Actuation & Instrumentation
Diaphragm Actuator
The diaphragm actuator is the classic pairing for throttling globe valves: a fabric-reinforced rubber diaphragm clamped between two casings, loaded on one side by instrument air and opposed by a range spring. Pressure moves the stem; the spring returns it. Simple, few moving parts, inherently fail-safe via the spring — reasons it has dominated for decades.
Strengths: very low friction (the diaphragm is nearly frictionless against the case), no leak path to atmosphere other than the packing, low cost, easy bench setting, and smooth response that suits positioner-driven throttling.
Limits: the diaphragm pressure is limited (~2–4 bar typical), so thrust is limited — large or high-ΔP valves move to piston actuators. The effective diaphragm area shrinks slightly with travel, and reverse-acting versions (pressure pushes up) re-arrange the same hardware.
Field checks: diaphragm ruptures show as a valve that fails to one end of travel and passes air continuously; stiffness changes show up as a shifted bench set.