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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The objective of the derivative term is to speed up the control response in very slow processes, as often encountered in some temperature controls.
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Generally, as PLCs sometimes do not handle PIDs well, I always do some tests on them to check on the operation of their control loops.
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The problems encountered in a heater outlet temperature control in a petrochemical refinery were causing considerable difficulties for the operators. Here is another wonderful example of the power of cascade control, even with a really bad control element.
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In the real world of industrial process control, P-only control should be used only on processes that are tuned with a large P gain value.
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To my amazement I find that people seem to think that because these days we are using ‘smart’ transmitters, the basic rules can be ignored.
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