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May
13

Review of Deminar #3 – PID Control of Slow Valves and Secondary Loops (How to Eliminate Bursts of Oscillations with the “Dynamic Reset Limit” PID option)

PID Control of Slow Valves and Secondary Loops Greg McMillan Deminar Series

You can click on the above to view and hear the recording of the Deminar. In Deminar #3 we explored the confusing situations that can develop for slow control valves and slow secondary loops. The loop can look fine but suddenly burst into oscillations and later go back to smooth sailing. The normal thought is “what changed in the process or the loop?” Well it turns out that nothing changed except the size of the upset or setpoint change. For large errors, the primary controller output starts changing faster than secondary loop or valve can respond. You could slow down the primary loop, but we know this correspondingly reduces control system performance as clearly quantified on slide 1 of Effects-of-Loop-Tuning-and-Dynamics-on-KPI.pdf. The best solutions of course are to make the valve faster and make the secondary measurement and tuning faster, but the “Quick Fix” that also offers long term protection is to enable the “Dynamic Reset Limit” option in the PID. Even if there is not a problem now, just simply turning on this option to protect against unforeseen deterioration in measurements or valves. For example, someone might try to make a secondary flow or pressure loop look smoother by the overzealous addition of a signal filter or transmitter damping setting in the middle of the night. Even more dramatically the time lag of an electrode in a secondary static mixer pH loop might go from 3 seconds to 300 seconds due to coatings or high temperatures. The Control magazine article “The Power of External-Reset Feedback” offers an excellent explanation of power of the “Dynamic Reset Limit” option which uses the PV of secondary loop as the external reset signal in the positive feedback implementation of the integral mode.

In the future, the Deminars will take only 45 minutes to reduce time commitments and audio and video recording file size. The current range of 8 to 10 demos per Deminar will be reduced to 4 to 5 demos per Deminar. Also, a “Quick Fix” will be discussed near the beginning for viewers who are short on time. The start time in June will be moved up from 1:00 pm to 10:00 am CDT to encourage European participation. I have added subtitles noting the process control improvement (PCI) in parentheses to the Deminars. Also, note that I have changed the topics for Deminars 6 through 8.

In particular, checkout the next Deminar on Thursday May 27 that shows how you can dynamically explore your own case histories and scenarios by you using the free online process control labs.

To attend the event, go to http://bit.ly/JC-LiveMeeting

Use the information below to connect (if you’re not using the available computer audio):

• Toll-free: +1 (877) 771-7176

• Toll: +1 (225) 383-1099

• Participant code: 264679

(1) PID Control of Sampled Measurements (How to Eliminate Oscillations from Analyzers and Wireless Measurements with a PID Enhancement) – April 7, Wed 1:00 pm CDT

(2) PID Control of Valve Sticktion and Backlash (How to Eliminate Continual Oscillations with the “Integral Deadband” PID option) – April 21, Wed 1:00 pm CDT

(3) PID Control of Slow Valves and Secondary Loops (How to Eliminate Bursts of Oscillations with the “Dynamic Reset Limit” PID option) – May 12, Wed 1:00 pm CDT

(4) Web Lab Access and Use Instructions (How to Use Free Online Process Control Labs for Fun and Profit and Become Famous by Friday or at Least Saturday) – May 27, Thurs* 1:00 pm CDT (* – Thursday date is to avoid conflict with the World Batch Forum)

(5) PID Tuning for Self-Regulating Processes (How to Compensate for Nonlinearities in Flow and Liquid Pressure Loops) – June 9, Wed 10:00 am CDT

(6) PID Tuning for Near-Integrating Processes (How to Reduce the Tuning Time for Column and Vessel Temperature and Pressure Loops by 90%) – June 23, 10:00 am CDT

(7) PID Control of True Integrating Processes (How to Reduce the Batch Cycle Time for Temperature and pH Loops by 25%) – July 14, 10:00 am CDT

(8) PID Control of Runaway Processes (How to Improve the Performance of Exothermic Reactor Temperature Loops) – July 21, Wed 10:00 am CDT