Control Loop Improvement

McMillan and Weiner Talk With Jacques Smuts About Improving Control Loops

By Greg McMillan, Stan Weiner

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Jacques: Some plants have operators tuning loops. Some operators have had a class on tuning, but when I ask, they tell me the math was too much. If the Zeigler-Nichols tuning relationships are over their heads, you have problems. We do need to take advantage of their first-hand knowledge. Operators know important aspects of equipment performance, but sometimes have their own interpretations of the causes for poor control. You need to get operator observations rather than conclusions and explore the cause-and-effect relationships. In a liquid-gas separator, operators said you needed to close the outlet valve first and then open it to lower the level; otherwise the level would keep going up. I was tempted to discount this requirement, but found out under certain conditions a vortex formed. Closing the valve broke the vortex.

Stan: Is there a considerable amount of hand-holding needed?

Jacques: Patience, explanations and time are essential, particularly for systems with significant dead time. Many years ago I worked at a production plant for whiskey. The dry distiller grain solids (DDGS) is a valuable by-product for cattle feed, and was dried in a kiln with a dead time of about 9 minutes. The main disturbance came from the level control of a syrup tank that feeds into the wet grain and was manually set by the operators because level control was deemed not possible. I tuned the level controller and added feed-forward control to the kiln. Operators said it wouldn't work. I had a difficult time getting the operators to leave the moisture controller in automatic because the control was not smooth because of clumps. I sat with the operators and showed them the controller had taken all the action they would take (e.g. 40%), and there was nothing more to do than wait 9 minutes.

Greg: Looks like what the operators needed was a moisture calculation one dead time into the future on a trend chart described in Control Talk Blog "Future PV Values Are the Future."  Dead time is the most difficult thing for humans for deal with. Tuning can take the dead time into account avoiding overreaction and providing a consistent response. Does tuning ever become boring?

Jacques: After working on thousands of loops, you would think you have seen everything, but almost every plant I work at I see something new. I found a controller with a derivative time of 300 minutes. The low output limit was 75%, and the high output limit was 78%, so the loop was essentially in manual. Recently I found loops on an oil rig that were never tuned since they were commissioned in 2004. No attempt was made to rectify severely oscillating loops. They just lived with them unless they caused a shutdown.

Greg: What about configuration problems?

Jacques: Configuration can prevent a loop from ever performing properly. Configuration engineers often haven't had time to learn tuning. I had an override control system that was not working. The initialization was done in the wrong blocks. I had to wait till shutdown to have it fixed. I don't need to know how to do configuration for each of the many types of DCS, but I do need to recognize when the configuration is a problem.

Stan: Before we retired, we saw a dramatic decline in technical training. A 12-week instrument and control course for new employees was decreased to 8 weeks, 5 weeks, 2 weeks, and finally no weeks. What do you see happening with training for your customers?

Jacques: One technical training company says they cannot fill their classes in the United States. Yet the need is greater than ever. Jobs are so spread out. The question is how do you make tuning a hot topic? If I can just get into a plant and tune a problematic loop or two, the results often convince the customer of the need for a consulting/training program.

Greg: What do you recommend as the best training program?

Jacques: The best results are gained from a class covering topics in my book and containing a hands-on component. A program must include users working hands-on with the loops, because if the training is just a class, the attendees go back to work and may not try out what they have learned for weeks or ever.

Stan: What do you say about the use of derivative?

Jacques: The benefit of derivative diminishes as the total loop dead time becomes greater than the open loop time constant. For derivative to be effective, the process response must predictably be in the same direction for at least as long as the derivative time. The improvement offered by derivative is evident, but not dramatic for most processes.

Greg: For slow integrating and, especially, runaway process responses with secondary time constants (e.g., heat transfer surface lags and thermowell lags in batch and exothermic reactor temperature loops), the use of derivative becomes more important. What do you do about noise, and what do you think about notch filters?

Jacques: I use filtering only when really necessary. Actually, I am more likely to take a filter out than to add one. Notch and higher-order filters may have some advantages in instruments where the signal noise is definitive. Process noise is too unpredictable for these more sophisticated filters to be generally applicable.

Greg: What did you find out from your tests of the tuning rules from the H.A. Fertik paper "Tuning Controllers for Noisy Processes" (ISA Transactions, Vol. 14, No. 4, 1975)?

Jacques: The response of his PID tuning rules on first-order plus dead time systems was very oscillatory. It turns out the Fertik tuning rules were applicable for systems with two or more equal time constants. As the secondary time constant became closer to the primary time constant, the response became much smoother, although the return to setpoint for a load disturbance was slow and had some hesitation.

Stan: How would you sum up tuning?

Jacques: You need to understand how to adjust a PID controller's response to match the process dynamics, know your process' control objective and recognize when configuration, equipment and valve problems need to be addressed.

Top Ten Signs Loops Need to be Tuned

(10) There is no process control engineer in sight or on-site.
(9) When asked what the controller gain setting is, the answer is given in %.
(8) When asked what the controller reset time setting is, the answer is given in repeats/min.
(7) The data historian compression setting is 25%.
(6) There is more recycle than product.
(5) Valves are wearing out.
(4) Tempers are wearing thin.
(3) Operators are placing bets on what loop will cause the next shutdown.
(2) The output limits are set to keep the valve from moving.
(1) The gain, reset and rate settings are all 1.0.

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