This Loop Signature series of articles would not be complete without sections on the problems of noise and filtering, which are two of the most misunderstood areas in process control.
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When analysing a control loop, one of the important things that one must do is to determine the dynamics of the process.
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The integral or I term in a controller is a brilliant thing. It is an extremely elegant and simple solution for eliminating offset in control. However, like everything else in this world, virtually everything good has its bad side as well.
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I was asked to investigate a problem of a drum pressure control that was cycling badly in a distillation column in a petrochemical refinery. The control team had spent a lot of time trying to stop the cycle by playing with the controller tuning. All to no avail.
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It is vital, when optimising a control loop, to establish at the outset if the process is self-regulating or integrating, as not only do these two process types behave completely differently, but also they are tuned very differently.
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People often have little realisation as to how badly a faulty valve can affect the performance of the control of the loop. So how can one make a real valve with its problems into a perfect valve? The answer is simple. One makes use of a technique called cascade control, whereby a second controller is used to control the flow of fluid through the valve.
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I recently encountered an interesting problem in a minerals recovery processing plant. The loop in question was a gas flow control to a burner. The operators reported that the loop cycled badly in automatic, and was very difficult to control manually. The C&I technicians had tried all sorts of tunings without any improvement.
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The full PID controller and response to setpoint or load changes.
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This is a wonderful example of how the use of a cascade flow control still allowed excellent control of the primary temperature control loop to be achieved, in spite of a valve with severe problems.
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This month’s article by Michael Brown covers two examples where all the tuning in the world would not have achieved good control.
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Why would a DCS manufacturer set a default value in the system that effectively switches one of the parameters off? It is completely senseless.
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Pressure control is one of the processes that can be self-regulating or integrating, and it is sometimes very difficult to determine which type it is.
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