Process Controls VALVE ACTUATORSThe bottom chamber contains a spring that forces the diaphragm against mechanical stops in theupper chamber. Finally, a local indicator is connected to the stem to indicate the position of thevalve.The position of the valve is controlled by varying supply air pressure in the upper chamber. Thisresults in a varying force on the top of the diaphragm. Initially, with no supply air, the springforces the diaphragm upward against the mechanical stops and holds the valve fully open. Assupply air pressure is increased from zero, its force on top of the diaphragm begins to overcomethe opposing force of the spring. This causes the diaphragm to move downward and the controlvalve to close. With increasing supply air pressure, the diaphragm will continue to movedownward and compress the spring until the control valve is fully closed. Conversely, if supplyair pressure is decreased, the spring will begin to force the diaphragm upward and open thecontrol valve. Additionally, if supply pressure is held constant at some value between zero andmaximum, the valve will position at an intermediate position. Therefore, the valve can bepositioned anywhere between fully open and fully closed in response to changes in supply airpressure.A positioner is a device that regulates the supply air pressure to a pneumatic actuator. It doesthis by comparing the actuator’s demanded position with the control valve’s actual position. Thedemanded position is transmitted by a pneumatic or electrical control signal from a controller tothe positioner. The pneumatic actuator in Figure 35 is shown in Figure 36 with a controller andpositioner added.The controller generates an output signal that represents the demanded position. This signal issent to the positioner. Externally, the positioner consists of an input connection for the controlsignal, a supply air input connection, a supply air output connection, a supply air vent connection,and a feedback linkage. Internally, it contains an intricate network of electrical transducers, airlines, valves, linkages, and necessary adjustments. Other positioners may also provide controlsfor local valve positioning and gauges to indicate supply air pressure and control air pressure (forpneumatic controllers). From an operator’s viewpoint, a description of complex internal workingsof a positioner is not needed. Therefore, this discussion will be limited to inputs to and outputsfrom the positioner.In Figure 36, the controller responds to a deviation of a controlled variable from setpoint andvaries the control output signal accordingly to correct the deviation. The control output signalis sent to the positioner, which responds by increasing or decreasing the supply air to theactuator. Positioning of the actuator and control valve is fed back to the positioner through thefeedback linkage. When the valve has reached the position demanded by the controller, thepositioner stops the change in supply air pressure and holds the valve at the new position. This,in turn, corrects the controlled variable’s deviation from setpoint.Rev. 0 Page 55 IC-07
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