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Stiction

Static friction that stops a valve stem moving until the signal change is large enough to break it free.

Valve stiction producing a limit cycle Trend showing valve stiction: the controller output ramps smoothly while the process variable moves in abrupt steps, producing a limit cycle that does not decay. high low PV SP Output PIDSnap time
Trend showing valve stiction: the controller output ramps smoothly while the process variable moves in abrupt steps, producing a limit cycle that does not decay. A sawtooth or square-cornered cycle on the process variable against a smooth, slowly ramping output. The cycle amplitude stays constant instead of decaying. Put the loop in manual and the cycle stops — that single test separates stiction from every tuning problem.

What it means

Stiction is static friction in the valve — usually in the packing. The stem resists movement until the actuator force exceeds the friction, then breaks free and slips past the requested position.

In a closed loop this produces a self-sustaining cycle. The controller winds up against a stem that will not move, the stem breaks free and overshoots, the controller corrects, and the cycle repeats at constant amplitude. Because the stiction band is the same size on every cycle, the oscillation does not decay — which is precisely what distinguishes it from a tuning problem.

Stroking a valve through its full travel will not reveal stiction. Large signal changes overcome it easily; it bites on the fractions of a percent a controller actually moves around a steady operating point.

Valve stiction producing a limit cycle Trend showing valve stiction: the controller output ramps smoothly while the process variable moves in abrupt steps, producing a limit cycle that does not decay. high low PV SP Output PIDSnap time
Trend showing valve stiction: the controller output ramps smoothly while the process variable moves in abrupt steps, producing a limit cycle that does not decay. A sawtooth or square-cornered cycle on the process variable against a smooth, slowly ramping output. The cycle amplitude stays constant instead of decaying. Put the loop in manual and the cycle stops — that single test separates stiction from every tuning problem.

Also called

On a plant you will hear this called static friction, sticking valve. They mean the same thing.

Related

Glossary

Where this comes up

Last reviewed 2026-09-18.

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