Pneumatic Actuators:

Pneumatic actuators as described in Figure that gives for automatic or semiautomatic valve operation.

These actuators translate an air signal within valve stem motion through air pressure acting on a diaphragm or piston linked to the stem. Pneumatic actuators are used in throttle valves for open-close positioning whereas fast action is needed. While air pressure closes the valve and spring action opens the valve, the actuator is term directacting.

While air pressure opens the valve and spring action closes the valve, an actuator is termed reverseacting. The Duplex actuators have air supplied to both sides of the diaphragm. The differential pressure across the diaphragm positions the valve stem. Automatic operation is given while the air signals are automatically controlled through circuitry. Semi-automatic operation is given through manual switches within the circuitry to the air control valves.