VALVE ACTUATORS Process ControlsFor example, as the control signal increases, a valve inside the positioner admits more supply airFigure 36 Pneumatic Actuator with Controller and Positionerto the actuator. As a result, the control valve moves downward. The linkage transmits the valveposition information back to the positioner. This forms a small internal feedback loop for theactuator. When the valve reaches the position that correlates to the control signal, the linkagestops supply air flow to the actuator. This causes the actuator to stop. On the other hand, if thecontrol signal decreases, another valve inside the positioner opens and allows the supply airpressure to decrease by venting the supply air. This causes the valve to move upward and open.When the valve has opened to the proper position, the positioner stops venting air from theactuator and stops movement of the control valve.An important safety feature is provided by the spring in an actuator. It can be designed toposition a control valve in a safe position if a loss of supply air occurs. On a loss of supply air,the actuator in Figure 36 will fail open. This type of arrangement is referred to as "air-to-close,spring-to-open" or simply "fail-open." Some valves fail in the closed position. This type ofactuator is referred to as "air-to-open, spring-to-close" or "fail-closed." This "fail-safe" conceptis an important consideration in nuclear facility design.IC-07 Page 56 Rev. 0
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