Flowserve PMV P3 - Manual (Page 7)

7
5. Function
Signal diaphragm Balance arm Flapper nozzle
To/from
actuator
Feedback arm
General
The P3 positioner operates on the force balance principle. Changes are initiated by the
signal pressure from the I/P converter. The functionality is described briefly below.
Relay
diaphragm
Signal
pressure
Decrease in signal pressure
When the air pressure on the signal
diaphragm is reduced, the balance arm is
lifted and the flapper nozzle opens. The
pressure on the relay diaphragm is then
reduced and the plug valve upper seal opens
and vents.
Air is now ventilated from the actuator
and its spindle moves. The actuator
movement goes back through the arm
mechanism (see figure) to the positioner
spindle, which rotates the feedback arm. The
balance arm is now lowered by the reduced
spring force and closes the flapper valve. An
equilibrium position is thus reached and the
valve being controlled is in the desired posi-
tion.
Increase in signal pressure
When the air pressure on the signal
diaphragm increases, the balance arm is pres-
sed down and the flapper nozzle closes. The
pressure on the relay diaphragm then
increases and the plug valve opens.
Air is now supplied to the actuator, the
spindle movement rotates the feedback arm,
the balance arm is lifted, and the flapper
nozzle opens. The pressure on the relay
diaphragm is reduced and the plug valve
reduces the air flow to the actuator. An
equilibrium position is thus reached and the
valve being controlled is in the desired posi-
tion.
Single action
Plug
valve
Air
supply
Venting