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Prepared by:
Kim Syro S. Borgonia

The electro-pneumatic action is a control

system for pipe organs, whereby air pressure,
controlled by an electric current and operated by
the keys of an organ console, opens and closes
valves within wind chests, allowing the pipes to
speak. This system also allows the console to be
physically detached from the organ itself. The only
connection was via an electrical cable from the
console to the relay, with some early organ
consoles utilizing a separate wind supply to operate
combination pistons.
 A valve Positioner is a device used to increase
or decrease the air load pressure driving the
actuator until the valve’s stem reaches a
“POSITION” balanced to the output SIGNAL
from the process variable instrument
 Valve positioners are used on controlling
valves where accurate and rapid control is
required without error or hysterises.
 Positioners are generally mounted on the side-yoke or
top casing of the pneumatic actuator for linear sliding
rotary control vales. For either basic design type,
“mechanical feedback linkage” connected directly to
the valve’s stem provides feedback to controller. The
process controller tells the positioner to “change”
position; the feedback linkage reports back to the
positioner confirming that a change has occurred and
gives a “sense” of the magnitude of the change in
Principle of Operation
 The Figure shows a valve positioner . The valve
positioner is a force-balanced instrument, with
pneumatic module installed on a double-acting
actuator for air to open action. Positioning is based
on a balance of two forces; one proportional to the
instrument signal and the other proportional to the
stem position.
Principle of Operation

 A downward force is activated as the signal

pressure acts upon the diaphragms in the
instrument signal capsule, through the
follower arm and cam, the motion of the
actuator stem is transmitted to the top end of
the feedback spring resulting in the varying of
tension in feedback spring as stem position
Principle of Operation

 The system will be in equilibrium and stem will

be in the position called for by the instrument
signal when these opposing forces balance
exactly. The balance will move up or down
and by means of the spool valve, will change
the output pressures and flow rate if these
opposing forces are not in balance. This will
lead to the piston to moving until the tension
on the feedback spring opposes exactly the
instrument signal pressure.
The detailed sequence of positioner
operations are as follows:
 An increase in the instrument signal forces the instrument
signal capsule and balance beam downward.
 This motion of the balance beam also pulls the pilot valve
spool downward from its equilibrium position.
 This opens the pilot valve ports, supplying air to port 1 and
exhausting air from port 2.
 This causes the actuator piston upward. Proportionally to
the valve position, to counter the force generated by the
instrument signal capsule, the piston continues to stroke
upwards until force in the feedback spring increases
The detailed sequence of positioner
operations are as follows:
 At this point the balance beam and spool begin to
return to equilibrium position.
 As the valve spool ports start to close, the air
flow rate to the actuator is decreased.
 The feedback spring tension force will equal the
force generated in the instrument signal capsule
after the piston has reached the required
The detailed sequence of positioner
operations are as follows:

 The balance beam and instrument signal capsule will

remain in their equilibrium positions with no air
flowing to the actuator until a change in the
instrument signal is made.
 A proportional downward movement of the actuator
piston and stem is affected by a decrease in the
instrument signal which reverses the described
Control Valve Positioner Assembly

 A valve positioner consists of a very high gain

amplifier- this may be pneumatic, electropneumatic
etc, and a feed back link which detects the actual
position of the valve.
 The required movement is for the valve to close. The
input pressure from the controller to the bellows
falls. The flapper moves away from the nozzle and the
pressure after the orifice falls. The pressure to the
diaphragm falls and the valve begins to close. The
feed back arm moves up rotating the cam clockwise.
This raises the beam increasing back pressure in the
nozzle until equilibrium is again achieved.
 The change over cock allows the signal from the
controller to be placed directly on the diaphragm
Positioner Advantages

 Precise positioning
 can cope with large variations in forces acting
on plug
 Rapid positioning
 Removes stiction and friction effects of gland
 Removes effects of large distances between
valve and positioner
 Eliminates hysterises
When should a positioner be fitted:
A positioner should be considered in the following
 When accurate valve positioning is required;
 To speed up the valve response. The positioner uses
higher pressure and greater air flow to adjust the
valve position;
 To increase the pressure that a particular actuator
and valve can close against. (To act as an amplifier);
 When the valve pressure drop at the maximum
operating flowrate, exceeds 5 bar for single seated
valves or 10 bar for double seated valves;
When should a positioner be fitted:

 To linearize a non-linear actuator;

 Where varying differential pressures within
the fluid would cause the plug position to
 When controlling with wide throttling range;
 When valves are handling sludge or solids in