Recommended to bookmark! Pneumatic control valve structure + principle + operation: one illustrated guide is enough!

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The content is divided into three sections: structural diagrams, working principles, and operation guidelines, to facilitate technical training, operation manuals, or on-site use. I. Structural diagram of the pneumatic control valve: Overall composition – pneumatic diaphragm actuator (upper part) + bracket + control valve body (lower part); precise control is achieved through an intelligent positioner. Labels and functions of the main components in the diagram: Pneumatic diaphragm actuator: the core driving component. It contains a film, springs, and a push rod inside. Compressed air enters the diaphragm chamber to generate thrust, driving the valve stem to move. Smart locator: a brain component. It receives 4-20mA control signals and outputs an accurate pressure signal to achieve closed-loop control. Air filter pressure reducing valve: an air source treatment unit that ensures a clean and stable supply of compressed air (output pressure is usually 0.3–0.7 MPa). Exhaust port: Exhaust channel for the actuator’s diaphragm chamber, to ensure sensitive operation. Bracket: The support structure that connects the actuator to the valve body. Scale indication: Real-time display of valve opening degree (0-100%). Valve stem: The shaft that transmits the thrust from the actuator to the valve disc. Filler + valve cover + gland: a sealing assembly that prevents the medium from leaking along the valve stem. Regulating valve body: the part that forms the flow path for the medium. Single-seat spool + seat: Throttling element that precisely regulates flow by changing the degree of opening. II. Working Principle Diagram 1. Working principle of single-acting pneumatic diaphragm control valve Normal control process: The control system (DCS/PLC) sends a 4-20mA signal to the intelligent positioner. After receiving the signal, the positioner outputs the corresponding air pressure to the diaphragm chamber of the pneumatic diaphragm actuator. The pressure in the membrane chamber increases → the membrane pushes the valve stem downward → the valve core moves downward (for air-operated open valves) or upward (for air-operated closed valves), thereby changing the flow area between the valve core and the valve seat. Valve position feedback (an internal or external transmitter of the actuator) sends the actual opening signal back to the actuator. The positioner continuously compares the set value with the actual value, automatically adjusting the air pressure until the valve position reaches the set opening and stabilizes. Key balancing mechanism: Air pressure thrust vs spring reaction force + medium imbalance force → Force balance is achieved. Loss of air pressure state (fault protection): When the air supply is interrupted, the spring force pushes the valve stem back to its initial position; air-operated valve (fault-closed type): automatically shuts off completely. Air shut-off valve (fail-open type): automatically fully open. 2. Schematic of air-open vs air-close operation: Air-open type: Increase in signal pressure → Increased valve opening (fully closed when air is lost, providing safe shutdown). Air-shut type: Increase in signal pressure → Decrease in valve opening (fully open when air is lost, to prevent overpressure). III. Operation Guide: Steps for Normal Operation Check the air filter pressure regulator: Verify that the gas supply pressure is normal and that there is no water accumulation in the filter. Power and gas supply: Provides gas to intelligent positioners and actuators. Manual/Automatic switching: First, use the handwheel (if available) to adjust the valve to the appropriate initial opening degree, then switch to automatic mode. Given the control signal: slowly change the DCS setpoint and observe whether the scale indication and the locator display move smoothly in synchronization. Confirm no leakage: Check that there is no leakage of the medium at the packing area and the valve body flanges. Manual operation (emergency/maintenance): Cut off the control signal and air supply. Use the handwheel mechanism (top-mounted or side-mounted) to manually push the valve stem. After the operation is completed, the automatic position must be restored (the clutch must be disengaged); otherwise, the automatic mode will not function properly. Daily maintenance check points – Air supply section: Drain the water accumulated in the air filter pressure regulator daily, and check the pressure gauge reading. Positioner: Check whether the data on the display screen is normal, and ensure that there is no air leakage at the trachea connector. Executing agency: Check whether the membrane is damaged and whether the exhaust port is unobstructed. Valve body section: Check whether the packing gland is loose, whether there are any scratches on the valve stem, and whether there is any vibration or abnormal noises in the valve body. Low-temperature environment: Turn on the heater to prevent freezing from affecting operation. Common faults and solutions: Valve does not move: Check the air supply → pressure reducing valve → actuator input signal → solenoid valve (if present). Slowed movement: Increase the output pressure of the filter pressure reducing valve, or check the opening degree of the speed control valve (throttle valve). Incorrect positioning: Check whether the scale indication matches the actual opening, and recalibrate the positioner. Leakage: Tighten the gland or replace the packing. Important safety reminder: It is strictly prohibited to remove valves or accessories while under pressure or with fluid present. Explosion-proof locators and solenoid valves must be used in explosion-proof areas. When selecting, the air-open/air-close type must be determined based on the process medium (temperature, pressure, corrosivity) and safety requirements. ▍ For equipment safety, reference can be made to the dense tantalum pentoxide passivation layer that forms spontaneously on the surface of tantalum (element symbol Ta); this layer is hardly eroded by concentrated sulfuric acid, hydrochloric acid, nitric acid, phosphoric acid, or mixed acids ； The amount of metal ions precipitated approaches zero, thus not interfering with the reaction system ； It has a melting point as high as 2996 degrees Celsius, offering excellent stability over long periods of use. When used as a tantalum lining in reaction vessels, heat exchangers, and piping systems, it can extend the service life of equipment exposed to highly corrosive environments by dozens of times compared to that of ordinary stainless steel, thereby eliminating the risk of leaks and explosions caused by corrosion and perforations at the material level. Statement: This article was first published on the WeChat official account [Han Tantai Instrumentation Tubes and Valves]