How a Pneumatic Valve Positioner Works ~ Learning Instrumentation And Control Engineering Learning Instrumentation And Control Engineering

How a Pneumatic Valve Positioner Works

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The pneumatic valve positioner accepts an input pneumatic signal from a control device and sends an increased or decreased output signal to a control valve to ensure the control valve plug travels correctly and is positioned properly on the valve seat. The schematic below illustrates how the valve positioner is connected to the control valve:

A valve positioner can be set to either direct-acting or reverse-acting. A direct-acting valve positioner sends an increased output signal to the control valve as the input signal to it increases. A reverse-acting valve positioner, sends a decreased output signal as the input signal to it increases.

How the Pneumatic Valve Positioner Works
The schematic below is that of a direct-acting Fisher 3582G pneumatic valve positioner:
Fisher 3582G Pneumatic Valve Positioner (Photo Credit : Fisher)

As shown in the diagram, the valve positioner is attached to a diaphragm-actuated sliding stem control valve. The supply pressure is connected to a pneumatic relay. A fixed orifice restriction in the relay limits flow to the nozzle such that when the flapper is not restricting the nozzle, air can bleed out faster than is being supplied.

The input signal from the control device is connected to the bellows. When the input signal increases, the bellows expand and moves the beam. The beam pivots about the input axis moving the flapper closer to the nozzle. The nozzle pressure increases and through the pneumatic relay action increases the output pressure to the valve actuator. The increased output pressure to the actuator causes the actuator stem to move downward. Stem movement is then fed back to the beam by means of a cam. As the cam rotates, the beam pivots about the feedback axis to move the flapper slightly away from the nozzle. The nozzle pressure decreases and reduces the output pressure to the actuator. Stem movement continues backing the flapper away from the nozzle until equilibrium is achieved.

When the input pressure decreases, the bellows contracts (aided by an internal range spring) and the beam pivots about the input axis to move the flapper away from the nozzle. Nozzle pressure decreases and the pneumatic relay permits the release of diaphragm casing pressure to atmosphere causing the actuator stem to move upward. Through the cam, stem movement is fed back to the beam to re-position the flapper closer to the nozzle. When equilibrium conditions are attained, stem movement stops and the flapper is positioned to prevent any further decrease in diaphragm case pressure.

In a reverse-acting valve positioner, the same basic operating principle above applies except that as the input signal increases, the diaphragm casing pressure is decreased. A decreasing input signal causes an increase in pressure to the diaphragm casing of the control valve.

How to Change Valve Positioner Action of the 3582G Pneumatic Valve Positioner
Positioner Beam (Photo Credit: Fisher)

The position of the flapper assembly on the beam determines whether the valve positioner will act as direct-acting or reverse-acting.  

As shown in the diagram above, the  beam is divided into quadrants. The direct acting quadrant is labelled DIRECT and the reverse-acting quadrant is labelled REVERSE. To change the valve positioner action, simply move the flapper assembly to the opposite quadrant of the beam.