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Jan
01

Top Ten Broken New Years Resolutions

(10) Stop talking about process feedback. I promise to do this right after this blog.

(9) Listen intently to my wife’s instructions. Why does my mind still jump to weighty matters like what is next for dinner?

(8) Stop making cheap control valve jokes. Could the next final element reputation I hurt be my own?

(7) Help make smart diagnostics smarter. Do I need to de-fussify my fuzzy logic?

(6) Stop lusting in my heart for more computing power. Is it the PC or me that is the constraint?

(5) Turndown the volume on my headphones. What did you say?

(4) Stop drinking cheap wine. Does good wine ever come in a size large enough?

(3) Read a college text on control theory. Can I watch Star Trek without setting up the state space equations?

(2) Stop answering a question with a question. Why should a consultant do this?

(1) Spend more time with my wife than with Control magazine. Whatever happened to my January issue?

Last Chance to Learn About Process Feedback

Processes can have negative feed back, no feedback, or positive feedback. A negative feedback process is a self-regulating process and will decelerate to a new steady state. A process with no feedback is an integrating process and will continually ramp. A process with positive feedback is a runaway process and will accelerate until hitting a relief device or safety interlock setting. As in circuits, positive feedback is problematic. Control systems use negative feedback for regulation and provide a correction to the process in the opposite direction of the excursion from set point to compensate for load changes. The control system has to be progressively more aggressive for integrating and ultimately positive feedback processes. The following file shows the response to of these three types of processes to a change in controller output with the controller in manual (open loop responses of self-regulating, integrating, and runaway processes).

Types of Process Feedback

Over 90% of the processes are self-regulating. However, many of the continuous and fed-batch processes involving large back mixed volumes with the greatest direct economic benefits behave in the time frame and control region of interest like they have an integrating response and can be best treated as “near integrating” processes. The classic integrating process is a pure batch or level process. Less than 1% of the processes are runaway. When these exist, understanding the runaway response is critical in terms of safety and control because of the propensity to accelerate and reach a point of no return. Runaway responses are almost exclusively associated with highly exothermic batch reactors used in plastics and specialty chemical production. I was in a control room when a batch reactor reached the point of no return and was going to blow over to a flare stack tank. There was nothing the operators could do except call for the evacuation of the reactor area and make sure the flare stack was ready.