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Symptom: Do you
ever observe a process variable signal (from a transmitter) with the
appearance shown in this figure?
Probable cause:
This is called a limit cycle, and it is caused by the control valve
having a stick/slip problem. This means that the valve dynamic friction is
greatly less than the static friction causing the valve to move suddenly,
and when it does move, it moves too far creating an error on the other side
of the setpoint. The flag to alert you of a problem are the sharp corners
on the PV plot.
In addition, the controller
has reset or integral action to continually ramp the controller output in
the direction trying to remove the error. A limit cycle will cause the
controller output signal to have a sawtooth pattern.
Corrective action:
Since the source of the problem is in the control valve and not the
controller tuning, the valve must be “fixed” to solve the source of the
problem. This could involve the following items:
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Perform maintenance to
remove process buildup causing friction
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Install new packing to
bring friction values back to normal levels
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Add a positioner to
minimize deadband effects
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Install a stiffer
spring to improve the valve resolution (this may also require larger
diaphragm area and/or higher diaphragm pressure).
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