Control Notes

Before I do step-testing to analyze and tune a control loop, I always take a look at the current tuning settings in the controller.

• The controller’s gain setting gives me some indication of the sensitivity of the process, e.g. if the controller gain is 0.1 the process could be very sensitive to controller output changes.
• The controller’s integral time gives me an idea of the speed of the process dynamics, i.e. a short integral time usually means fast process dynamics and vice versa.
• The derivative time (if used) can reveal if the last person tuning the loop lacked understanding of the tuning process, e.g. if the derivative time is set to more than half the integral time, or less than one-eighth of it.

Earlier this month I optimized control loops on an oil platform. A few of the loops were oscillating. One of the oscillating loops, a gas pressure control loop on a separator, had a controller gain of 16! I facetiously told the control engineer: “Well, there’s your problem!” A value of 16 did seem like an abnormally large controller gain, but I know there are many exceptions from “normal” in process control.

A closer look revealed the reason for the high controller gain. Even though the set point was set to the normal operating pressure of 200 PSI, the pressure transmitter was calibrated to measure between 0 and 4000 PSI. So the operating pressure was at only 5% of the measurement range! A more appropriate measurement range would have been 0 – 400 PSI, since the maximum design pressure for the vessel was 380 PSI. In this case, the calibration range was ten times larger than it should have been. Considering that the measurement span was ten times over-ranged, the controller gain had to be ten times larger than “normal” to compensate. This means the effective gain of the controller was only 1.6, which is a reasonable value, especially for gas pressure control. In other words, the high controller gain was not responsible for causing the control loop instability. It turned out that the control loop was oscillating because of control valve stiction.

Based on these findings I recommended a replacement / recalibration of the pressure transmitter and the subsequent reranging of the signals in the DCS. After doing this, the controller gain must be set to 1.6. I also recommended that the sticky control valve be repaired or replaced to fix the oscillations.

The high controller gain cancelled out by the large measurement span reminded me of the warning on a passenger-side rear-view mirror: “Objects in the mirror are closer than they appear.”

Objects in the mirror are closer than they appear.

Learn more about controller settings from the book Process Control for Practitioners.
Try it out for yourself using the OptiControls Loop Simulator.

Stay Tuned!

Jacques Smuts
Founder and Principal Consultant
OptiControls Inc.