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Symptom: Do you observe that some control
valves require abnormally frequent trim replacement? Or do you observe that
a loop gets very unstable at the lower flow conditions?
Probable cause: Both of these symptoms could be
cause by control valve oversizing. This common problem is observed by the
valve always operating at the low end of travel. The high rate of trim
wear can be caused by high fluid velocity directing erosive particles
against metal components.
The unstable loop can be caused by several
effects:
·
The oversized valve is “bouncing” off the seat causing the
process to cycle.
·
An oversized equal percentage valve behaves as a linear
valve. This linear characteristic may be the wrong characteristic for the
process.
·
The oversized valve has very high gain.
·
If the valve has stick/slip, a limit cycle could more easily
develop.
Corrective actions: Installing a smaller valve
or a reduced port size trim in the existing valve can bring the valve back
into its proper travel range. Control valves, in general, are designed to
throttle between 10% and 90% travel. This is especially true of equal
percentage valves. A good guideline is to carefully look into any control
valve that is the same size as the pipe. This is often the sign of an
oversized valve. |
