The key
to working with control valves and controllers is to remember that there must always be a balance maintained in the system. "Direct" and "reverse" are kind of
like "positive" and "negative" in that where you find one
you will usually find the other.
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While
control valve bodies and control valve actuators can be described as being
direct acting or reverse acting, thinking about such things when working
through a system problem only adds to the confusion. Therefore, it is always best to consider the FAIL SAFE mode of the
valve and simply let the control
valve be what it may be.
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Positioners, 99% of the time, will usually mimic the input signal from the
controller. That is, they will be DIRECT ACTING.
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Direct-Acting
Positioner
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Another
reason the direct-acting pneumatic positioner is so popular is that it can be
by-passed and the control valve will respond to the input signal from the
controller as though the positioner were in the control loop. If a positioner
malfunction occurs or if the positioner causes the control valve to become
unstable, it can be easily by-passed. Many control valves in the field are
operating with a by-passed positioner.
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Reverse-acting positioners are sometimes used on control valves, but their
appearance is rare. Occasionally one will be found in a split-ranging
sequence.
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Reverse-Acting
Positioner
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Tuesday, December 11, 2012
Direct and Reverse Action Positioners
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