The following tip is from the ISA book by Greg McMillan and Hunter Vegas titled 101 Tips for a Successful Automation Career, inspired by the ISA Mentor Program. This is Tip #99, and was written by Greg.
Simulators for system acceptance testing (SAT), operator training systems (OTS), and process control improvement (PCI) should be part of a virtual plant, where a virtual version of the actual control system configuration and graphics, including historian and advanced control tools, interfaced to the simulator are running in a personal computer. The use of a virtualized rather than an emulated control system is necessary to allow the operators, process engineers, and automation engineers and technicians to use the same graphics, trends, and configuration as the actual installation. This fidelity to the actual installation is essential. The emulation of a PID block is problematic because of the numerous and powerful proprietary features, such as anti-reset windup and external-reset feedback.
The simulation used to develop, test, and quantify the benefits of process control improvements can be used as an OTS to help increase onstream time by operator buy-in and understanding particularly for advanced process control (APC). The supportive participation of specialists in the control room is critical as the inevitable questions arise as to what the APC system is doing. An APC system may appear to be taking actions that seem wrong to the operators due to human limitations in understanding complex relationships, delayed effects, and trajectories.
The virtual plant’s processes must be able to be speeded up so that simulations for slow processes can be run within a reasonable time frame (e.g., 1 hour). The speedup can be obtained by increasing kinetics, mass transfer rates, and heat transfer rates and increasing the step size used in the integration of the differential equations for the mass and energy balances. For bioreactors, the speedup requirement is extraordinary because batch cycle times are 7 to 10 days. For bioreactors, the mass transfer rates, heat transfer rates, cell growth rate, and product formation rates are speeded up by a factor of 10. Material and energy balances are speeded up by increasing the integration step size by a factor of 50. The effect is multiplicative, so the total speedup is 500 times real time and a simulated 10 day batch is completed in less than an hour. The flow ranges for gases, nutrients, and reagents must be increased by the same factor as the transfer rate and kinetic speedup. The integrating process gains are increased and the process time constants are decreased by the same factor as the material and energy balance speedup factor. If the loop deadtime is not affected by speedup, the controller gains must be decreased by this same factor to account for the change in integrating process gains or time constants
Concept: The actual displays and trend charts files used in the control room must be copied and the actual configuration downloaded into the virtual plant to provide physical fidelity. The models are speeded up faster than real time to ensure scenarios don’t take too long. The corresponding flow measurement spans and final control element capacities are increased by the same factor as the kinetics and mass transfer rates and heat transfer rates factor. The controller gains are decreased by the same factor as the integration step size factor if the deadtimes are not affected by speedup. The controller gain remains unchanged for step response models where both the deadtime and time constant are speeded up. The controller reset time that is proportional to deadtime is speeded up.
Details: Use step response and first principle models to achieve the fidelity needed (Tip #98) in a virtual plant. Use speedup factors as necessary so entire simulations take less than an hour. Increase the flow measurement span and final control element capacity by the same factor used to speed up kinetics, mass transfer rate, and heat transfer rate in first principle models. Increase the integration step sizes in these same models to speed up the process time constants and integrating process gains. Decrease the controller gains by the integration speedup factor, assuming that deadtime does not change with speedup. The controller gains are not affected by kinetics and transfer rate speedup if the flow measurement and final control elements are scaled up accordingly. The total model speedup is the kinetics and transfer rates speedup factor multiplied by the integration speedup factor. For step response models, decrease both the deadtime and the time constant so the controller gain does not change with speedup. For these step response models, decrease the reset time with the speedup factor since the reset time is proportional to the deadtime. See the Control Talk blog “The ABCs of Controller Tuning” to better understand the implications of speedup on tuning.
Watch-outs: The proper simulation of deadtime is notoriously difficult. If the step response deadtime is speeded up, module execution times and measurement sensor responses must also be speeded up unless these are factored into the speedup of the deadtime.
Exceptions: The real-time simulation of momentum balances and water hammer may not be possible because these require exceptionally small integration step sizes. It may be possible to simulate surge by slowing down the dynamics by scaling down the integration step size and implementing the model in a module with an execution time of 0.1 sec.
Insight: A virtual plant provides a level of physical fidelity by the use of the actual plant’s control system and operator interface that is crucial for all types of training and all levels of process control improvement.
Rule of Thumb: Use the actual configuration, displays, and trends that will be in the control room in a virtual plant for the training of operators, maintenance engineers, process engineers, and control engineers and the development, testing, and prototyping of process control improvements.
About the Author
Gregory K. McMillan, CAP, is a retired Senior Fellow from Solutia/Monsanto where he worked in engineering technology on process control improvement. Greg was also an affiliate professor for Washington University in Saint Louis. Greg is an ISA Fellow and received the ISA Kermit Fischer Environmental Award for pH control in 1991, the Control magazine Engineer of the Year award for the process industry in 1994, was inducted into the Control magazine Process Automation Hall of Fame in 2001, was honored by InTech magazine in 2003 as one of the most influential innovators in automation, and received the ISA Life Achievement Award in 2010. Greg is the author of numerous books on process control, including Advances in Reactor Measurement and Control and Essentials of Modern Measurements and Final Elements in the Process Industry. Greg has been the monthly "Control Talk" columnist for Control magazine since 2002. Presently, Greg is a part time modeling and control consultant in Technology for Process Simulation for Emerson Automation Solutions specializing in the use of the virtual plant for exploring new opportunities. He spends most of his time writing, teaching and leading the ISA Mentor Program he founded in 2011.
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Hunter Vegas, P.E., holds a B.S.E.E. degree from Tulane University and an M.B.A. from Wake Forest University. His job titles have included instrument engineer, production engineer, instrumentation group leader, principal automation engineer, and unit production manager. In 2001, he joined Avid Solutions, Inc., as an engineering manager and lead project engineer, where he works today. Hunter has executed nearly 2,000 instrumentation and control projects over his career, with budgets ranging from a few thousand to millions of dollars. He is proficient in field instrumentation sizing and selection, safety interlock design, electrical design, advanced control strategy, and numerous control system hardware and software platforms.
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