Pneumatic valve positioner

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A pneumatic valve positioner (electrical positioner) is a conversion device that transforms electrical signals into pressure signals; it uses compressed air or nitrogen as the operating medium to control the degree of opening of the valve. It is one of the extremely important accessories in valves, serving to position the valve. Pneumatic valve positioners are typically used together with pneumatic actuators; after being connected and tuned, they form pneumatic control valves that are employed in various fields of industrial automation process control. Principle of pneumatic valve positioner: The pneumatic valve positioner is a key accessory for control valves; it operates on the principle of torque balance. It uses the signal indicating the displacement of the valve stem as an input feedback signal, while using the output signal from the controller as the setpoint signal. By comparing these two signals, any discrepancy leads to adjustments in the output signal sent to the actuator, thereby causing the actuator to move. This establishes a one-to-one relationship between the displacement of the valve stem and the output signal from the controller. Therefore, the valve positioner forms a feedback control system that uses the stem displacement as the measurement signal and the controller output as the setpoint signal. The control variable of this control system is the output signal from the valve positioner to the actuator. https://ss0.bdstatic.com/70cFvHSh_Q1YnxGkpoWK1HF6hhy/it/u=3717724137,2168176843&fm=26&gp=0.jpg As the input signal increases, that is, as the 4-20MA signal rises, the torque motor generates an electromagnetic field, and the baffle is pushed away from the nozzle by the force of this electromagnetic field. Increasing the distance between the nozzle and the baffle releases the air pressure above the spool inside the pilot valve. Affected by this, its axis shifts to the right, pushing the valve element that blocks the base, allowing air pressure to be introduced into the actuator through the base. As the pressure inside the actuator’s air chamber increases, the actuator’s push rod moves downward, and the displacement changes of this push rod are transmitted to the slider through a feedback rod. This displacement change is then transmitted to the range feedback rod, which pulls on the range spring. When the force of the range spring and that of the torque motor are in balance, the baffle returns to its original position, reducing the distance from the nozzle. As the amount of air discharged through the nozzle decreases, the air pressure above the spool increases. The spool returns to its original position, the valve core once again blocks the base, and the supply of air pressure to the actuator is stopped. When the movement of the actuator stops, the positioner remains in a stable state.