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To ensure safe, stable, and long-term rational operation of its production process, the precise and stable functioning of control valves is essential. Therefore, it is important to strengthen the maintenance of control valves; poor maintenance of these valves or delayed repair of any faults can affect the proper functioning of the entire control system. Which parts should be checked carefully during maintenance? The inner wall of the valve body: In situations with high pressure differences and corrosive media, the inner wall of the valve body and the diaphragm in diaphragm valves are often subjected to stress and corrosion from these media; it is therefore necessary to carefully check their resistance to pressure and corrosion ； Valve core: The valve core is one of the moving parts of a control valve, and it is subject to severe erosion by the medium. During maintenance, it is necessary to carefully check whether various parts of the valve core have been corroded or worn. Especially under high pressure differences, wear on the valve core becomes more severe due to cavitation. Valves with severely damaged valve cores should be replaced. Check the sealing packing: Verify whether the packing made of asbestos rope is dry; if PTFE packing is used, pay attention to checking whether it has aged and whether its mating surfaces are damaged ； Due to the infiltration of medium during operation, the internal surface of the threads used to secure the valve seat is prone to corrosion, which causes the valve seat to become loose ； Diaphragms and O-rings are vulnerable components; check for signs of aging ； Packaging: When graphite or asbestos is used as the packaging material in control valves, lubricant should be applied to it approximately every three months to ensure smooth operation of the valve ； If it is found that the packing gland cap is pressed too low, the packing should be replenished ； If the polytetrafluoroethylene filler becomes hardened, it should be replaced promptly ； Air supply: For control valves equipped with positioners, the air supply should be checked regularly to ensure that it is of high quality, free from water and other impurities. Common faults and countermeasures: Unlike manual valves, control valves are in a state of continuous movement and adjustment during operation. They have numerous moving parts and must withstand various forces such as those resulting from imbalances in the fluid flow, which can lead to various unexpected faults. These faults can originate from the actuator or control mechanism, or from the connected auxiliary devices. The control valve does not function. Reason 1: No air supply or insufficient air pressure. Action: First, check whether the air supply (instrument air) is unobstructed and whether the air pressure meets the requirements for operating this valve. Reason 2: There is air supply, but no output signal air pressure. Solution: 1) Replace the mechanical controller or positioner with an air pressure controller or positioner. 2) For the control valves of mechanical controllers, check whether the valves in the signal acquisition pipelines of the medium in the installation pipelines of these control valves are fully open or suffering from severe leakage; any abnormalities should be addressed promptly. Reason 3: The air pressure of the output signal is normal, but it still does not operate. Measures: 1) Check whether the diaphragm of the pneumatic diaphragm actuator has severe air leakage; if there is leakage, the diaphragm and related seals should be replaced promptly. 2) If it is checked that the diaphragm is intact and there is no air leakage, then it is likely that the valve core of the main valve is stuck to the bushing and seat; the main valve should be disassembled for inspection and any debris removed. 3) If the valve stem is severely bent, the main valve should be disassembled for inspection, and replacement or repair should be carried out based on the results of the inspection. 4) There is a leak in the signal gas supply pipeline; check and repair the leak. 5) If the signal amplifier is faulty or not properly adjusted, the main air supply does not reach the diaphragm chamber through the amplifier; it should be adjusted or replaced promptly. Regulating valves experience regulating oscillations and unstable control during operation. Reason 1: Large fluctuations in air supply pressure, or abnormal operation of the filter or pressure regulator. Measures: 1) Check the operating status of the compressed air system. 2) Check and replace the filter or pressure reducing valve. Reason 2: The air supply pressure is stable, while the signal pressure is not. Measures: 1) If stability is still not achieved after replacing the intelligent locator, the PID parameters should be adjusted. 2) For the mechanical controller or positioner, it should be replaced, and the controller should be adjusted. 3) If instability persists after replacing the mechanical controller or positioner, check and adjust the replaced signal air supply amplifier. Reason 3: Both the air supply and signal pressure are stable, but the control valve still operates unstably. Measures: 1) Check the airtightness of the pneumatic diaphragm actuator. Carefully check the drive rod seal for any slight air leakage; if there is leakage, the seal should be removed and replaced ； Inspect the diaphragm for any scratches or punctures that may cause minor air leakage; if there is leakage, the diaphragm should be replaced. 2) Carefully check whether there are any gaps at the connection point between the locator and the main valve; the relevant connectors between them should be replaced or tightened. 3) Carefully check the signal air pipeline with soapy water for any slight leaks; if any are detected, they should be addressed promptly. 4) The signal gas source amplifier is not adjusted properly; adjust the balance screw of the signal gas source amplifier. 5) If the resistance of the actuation mechanism of the control valve (including the main valve and the pneumatic diaphragm actuator) is too high, the main valve should be disassembled to inspect and replace any faulty components, including the valve core, valve seat, valve stem, valve stem seal, and drive rod. 6) Check whether the pressure balance spring in the pneumatic diaphragm actuator is damaged, and inspect whether the spring has suffered fatigue deformation; if any abnormalities are found, the pressure balance spring should be replaced. Slow operation of the control valve: Reason 1: Accumulation of sticky substances inside the valve body. Measure: Disassemble the main valve body and clean the residues inside it. Reason 2: The valve stem packing has hardened due to deterioration, or the lubricant in the graphite or asbestos packing has dried out. Measure: Disassemble the main valve body and replace the valve stem packing. Reason 3: The filler is tightened too much, increasing frictional resistance. Measurements: 1) Loosen the valve stem packing compression nut, operate the main valve back and forth several times, then adjust the valve stem packing compression nut to the appropriate torque. 2) If Measure 1 does not work, replace the valve stem packing and adjust the packing compression nut to the appropriate torque. Reason 4: An uneven valve stem leads to high frictional resistance. Measure: Disassemble the main valve and replace the valve stem. Reason 5: There is slight air leakage in the diaphragm of the pneumatic diaphragm actuator or in the signal air pipeline. Measurements: Replace the diaphragm of the pneumatic diaphragm actuator, and address leaks in the signal air pipeline. The control valve is operating normally, but the process control parameters are abnormal. Cause 1: The valve spool of the control valve has fallen off. Measure: Disassemble and inspect the main valve body of the control valve, and restore the valve spool in case it has fallen out, based on the results of the inspection. Reason 2: Relative displacement has occurred at the connection between the valve spool and the valve stem, but they have not separated. Measure: Disassemble and inspect the main valve body of the control valve, and restore the position of the valve spool based on the inspection results. Reason 3: Breakage of the control valve stem. Measurements: Disassemble and inspect the main valve body of the control valve, and replace the valve stem. Reason 4: There is dirt blocking inside the valve spool of the control valve. Measure: Disassemble and inspect the main valve body of the control valve, and clean the dirt inside the valve core. Reason 5: Other control parameters of the controlled medium system are inappropriate, such as the medium flow rate and pressure being outside the design specifications. Measure: Adjust the system control parameters within the specified design range.