Actuation & Instrumentation
Piston Actuator
A piston actuator replaces the diaphragm with a sealed piston sliding in a cylinder, fed by plant air (often 4–7 bar, sometimes much higher) on one or both sides. Because the full supply pressure acts on the full piston area, it delivers several times the thrust of a same-size diaphragm actuator — the choice for large globe valves, high differential pressures, and tight shutoff requiring high seat load.
Configurations: single-acting with spring (inherent fail-safe), double-acting (both sides pressurized — more thrust per size, but fail action must be engineered with trip tanks, accumulators or springs). Stroke lengths are longer than diaphragm units, useful for long-travel trim.
Trade-offs vs diaphragm: more friction (piston seals), so positioner quality matters more; higher air-quality sensitivity; cost. Maintenance focuses on seal rings and cylinder bore condition — scoring a cylinder converts a crisp actuator into a hunting one.