Masoneilan 4700P/E & 4800P/E Positioners Manual =| 15
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General Description and Operation
The 4700P/4700E and 4800P/4800E positioners function to make a valve stroke proportional to
a pneumatic or electrical control signal from a controller, or to modify the natural flow charac-
teristic of the valve itself through the use of a characterized cam. They can be configured to pro-
vide split-ranging of valves and may be used with supplemental air supplies to achieve greater
valve pressure drop. The 4700P/4800P pneumatic positioner can also be configured to reverse
valve response to a control signal (i.e. control signal can either open or close valve). The 4700E/
4800E electropneumatic positioner is not available with reverse action.
The Model 4700P/4700E and 4800P/4800E pneumatic positioner design is based on the
force-balance principle: the signal pressure exerted on a diaphragm is opposed by a feedback
spring. In the balanced state, when the pneumatic signal varies, the diaphragm assembly moves.
This movement is followed by the pilot plug which is opposed by the pilot spring.
Movement of the pilot plug alternately connects the output circuit to the supply circuit or the ex-
haust port, thus modifying air pressure to the actuator. The cam transmits valve plug movement
to the feedback spring. The valve plug continues to move until the spring force exactly balances
the force of the instrument signal on the diaphragm. In the new balanced state, the valve plug is
positioned in a programmed relationship to the instrument signal.
Figure 1 Model 4700P/4700E and 4800P/4800E Positioner Numbering System
Introduction
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