A process plant consultants, our services include designing robust process control system for clients. This short article provide an example of how our process control and instrumentation design overcame an engineering challenge. A mixer requires a precise temperature control in the accuracy of ± 1 degree Celsius. To achieve this process control, we designed a cascade control system as shown in the piping and instrumentation diagram (P&ID). Cascade control system consists of two or more controllers in series and have only a single, independently adjustable set point, that of the primary (master) controller. The main value of having secondary (slave) controllers is that they act as the first line of defence against process perturbations, preventing these upsets from entering and upsetting the primary process. If there were no slave controller, an upset due to a change in water temperature would not be detected until it had upset the master measurement. In this configuration, the cascade slave detects the occurrence of such upsets and immediately counteracts them so that the master measurement is not upset and the primary loop is not even aware that an upset occurred in the properties of the utilities.
In this particular project, to achieve a precise mixer temperature, the controlled process variable, mixer temperature, whose response is slow to changes in the heat transfer medium flow, manipulated variable, is allowed to adjust the set point of a secondary loop, whose response to hot water flow changes is rapid. In this scenario, the mixer temperature controller (master) varies the set point of the jacket temperature control loop (slave). The objective of the slave loop in this scenario is to correct for all outside disturbances, without allowing them to affect the mixer temperature. For example, on a basic control system, if the control valve is faulty or if the temperature or pressure of the heat transfer media changes, this would eventually upset mixer temperature. However, with a cascade control system, the slave loop would detect the resulting upset at the jacket outlet earlier and would correct it before it had a chance to upset the master. Since cascade control system will not function properly if the master loop is faster than the slave loop, the process lags were distributed in such a way that the time constant of the slave is one tenth that of the master. In this scenario, the slave controller is used to maintain the jacket outlet (and not inlet) temperature, because in this way the jacket and its dynamic response is included in the slave loop. Another advantage of this configuration is that it removes the principal nonlinearity of the system from the master loop, because mixer temperature is linear with jacket-outlet temperature. The end result is the temperature of the mixer is highly consistent as monitored in the SCADA system.