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Aug
17

Tips-N-Techniques (TNT) – Tuning Furnace and Incinerator Pressure Loops

If the pressure set point is a fraction of an inch of water column, you have a high integrating process gain. The response is often a high speed ramp in the control region. For a waste incinerator and a phosphorous furnace I worked on decades ago, the pressure could ramp off scale in 0.2 and 5 seconds, respectively. Trying to control the incinerator and furnace pressure was reported to be like trying to control an explosion when there was a shutdown or a slag slide, respectively, and a corresponding burst of vapors and gases. Needless to say these pressure loops could never go to manual and open loop tuning methods were down right dangerous. In the old days I used a modified ultimate oscillation method and a high speed recorder. As with runaway reactors, the reset time (e.g. sec/repeat) was increased by 100x to make reset action negligible and the controller gain was increased until there was the start of an oscillation. The reset time was set equal to the period of the damped oscillation and the controller gain was halved. A set point change was then made and if the response was more oscillatory than dictated by valve limit cycles from stick-slip or deadband, the controller gain was decreased. If damped oscillations persisted and got worse or slower, then the reset time was increased until the oscillations period and decay rate were faster. This test was repeated and the gain decreased or the reset time increased until the response was sufficiently smooth.

Before we go further, one should realize that the original ultimate oscillation method asked the user to increase the controller gain until there were equal amplitude oscillations. This was too exciting and gave controller tuning settings that were too oscillatory especially if there was an increase in the loop dead time or process gain or a decrease in the process time constant. The damped oscillations mentioned here are rapidly decaying where each succeeding peak is less than ¼ the previous peak.

The damped oscillation period is larger than the ultimate oscillation period and the damped oscillation controller gain is smaller than the ultimate oscillation gain and the factors of 1.0 for period and 0.5 for controller gain are not per the textbook definitions of the Ziegler-Nichols ultimate oscillation method. Using the text version of the closed loop (ultimate oscillation) or open loop (reaction curve) Ziegler-Nichols tuning method and thinking that tuning settings with more than one significant digit are practical, is a great way to reject the pioneering work of Ziegler and Nichols and to glorify new tuning methods. What I found early in my career is a simple change of using damped oscillations instead of ultimate oscillations and using easy to remember rounded off factors, gave me the proportional mode action needed for these loops that lack self-regulation and can be headed for a trip point. I also quickly realized that the nonlinear and non-stationary nature of chemical processes and valve stick-slip and backlash meant that the long term tuning setting accuracy of better than 50% was wishful thinking.

Today, integrated online adaptive tuning tools that look at set point changes, such as DeltaV Insight, should be able to automatically identify tuning settings of most fast integrating processes. However, some pressures can be so fast (e.g. the cited incinerator) digital delays must be eliminated and tuning tools that directly connect to the I/O, such as those used by EnTech, are needed. It is important that the module execution time, the tuning tool, and the trend chart update time not cause aliasing or an extra observed dead time. The controller, final element, and pressure sensor must also be extremely fast. Finally, it is particularly critical to test and observe new tuning settings for these and other types of loops that require aggressive feedback control.

If you want to get more details on the importance of making the loop fast enough, check out the chapter “Pressure Control: Without Deadtime I Might be Out of a Job” in the free E-book A Funny Thing Happened on the Way to the Control Room on pages 31-41: http://www.modelingandcontrol.com/FunnyThing/.