The Control Talk Blog provides guidance from a user's viewpoint on the design of automation systems, equipment, and piping for process control improvement. Details are offered on the selection and installation of PID controllers, control valves, variable speed drives, and measurements to maximize loop performance. The blogs are often more intensive and extensive and less vendor specific than a white paper. The goal is an advancement of the profession by sharing conceptual principle based knowledge.
Greg McMillan is a retired Senior Fellow from Solutia/Monsanto and an ISA Fellow. At present, McMillan 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. He received the ISA Kermit Fischer Environmental Award for pH control in 1991, received 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.
The value of a key PID feature is increasingly becoming apparent. I got a preview of the importance when I found the 1980s vintage DCS required a fix for override control that was inherently addressed in the next generation of DCS by this feature.
The questions submitted by the 10 worldwide participants in ISA Mentor program are refreshing and cause one to stand back and think about how we do things and how can we convey knowledge we take for granted.
We conclude this series with a look at control schemes that can increase process efficiency for distillation control, compressor control, and neutralizer control. These control schemes can inherently reduce process variability and controller tuning difficulty and for compressors increase onstream time.
What configuration changes to control strategies can readily take advantage of improvements in measurements and final control elements? Control strategies can inherently improve production rate by maximizing feed and efficiency by enforcing material and energy balances.
Process efficiency can be increased by eliminating the excess use of reactants, reagents, and energy, eliminating the production of waste and off-spec material, and taking advantage of low energy and raw material sources. Not well recognized is effect of sensor drift and location, missing measurements, abnormal operation, process understanding, control...
When we saw an increase in production rate needed during opportunity assessments at Monsanto and Solutia, we would salivate. Impressive results could be obtained in a matter of a few days by a simple change in setpoint, tuning, and/or configuration.
The best wireless settings provide an effective compromise between the need to maximize battery life and meet loop performance objectives. Since power consumption is greatest in transmitting an update, battery life can be significantly extended by reducing unnecessary updates.
Temperature loops benefit more than most loops from the use of derivative action. The rate time can be 1/2 of the total loop deadtime or larger by the use of a wireless smart transmitter, the enhanced PID for wireless, and a proper installation location and thermowell design.
A transmitter damping setting or signal filter setting that is too slow can not only degrade performance but also cause trips. Less understood is that a large filter setting can mislead operations into thinking the filter has helped rather than hurt the process.
An execution time that is too slow can degrade control loop performance. An execution time too fast can unnecessarily increase the loading in the distributed control system (DCS) or programmable logic controller (PLC). Less recognized is an excessively fast module execution time can cause the module to run out of...