Figure 17 Combined Controller and Final Control Element Action

Process Controls PROPORTIONAL CONTROL SYSTEMS Steam is admitted to the heat exchanger to raise the temperature of the cold water supply. The temperature detector monitors the hot water outlet and produces a 3 to 15 psi output signal that represents a controlled variable range of 100^o to 300^oF. The controller compares the measured variable signal with the setpoint and sends a 3 to 15 psi output to the final control element, which is a 3-in control valve. The controller has been set for a proportional band of 50%. Therefore, a 50% change in the 200^oF span, or a change of 100^oF, causes a 100% controller output change. The proportional controller is reverse-acting so that the control valve throttles down to reduce steam flow as the hot water outlet temperature increases; the control valve will open further to increase steam flow as the water temperature decreases. The combined action of the controller and control valve for different changes in the measured variable is shown in Figure 17. Initially, the measured variable value is equal to 100^oF. The controller has been set so that this Figure 17 Combined Controller and Final Control Element Action value of measured variable corresponds to a 100% output, or 15 psi, which in turn, corresponds to a "full open" control valve position. At time t₁, the measured variable increases by 100^oF, or 50%, of the measured variable span. This 50% controller input change causes a 100% controller output change due to the controller’s proportional band of 50%. The direction of the controller output change is decreasing because the controller is reverse-acting. The 100% decrease corresponds to a decrease in output for 15 psi to 3 psi, which causes the control valve to go from fully open to fully shut. Rev. 0 Page 25 IC-07