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Symptom:
Do you find that some loops never perform well no matter what controller
tuning is used?
Probable cause:
A final control element, such as a control valve, is often the source of
high friction and/or lost motion. These factors are control problems which
controller tuning cannot correct. High friction can come from stem packing,
process buildup, and obstructions in the valve. Lost motion is caused by
worn parts, such as linkages or other attachment points, that can become
worn or loose over time.
Corrective action:
In order to know if a final control element needs corrective action, the
deadband needs to be tested. This can be done from the control room using
the following procedure.
The
first step is to place the controller in manual. Then bump the output in
small increments in one direction until movement of the PV is seen. This
means that the actuator force has gone through the deadband and the valve
moved. Then bump the output the other direction, again in small increments,
until the PV moves the other direction.
The
difference in controller output in percent is the field equipment deadband.
And the control valve is usually the source of that deadband. If the
deadband is greater than 1%, high performance control will not be achieved.
If this is the case, some maintenance on the valve or positioner will be
required. |
