How Do You Handle Manual Operations in Industrial Process Control?

In the ISA Mentor Program, I am providing guidance for extremely talented individuals from countries such as Argentina, Brazil, Malaysia, Mexico, Saudi Arabia, and the USA. This question comes from Danaca Jordan.

Danaca Jordan’s Question

How should the need for manual operation be handled?

Greg McMillan’s Answer

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 article in Control magazine, The Power of External-Reset Feedback. Please refer to the ISA blog PID Controller Tuning Rules and click this link to download the accompanying PID Tuning appendices for information on how to get the most out of your PID controllers.