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Symptom:
A control valve with a spring and diaphragm actuator has good control
characteristics may have poor shutoff; or a valve with good shutoff seems to
have poor control characteristics.
Probable cause:
The actuator spring rate
will influence both the shutoff and control characteristics of the valve.
Many actuators are specified with the smallest actuator possible, but this
approach may cause poor controlability. The spring rate establishes the
actuator gain, which is part of the process gain (see
LOOPtip #3).
The compromise between
shutoff and better control is shown on this drawing. The weak spring
(dashed line), has sufficient shutoff force, but the slope is indicative of
a valve with poor resolution and a small bench set span. The stronger
spring (solid line), has better resolution for better control, but
sacrifices on shutoff force available.
In general, the lower
spring rate results in higher actuator gain, which also raises the process
gain. This higher process gain would require lower controller gain to
achieve the same performance.
Corrective action:
The only way to get
tighter shutoff and better controlability at the same time is to use a
stronger spring or a higher actuator pressure range, but this may require a
larger actuator size. |
