The working principle of pneumatic valve positioners

2026-01-18

    The pneumatic valve positioner operates based on the principle of torque balance. When the signal pressure P1 introduced into the bellows 2 increases, the main lever 3 rotates around the fulcrum, causing the nozzle baffle 9 to approach the nozzle. The back pressure of the nozzle is amplified by the unidirectional amplifier 8, and the pressure introduced into the diaphragm chamber of the actuator increases, causing the valve stem to move downward. It drives the feedback rod to rotate around the fulcrum, and the feedback CAM also rotates counterclockwise. Through the roller, the secondary lever 4 rotates around the fulcrum and stretches the feedback spring. When the pulling force of the spring on the main lever 3 and the force of the signal pressure applied to the bellows reach torque balance, the instrument reaches a balanced state. The valve position of the actuator is maintained at a certain opening degree, and a certain signal pressure corresponds to a certain valve position opening degree. The above mode of action is positive action. If you want to change the mode of action, just flip the CAM to change the A direction to the B direction, etc. The so-called positive action positioner means that when the signal pressure increases, the output pressure also increases. The so-called reaction positioner is one where the output pressure decreases as the signal pressure increases. As long as a direct-acting actuator is equipped with a reverse-acting positioner, it can achieve the action of a reverse-acting actuator. On the contrary, a reaction actuator can perform the actions of a positive actuator as long as it is equipped with a reaction positioner.