Process Controls PROPORTIONAL CONTROL SYSTEMSthat the actual flow rates cannot be predicted. Therefore, the system is designed to control tanklevel within a narrow band in order to minimize the chance of a large demand disturbancecausing overflow or runout. A fulcrum and lever assembly is used as the proportional controller.A float chamber is the level measuring element, and a 4-in stroke valve is the final controlelement. The fulcrum point is set such that a level change of 4-in causes a full 4-in stroke ofthe valve. Therefore, a 100% change in the controller output equals 4-in.The proportional band is the input band over which the controller provides a proportional outputand is defined as follows:Proportional band% change in input% change in outputx 100%For this example, the fulcrum point is such that a full 4-in change in float height causes a full4-in stroke of the valve.P.B.100% change in input100% change in outputx 100%Therefore:P.B. = 100%The controller has a proportional band of 100%, which means the input must change 100% tocause a 100% change in the output of the controller.If the fulcrum setting was changed so that a level change of 2 in, or 50% of the input, causes thefull 3-in stroke, or 100% of the output, the proportional band would become 50%. Theproportional band of a proportional controller is important because it determines the range ofoutputs for given inputs.ExampleofaProportionalProcessControlSystemFigure 16 illustrates a process system using a proportional temperature controller for providinghot water.Rev. 0 Page 23 IC-07
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