Actuation & Instrumentation
Stiction
Stiction (static friction) is the extra force needed to start a valve moving from rest, over and above the force needed to keep it moving. A valve with stiction sits still while the controller output creeps, then breaks free and jumps past the target — the "stick-slip" motion that shows up as jagged, staircase-like position traces and limit cycles in the loop.
Typical origins: packing gland load (over-tight or graphite packing that has dried or been thermally cycled), plug-to-seat adhesion (process deposits, cold welding of metal seats under high seat load), guide galling, and thermal binding of the stem in the guide bushings. Stiction often worsens after a shutdown — thermal soak and deposit hardening raise it until the valve is re-worked.
Detection: smart positioners trend the valve signature (command vs position plots); the classic manual test is a slow ramp with a dial gauge, watching for jump motion. Mitigation: correct packing load (live loading helps), lubrication where service permits, guide material upgrades, and — in dirty services — periodic stroking programs that keep the valve exercised.