Improved process control solves PID tuning issue for food processor

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A major food processor was having problems with two scraped surface heat exchangers (SSHEs) in a continuous process. PID temperature regulators were not adequately tuned, and the process temperature would drift well above the upper limit, causing the product to overcook. This meant the product had to be scrapped, but it also caused a burned residue to stick to the heat exchanger surface. That reduced heat transfer, which made the controller use even more steam to compensate. Operators would have to stop the process and clean the exchanger before trying to stabilize it again.

The process used two SSHEs arranged in series. The first heated the product to an intermediate point before sending it to the second where it reached the final temperature. Each unit has a control valve that adjusts the steam flow. This production unit was used for a group of products, and there was one that caused most of the problems. The boiling point of this product was only 4 °F above the desired outlet temperature, so there was a very narrow operational window. Any time the product reached its boiling point, it would be ruined and begin to foul the heat exchanger.

Engineers trying to tune the PID loop found that the control valves for both heat exchangers operated saturated at 100% of output almost 100% of the time. This made control of the system difficult regardless of the PID tuning. The control valve of the second SSHE was acting more like an on/off controller because of a very high proportional term. As such, the controller was not effective, and was apt to oscillate and overshoot the outlet temperature, particularly in reaction to disturbances.

ControlSoft was given the task of stabilizing the system, with tighter control capable of responding to changes in product rates and inlet temperature disturbances. Advanced process controllers (APC) met the challenge of eliminating final product temperature oscillation and providing more responsive and tighter temperature control than their existing PID controllers. After implementing the new control strategy, temperature variations were reduced by 60 to 80%.

Related resources on PID and loop tuning You can read a more detailed discussion of the problem and the solution from ControlSoft . Scroll down to the food & beverage section and request paper AN 321.

Also read, from Control Engineering : Advances in Control Loop Optimization and search PID or loop tuning atop www.controleng.com for many other related resources on process optimization.

—Edited by Peter Welander, process industries editor, PWelander@cfemedia.com , Process & Advanced Control Monthly eNewsletter Register here to select your choice of free eNewsletters .

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