How does a pneumatic actuator work on a control valve?

Pneumatic actuators convert air pressure (typically 3-15 psi or 6-30 psi) into linear or rotary motion to position the valve. Diaphragm actuators use flexible diaphragm with spring return - air pressure on diaphragm moves stem against spring. Piston actuators use cylinder with air on one or both sides for higher thrust. Key terms: signal pressure (from positioner), bench set (spring range), and fail action (air-to-open or air-to-close). Pneumatic actuators are preferred for process control due to reliability, fire-safety, and availability of compressed air.