Actuation & Instrumentation
Hunting (Valve Oscillation)
Hunting is the sustained cyclic motion of a valve — typically a few seconds to a minute in period — in which the valve chases the set point and repeatedly overshoots in both directions. It is one of the most common field complaints, and it usually has a mechanical, not a controller, cause.
Mechanical causes: excessive dead band (the controller integrates while the valve sits still, then the valve jumps), stiction, a positioner gain too high for the actuator volume, an undersized positioner or booster combination, elastic linkages, or compressible air trapped where the design assumed liquid.
Process-side causes: the loop gain is too high where the valve currently operates — often because an equal-percentage valve was specified where the installed curve is steeper than linear, or because the process itself has variable gain.
Diagnosis order: check positioner tuning → measure dead band (ramp test) → check actuator/booster sizing → only then touch controller tuning. Treating mechanical hunting with retuning hides the problem until the next load change.