A high-pressure steam flow control in a reboiler on a column in a petrochemical refinery continually cycled when placed in automatic. Several attempts had been made to tune the controller, but these had been unsuccessful.
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A couple of SWAG methods of tuning were given in the previous Loop Signature article. I have tuned a simple self-regulating process using those methods, and two other tuning methods, one of them being the sophisticated Protuner tuning package, which is the system I employ. The tests were performed on a very accurate and powerful simulation package, and the results are compared below.
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A boiler level control in a petrochemical refinery was causing problems, as the level was cycling badly and taking a long time to recover from sudden load changes. In addition, there were frequent high-level alarms.
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This article gives an example of an important flow control system for flare gas in a petrochemical refinery. The control was suffering from serious problems, often cycling severely, and at other times the flow process variable (PV) would suddenly move quite a lot away from the setpoint (SP).
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This is the first of several articles dealing with the subject of tuning. I have found that many people think that optimisation consists solely of tuning. I would stress once again that tuning is the last thing one should do when optimising regulatory controls.
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When tuning noisy loops, we recommend in our courses that one should eliminate the noise by editing it out, so the tuning will be done only on the true process response, free of any noise. The controller is controlling the process, and is not controlling the noise.
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Which is the weakest link in a control loop? The answer, without any doubt is that, in most cases, the final control element is the weakest link.
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Reminiscences from Michael Brown on a long and rich journey in the world of automation together with SAIMC.
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The first law of process control could be stated as saying that you cannot control if your measurement is incorrect. We all know this law. However do we all remember it in the heat of the moment?
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The widely held belief in many plants that tuning will solve all base layer control problems is completely fallacious. Bad tuning is generally not the main reason for loops to perform badly. It is important when performing optimisation that all elements in a loop are considered, in addition to the control strategy, before even thinking of tuning.
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It is generally better to live with noise rather than filtering it out, provided it does not cause the final control element to jump around excessively.
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Many process engineers do not appreciate the importance of flow loops in their unit controls. A senior process engineer once told me that flow loops need not be tuned well, as they generally have little effect on the more important and much slower loops like temperatures and pressures.
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