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Valve
Positioners
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| A valve Positioner is a device used to increase
or decrease the air pressure operating the actuator until the valve stem
reaches the position called for by the instrument controller. |
| Positioners are generally mounted on the side
or top of the actuator. They are connected mechanically to the valve stem
so that stem position can be compared with the position dictated by the
controller. |
| A positioner is a type of air relay which
is used between the controller output and the valve diaphragm. The positioner
acts to overcome hysteresis, packing box friction, and valve plug unbalance
due to pressure drop. It assures exact positioning of the valve stem in
accordance with the controller output. |
| Reasons To Use
Positioners |
Increase control system resolution: i.e. fine control.
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Allow use of characteristic cams.
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Minimize packing friction effects: i.e. high-temperature packing.
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Negate flow-induced reactions to higher pressure drops.
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Increase speed of response to a change in process.
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Overcome seating friction in rotary valves.
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Allow distances between controller and control valve.
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Allow wide range of flow variation: i.e. operate at less than 10% travel
under normal conditions.
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Allow increased usage of 4-20 mA electronic signal.
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Increase fast venting (unloading) capability.
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Permit use of piston actuators.
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Facilitate operation when the higher number in the bench-set range
is greater than 15 psig: i.e. 10-30 psig, 6-30 psig, etc.
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How
Positioners Work
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| Although there are many different types of
positioners, the basic principles of operation are similar for all of them. |
| Principle of
Operation: |
| The positioner is mechanically connected to
the stem of the valve. This stem position is compared with the position
called for by the instrument controller, i.e. by the instrument output
air signal. A separate air supply is brought into the positioner for positioning
the valve at exactly the point called for by the controller. |
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Control Valve Concepts
/ Actuator Operating Modes / Positioner
and Controller Operating Modes / Flow
Characteristics / Seat Leakage
/ Packing / Helpful
Hints / Cashco Terminology /
Advanced Topics
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