ValvesDOE-HDBK-1018/2-93VALVE ACTUATORSSpeedof PowerActuatorsPlant safety considerations dictate valve speeds for certain safety-related valves. Where a systemmust be very quickly isolated or opened, very fast valve actuation is required. Where theopening of a valve results in injection of relatively cold water to a hot system, slower openingis necessary to minimize thermal shock. Engineering design selects the actuator for safety-related valves based upon speed and power requirements and availability of energy to theactuator.In general, fastest actuation is provided by hydraulic, pneumatic, and solenoid actuators.However, solenoids are not practical for large valves because their size and power requirementswould be excessive. Also, hydraulic and pneumatic actuators require a system for providinghydraulic or pneumatic energy. The speed of actuation in either case can be set by installingappropriately sized orifices in the hydraulic or pneumatic lines. In certain cases, the valve isclosed by spring pressure, which is opposed by hydraulic or pneumatic pressure to keep thevalve open.Electrical motors provide relatively fast actuation. Actual valve speed is set by the combinationof motor speed and gear ratio. This combination can be selected to provide full valve travelwithin a range from about two seconds to several seconds.ValvePositionIndicationOperators require indication of the position of certain valves to permit knowledgeable operationof the plant. For such valves, remote valve position indication is provided in the form ofposition lights that indicate if valves are open or closed. Remote valve position indicationcircuits use a position detector that senses stem and disk position or actuator position. One typeof position detector is the mechanical limit switch, which is physically operated by valvemovement.Another type is magnetic switches or transformers that sense movement of their magnetic cores,which are physically operated by valve movement.Local valve position indication refers to some visually discernable characteristic of the valve thatindicates valve position. Rising stem valve position is indicated by the stem position. Nonrisingstem valves sometimes have small mechanical pointers that are operated by the valve actuatorsimultaneously with valve operation. Power actuated valves typically have a mechanical pointerthat provides local valve position indication. On the other hand, some valves do not have anyfeature for position indication.Rev. 0ME-04Page 49
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