Process Controls VALVE ACTUATORSHydraulicActuatorsPneumatic actuators are normally used to control processes requiring quick and accurate response,as they do not require a large amount of motive force. However, when a large amount of forceis required to operate a valve (for example, the main steam system valves), hydraulic actuatorsare normally used. Although hydraulic actuators come in many designs, piston types are mostcommon.A typical piston-typeFigure 37 Hydraulic Actuatorhydraulic actuator is shownin Figure 37. It consists ofa cylinder, piston, spring,hydraulic supply and returnline, and stem. The pistonslides vertically inside thecylinder and separates thec y l i n d e r i n t o t w ochambers. The upperchamber contains thespring and the lowerchamber contains hydraulicoil.The hydraulic supply andreturn line is connected tothe lower chamber andallows hydraulic fluid toflow to and from the lowerchamber of the actuator.The stem transmits themotion of the piston to avalve.Initially, with no hydraulic fluid pressure, the spring force holds the valve in the closed position.As fluid enters the lower chamber, pressure in the chamber increases. This pressure results ina force on the bottom of the piston opposite to the force caused by the spring. When thehydraulic force is greater than the spring force, the piston begins to move upward, the springcompresses, and the valve begins to open. As the hydraulic pressure increases, the valvecontinues to open. Conversely, as hydraulic oil is drained from the cylinder, the hydraulic forcebecomes less than the spring force, the piston moves downward, and the valve closes. Byregulating amount of oil supplied or drained from the actuator, the valve can be positionedbetween fully open and fully closed.Rev. 0 Page 57 IC-07
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