the process controller tells the positioner to “change” position; the feedback linkage reports back to the positioner confirming that a change in the position of the valve stem has occurred and gives a sense of the magnitude of the change in position. Note that for continuous control over the entire stroke of the valve, the use of positioners is usually necessary if the valve position is

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