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Symptom:
Do you
observe that controllers are often retuned? This controller retuning usually
occurs after a change in the process such as a flow demand change.
Probable cause:
Process
non-linearity is a fairly normal situation. (See
Loop Tip #10) This is also shown in the process curve below. The
changing process gain will require controller retuning to keep the same loop
performance which can require a significant time commitment.
Corrective action:
If adaptive gain
techniques cannot be utilized, the selection of the control valve
characteristic can have a significant effect on the closed loop controller
tuning. The control valve characteristic is used to compensate for the
process gain to achieve an approximate installed linear process. A linear
process has constant gain.
Notice that the high
gain of the process (steep slope of the curve) is combined with the low gain
of the equal percentage control valve. This combination of the high and low
gains of each portion of the loop is trying to achieve a linear curve
meaning a truly constant gain process. In reality, it is difficult to have a
perfectly matched valve to the process, but at least the right valve
characteristic can help to approximate a linear constant gain process.
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