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Direct-Acting
and Reverse-Acting Positioners
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| The terms "direct" and "reverse" are frequently
used when discussing control valves, positioners, and controllers. While
the definitions of direct and reverse seem pretty straightforward, they
cause quite a bit of confusion - especially when split-ranging is done. |
| 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. |
| 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. |
| 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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Input Increases
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Output Increases
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Equals
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Increasing Signal from Controller
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Increasing Output from Positioner
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Input Decreases
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Output Decreases
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Equals |
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Decreasing
Signal From
Controller |
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Decreasing
Output From 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. |
| 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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Input Increases
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Output Decreases
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Equals
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Increasing Signal from Controller
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Decreasing Output from Positioner
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Input Decreases
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Output Increases
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Equals |
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Decreasing
Signal From
Controller |
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Increasing
Output From Positioner |
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Direct-Acting
and Reverse-Acting Controllers
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| Controllers can be set up in either direct
or reverse modes. It was stated that 99% of the positioners are direct
acting, and it follows that if a balance is to be maintained in the control
loop that 99% of the controllers will be reverse
acting. If the control valve and its controller are not
in balance, the control valve will either go to the wide-open position
and stay there, or it will stay closed and act as though it is not responding.
This situation can normally be corrected by reversing the action of the
controller. |
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Direct-Acting
Controller
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Setpoint Increases
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Output Increases
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Equals
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Increase
in
Setpoint
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Increase
in
Output
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Setpoint Decreases
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Output Decreases
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Equals |
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Decrease
in
Setpoint |
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Decrease
in
Output |
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Reverse-Acting
Controller
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Setpoint Increases
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Output Decreases
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Equals
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Increase
in
Setpoint
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Decrease
in
Output
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Setpoint Decreases
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Output Increases
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Equals |
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Decrease
in
Setpoint |
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Increase
in
Output |
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| Two of the more common control valve uses
are for pressure control. In both instances, the controllers are reverse
acting. Most pressure-reducing valves will be fail-closed and most back-pressure
control valves will be fail-open. If the pressure-reducing valve were fail-open
or the back-pressure valve fail-closed, then the controllers would have
been direct acting. |
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The
key is to start with the fail-safe mode of the control valve.
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Control Valve
Concepts / Actuator Operating Modes
/ Positioners / Flow
Characteristics / Seat Leakage
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