A large petrochemical refinery asked me to perform an audit on several critical base layer control loops. This article deals with a problem found on a valve controlling the flow of fuel to a heat exchanger.
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I was giving a course at a remote mine in the middle of the Namibian desert. We were discussing tuning responses, and as I always do on my courses, I mentioned that in my opinion ¼ amplitude damped tuning is not desirable, and is in fact not good.
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It is very important that reboiler temperature controls operate well in petrochemical refineries, or the product quality can really suffer. I was asked to check such a control in a refinery where they were having problems with one of these controls.
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The operators in a petrochemical refinery were having great trouble in trying to stabilise the condensate level in a vessel, and this was adversely affecting other loops downstream. Several unsuccessful attempts had been made to retune the controller.
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The purpose of this particular article is to try and give those unfortunate enough to have to use SWAG (scientific wild ass guess) tuning a bit of an idea of how to go about it, and even more importantly some understanding of a couple of basic principles.
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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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