Process Controls CONTROL LOOP DIAGRAMSThe actuating signal passes through the two control elements: the temperature controller and thetemperature control valve. The temperature control valve responds by adjusting the manipulatedvariable (the cooling water flow rate). The lube oil temperature changes in response to thedifferent water flow rate, and the control loop is complete.ProcessTimeLagsIn the last example, the control of the lube oil temperature may initially seem easy. Apparently,the operator need only measure the lube oil temperature, compare the actual temperature to thedesired (setpoint), compute the amount of error (if any), and adjust the temperature control valveto correct the error accordingly. However, processes have the characteristic of delaying andretarding changes in the values of the process variables. This characteristic greatly increases thedifficulty of control.Process time lags is the general term that describes these process delays and retardations.Process time lags are caused by three properties of the process. They are:capacitance,resistance, and transportation time.Capacitanceis the ability of a process to store energy. In Figure 9, for example, the walls ofthe tubes in the lube oil cooler, the cooling water, and the lube oil can store heat energy. Thisenergy-storing property gives the ability to retard change. If the cooling water flow rate isincreased, it will take a period of time for more energy to be removed from the lube oil to reduceits temperature.Resistanceis that part of the process that opposes the transfer of energy between capacities. InFigure 9, the walls of the lube oil cooler oppose the transfer of heat from the lube oil inside thetubes to the cooling water outside the tubes.Transportation time is time required to carry a change in a process variable from one point toanother in the process. If the temperature of the lube oil (Figure 9) is lowered by increasing thecooling water flow rate, some time will elapse before the lube oil travels from the lube oil coolerto the temperature transmitter. If the transmitter is moved farther from the lube oil cooler, thetransportation time will increase. This time lag is not just a slowing down or retardation of achange; it is an actual time delay during which no change occurs.StabilityofAutomaticControlSystemsAll control modes previously described can return a process variable to a steady value followinga disturbance. This characteristic is called "stability."Stability is the ability of a control loop to return a controlled variable to a steady, non-cyclicvalue, following a disturbance.Rev. 0 Page 13 IC-07
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