What are the functions of the three major components of the valve pneumatic actuator?

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In industrial process control, pneumatic actuators mainly drive ball valves, butterfly valves, plug valves, etc. to complete 90° rotation to achieve on-off or adjustment of the medium. As the three core components of pneumatic control, the air filter, solenoid valve, and limit switch (valve transmitter) form a standard open-loop control loop. The collaborative work of the three is the key to the stable operation of the valve. Their selection, configuration, and maintenance directly affect control accuracy and equipment life. As the core unit of air source purification, the air filter is used to remove moisture, rust and other impurities in the compressed air, avoid corrosion of the internal rack of the actuator, accelerate the aging of seals, and ensure the cleanliness of the air source. Its core configuration requirements: The filtration accuracy needs to be controlled at 5-40μm, and the bottom is equipped with an automatic drain valve as standard to drain the condensed water in time. ； The diameter needs to be selected according to the air consumption of the actuator (calculated according to the action cycle). The commonly used interface is G1/4" or G1/2" ； In low-temperature environments (such as below -10°C), electric heating or manual drainage is required to prevent the filter cup from freezing and clogging. The solenoid valve is responsible for the gas path reversal control function and is the key to connecting the control signal and executing the action. Selection must strictly match the actuator type: Double-acting actuator (remains in position after power loss), equipped with a two-position five-way solenoid valve, which controls the opening and closing of the valve by switching between power-on and power-off states. ； The single-acting actuator (returns to a safe position by a spring after losing power) is equipped with a two-position three-way single-coil solenoid valve. When powered, the air inlet drives the valve to move, and when powered off, the exhaust completes the reset. Selection taboos: Insufficient Cv value of the solenoid valve will prolong the valve action time and easily cause water hammer and seal damage. ； Explosion-proof sites require corresponding Ex-level solenoid valves and matching coils to ensure that the explosion-proof performance reaches the standard. The limit switch (valve transmitter) is a valve position feedback unit, used to feed back the actual opening and closing status of the valve to the control system (such as DCS) to ensure closed-loop control. It is installed in the NAMUR standard slot on the top of the actuator. It is divided into mechanical type and inductive type. The main shaft rotates synchronously with the valve, driving the internal cam to press the micro switch or trigger the proximity sensor, and outputs two independent sets of opening and closing position signals. Common faults are mostly caused by cam angle adjustment deviation (such as the valve is fully closed but the switch is not triggered), and water in the junction box causes false alarms. During debugging, you need to manually adjust the valve to fully closed, calibrate the closed cam to just trigger the switch, and then adjust it to fully open to calibrate the open cam. working sequence: DCS issues a valve opening command → the solenoid valve coil is energized and reversed → the compressed air enters port A of the actuator after being purified by the filter → the actuator drives the valve to rotate → when the valve is fully open, the opening cam presses the micro switch → the switch outputs a closing signal to DCS → DCS synchronously turns off the valve opening indicator light to complete a valve opening cycle. Routine maintenance inspections need to focus on: Check the water line of the filter cup every month, and it should not exceed 1/2 of the cup body. Replace the filter element every year. ； The solenoid valve is tested every quarter by manually pressing the reversing button to clean the dust agglomeration at the exhaust port. ； Tighten the terminal block of the limit switch every six months, check that the cam screw is not loose, and ensure that the switching action is synchronized with the actual position of the valve. Selection principles: Determine the solenoid valve model according to the actuator type (double acting/single acting) ； The explosion-proof level of the solenoid valve and limit switch must match the on-site environment ； When the valve rotation angle is 90°, the adjustable range of the limit switch cam must be ≥90° ； In low-temperature environments (below -20°C), low-temperature silicone seals and cold-resistant cable joints must be used to avoid component failure.