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Symptom:
Do you
ever observe that a loop behaves in a different manner at different
times? It may be stable one day and unstable the next. Or, the PV can be on
setpoint one day and off setpoint the next.
Probable cause:
Most
processes have a changing gain characteristic. The process gain can change
because centrifugal pump curves and compressor curves have decreasing output
pressure with increasing flow. These changes in pressure will affect the
pressure at the inlet of a control valve. Also, the pressure at the valve
outlet will change because of a changing back pressure caused by the fluid
friction with the downstream piping. This changing valve pressure drop will
cause the process to have a changing gain.
This curve indicates the
process gain by the slope of the flow vs. Cv curve.
Corrective action:
The tuning of the
controller gain is a function of the process gain. (See Loop Tip # 7) In
general, if the process gain increases, the controller gain should decrease
by the same proportion to keep the same performance. This controller gain
can be changed manually or through adaptive gain found in most
microprocessor controllers. Adaptive gain is a technique where another
variable is measured, and based on that value, the controller gain is
automatically changed to a predetermined value based on the process gain.
If adaptive gain
techniques are not available, the controller will require new tuning to
maintain the same performance. |
