How do you handle manual operations?

This guest post is authored by Greg McMillan.

In the ISA Automation Week Mentor Program, I am providing guidance for extremely talented individuals from Argentina, Brazil, Malaysia, Mexico, Saudi Arabia, and the USA. We will be sharing a question and the answers each week. If you would like to provide additional answers, please send them to Susan Colwell at ISA. The twelfth question in the ISA Mentor program is from Danaca Jordan (USA):

“How should the need for manual operation be handled?”

If at all possible, manual actions should be eliminated to remove the discontinuity, inconsistency, and delay inherent in any human correction. The fastest and most abrupt and unpredictable disturbance is an operator action. The best of the operator actions can be automated and from increased pattern recognition from a consistent response be continuously improved. The magnitude of the opportunity was exemplified in the Control Talk column “Show Me the Money – Part 1.” Operator manual activity is most intense during startup, transitions, and abnormal operations. The more an operator says a manual action cannot be automated the greater the opportunity as discussed in my modeling and control blog from yesteryear “Exceptional Opportunities in Process Control – Startup and Abnormal Operation.”

Given that there is a manual action required, consider slowing down transitions in manual operations by the use of velocity limits on the setpoints of PID and analog output (AO) blocks. Be sure to use the PID dynamic reset limit option in any controller trying to manipulate a velocity limited valve or flow setpoint to prevent the burst of oscillations when a PID output tries to change faster than the final control element or secondary loop can respond. For  an example of the importance of the dynamic reset limit see Shinskey’s May 2006 article in ControlThe Power of External-Reset Feedback.”

Refer to the InTech article PID tuning rules and its accompanying appendices for information on how to get the most out of your PID controllers.


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