This article is a continuation of Loop Signature 30 published in the last issue in this series, exploring reasons for non-linearities which may be encountered in feedback control loops
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For this final article I have chosen to relate a problem that existed in a desuperheater temperature control on a boiler in a petrochemical refinery.
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If nonlinearity occurs it means that if one is to carry on controlling with the same response to changes in load or setpoint, then the tuning of the controller will also need to be adjusted to meet the new conditions.
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In my last article I wrote about how cascade control systems can effectively overcome valve problems. This article gives another example of how a temperature control was able to perform well, in spite of really severe valve problems.
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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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