|
Symptom:
Is it difficult to make some loops control accurately no matter what
controller tuning is used?
Probable cause:
A possible
cause could be that the final control element, usually a control valve, has
significant deadband. This deadband can be caused by friction and lost
motion.
Corrective action:
A positioner can be
installed on a control valve to reduce the deadband caused by friction.
This friction from valve packing, seals, or process buildup can cause valve
deadband to be at least 5%. Almost any control valve driven by just an I/P
transducer will have this magnitude of deadband. Any large deadband is a
control problem, because the process gain is zero within that band.
A positioner will
usually reduce the frictional deadband to less than 1%. Deadband caused by
lost motion requires maintenance to ensure that all connections are not
loose.
Adding a positioner
will not solve all of the loop problems, but will provide the opportunity
for good control once the controller tuning is correct. |
