Tight packing, a dirty stem or worn guides make the valve ignore small signal changes until actuator force builds enough to break it loose, after which it overshoots. The result is stick-slip motion and a sawtooth limit cycle in the loop. Techs diagnose it with a valve signature or a small bump test in manual, and fix it with packing adjustment, live-loading, or a more powerful actuator.
Bump it two percent and nothing, bump it again and it jumps five, that's stiction, not tuning.
Also heard as
- stick-slip
- sticky valve
Related terms
Hunting
A control valve or loop swinging back and forth around setpoint instead of settling.
Packing
Compressible rings packed around a valve stem to stop process fluid leaking out to atmosphere.
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.