Reflections of a Process Control Practitioner
Overview Gain-Anchored Tuning (GAT) is a PID tuning method for integrating processes, developed by Jacques Smuts of OptiControls. It is particularly suited for level control in large tanks, vessels, and reservoirs with slow process dynamics. Traditional tuning methods calculate controller gain based on process dynamics and often produce excessively high gain values for slow-responding processes, […
Background A few weeks ago, a refinery got me out to tune the control loops on their thermal oxidizer. A thermal oxidizer (TOX) is a specialized combustion system that uses heat to destroy volatile organic compounds (VOCs) and hazardous air pollutants so that they aren’t released into the atmosphere. This particular TOX had two distinct [ ]
Controller gains of properly tuned loops are typically within the range of 0.1 to 10. During my process control training workshops, I advise participants to be careful when applying controller gains greater than 10 – even when a tuning rule recommends it. The reason why I recommend this is that changes in the error signal [ ]
A customer of mine owns a combined-cycle power plant that runs well most of the time, but when they start up or shut down the power augmentation system, the latter oscillates wildly. They have already tried to retune the relevant controllers, but with no success. Power augmentation is done by injecting water or steam into [ ]
Weird control strategies are sometimes used in lieu of proper tuning to address process interactions. A customer contacted me with a question about such a control strategy. She explained that a blower was being used for maintaining (negative) air pressure in a common clean-air header, and to deliver air flow to process equipment. The blower [ ]
Once in a while you’ll have to tune a control loop on a process that takes seemingly forever to settle out at its final value. This specifically applies to self-regulating processes where you traditionally have to wait for the process response to fully complete before you can analyze the data for tuning (see: Cohen-Coon Tuning [ ]
This discussion is as old as controller tuning itself: should one tune a controller to respond well to setpoint changes or to do well at rejecting disturbances. Different tuning rules have been developed for the two cases, and much research has been done to find the silver bullet of a tuning rule that would do [ ]
What is a Filter? In a modern control system, a filter is an algorithm (or function block) used mainly for the reduction of noise on a process measurement signal (Figure 1). But that is not its only use as we will see later. Figure 1. Noise filter. Types of Filters Control systems generally provide first-order [ ]
Process design flaws often lead to poor control performance. Then all eyes are on the control engineer to make the system work. Case in point is one level control loop at a chemical company that contracted me to improve the control performance of one of their process units. This particular level control loop was always [ ]
For a long time I suspected that control valves take longer to respond to small changes in controller output than to large ones. I have now confirmed this by doing tests on several control valves and analyzing the results. Most of my control loop optimization work has been on live running processes, so I never [ ]