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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There is a commonly held belief in control circles that all PID controllers are similar and relatively simple. This is a dangerous fallacy.
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The problem with the pressure control was that it seemed to work intermittently and seldom got to setpoint, resulting in large and unacceptable variance on the control.
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There is a commonly held belief in control circles that all PID controllers are similar and relatively simple. This is a dangerous fallacy.
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The example given in this article illustrates some mistakes made by the system integrators and control engineers at a metals extraction plant that used a well-known make of PLC and scada for its controls.
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An inherent phenomenon occurring in most control valves that few people are aware of is ‘stick-slip’.
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