Ask Greg McMillan

Greg's Response:

Abnormal operating conditions not only pose process safety and performance issues but also potentially cause equipment damage and environmental violations. Often operator actions are not timely and not sufficient, possibly even wrong making the problem worse. State-based control developed and tested using a Digital Twin with dynamic first principle models can find and implement the best solution and provide training of operators, engineers, and technicians enabling continuous improvement. 

Classic cases are state-based control to prevent compressor surge and Resource Conservation and Recovery Act (RCRA) pH violations. Compressor flow can completely reverse direction in a few hundredths of a second and cause fast (<1 sec period) nearly full scale forward and reverse flow oscillations. PID controllers can not deal with these oscillations and will not be able to stop the surge cycles. Fortunately, the pressure versus suction flow characteristic curve to the left of the surge curve including negative flow normally not seen in the literature or provided by the compressor manufacturer, can be modeled based on equations and figures in my free book Centrifugal and Axial Compressor Control and implemented in a Digital Twin dynamic simulation. Compressor surge can potentially damage rotors in axial compressors and seals in all compressors and cause downstream shutdowns. Future-value blocks can predict a surge event before the operating point crosses the surge curve enabling state-based control to put the PID surge controller in remote output or output tracking (e.g., open-loop backup) with an output large enough to prevent surge. The optimum point of return to closed loop control can also be predicted to bumplessly put the PID surge controller back in the automatic mode. 

Similarly, the Digital Twin simulation using a charge balance with carbon dioxide added to match the lab titration curve can use state-based control and a future-value block to predict a possible RCRA pH violation and trigger an open-loop backup and bumpless return to automatic mode of pH PID effectively addressing the pH disturbance as detailed in my 2024 ISA book Advanced pH Measurement and Control Fourth Edition. The Digital Twin can also update the surge curve and titration curves based on observed changes in the controlled and manipulated variables.