ValvesDOE-HDBK-1018/2-93VALVE ACTUATORSPneumaticActuatorsFigure 34 Pneumatic ActuatorPneumatic actuators asillustrated in Figure 34 providefor automatic or semi-automatic valve operation.These actuators translate an airsignal into valve stem motionby air pressure acting on adiaphragm or piston connectedto the stem. Pneumaticactuators are used in throttlevalves for open-closepositioning where fast action isrequired. When air pressurecloses the valve and springaction opens the valve, theactuator is termed direct-acting. When air pressureopens the valve and springaction closes the valve, theactuator is termed reverse-acting. Duplex actuators haveair supplied to both sides ofthe diaphragm. Thedifferential pressure across thediaphragm positions the valvestem. Automatic operation isprovided when the air signalsare automatically controlled bycircuitry. Semi-automaticoperation is provided bymanual switches in thecircuitry to the air controlvalves.HydraulicActuatorsHydraulic actuators provide for semi-automatic or automatic positioning of the valve, similar tothe pneumatic actuators. These actuators use a piston to convert a signal pressure into valvestem motion. Hydraulic fluid is fed to either side of the piston while the other side is drainedor bled. Water or oil is used as the hydraulic fluid. Solenoid valves are typically used forautomatic control of the hydraulic fluid to direct either opening or closing of the valve. Manualvalves can also be used for controlling the hydraulic fluid; thus providing semi-automaticoperation.Rev. 0ME-04Page 47
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