Pneumatic control valve is one of the instruments widely used in petrochemical enterprises. Its accurate and normal operation is of great significance to ensure the normal operation and safe production of process units. Therefore, it is necessary to strengthen the maintenance of pneumatic control valve.

1、 Key inspection parts during maintenance

Check the inner wall of the valve body: in the case of high pressure difference and corrosive media, the inner wall of the valve body and the diaphragm of the diaphragm valve are often impacted and corroded by the media, and the pressure resistance and corrosion resistance must be checked;

Check the valve seat: due to the medium infiltration during the operation, the inner surface of the thread used to fix the valve seat is easy to be corroded and make the valve seat loose;

Check the valve core: the valve core is one of the movable parts of the regulating valve, which is seriously eroded by the medium. During maintenance, carefully check whether the parts of the valve core are corroded and worn, especially under the condition of high pressure difference, the wear of the valve core is more serious due to cavitation. The seriously damaged valve core shall be replaced; Check the sealing packing: check whether the packing asbestos rope is dry. If PTFE packing is used, pay attention to check whether it is aging and whether its mating surface is damaged;

Check the rubber film in the actuator for aging and cracking.

2、 Routine maintenance of pneumatic control valve

When the control valve uses graphite-asbestos as the packing, lubricating oil should be added to the packing once every three months to ensure that the control valve is flexible and easy to use. If it is found that the pressure of the packing gland is very low, the packing shall be supplemented. If it is found that the polytetrafluoroethylene dry packing is hardened, it shall be replaced in time; Pay attention to the operation of the regulating valve during the patrol inspection, and check whether the valve position indicator matches the output of the regulator; For the regulating valve with positioner, check the air source frequently and deal with the problems found in time; The regulating valve shall be kept clean and all parts shall be complete and easy to use.

3、 Common faults and causes

（1） The regulating valve does not operate. The fault phenomena and causes are as follows:

1. No signal or air source. ① The air source is not turned on, ② the air pipe is blocked or the filter and pressure reducing valve are blocked due to the water content in the air source icing in winter, ③ the compressor is faulty; ④ The main air supply pipe leaks.

2. There is air source and no signal. ① Regulator failure, ② signal pipe leakage; ③ The positioner bellows leaks; ④ The adjusting mesh is damaged.

3. The positioner has no air source. ① The filter is blocked; ② Pressure reducing valve failure I ③ pipeline leakage or blockage.

4. The positioner has air supply and no output. The orifice of the positioner is blocked.

5. There is signal and no action. ① The valve core falls off, ② the valve core is stuck with the society or the valve seat; ③ The valve rod is bent or broken; ④ The valve seat core is frozen or burnt and dirty; ⑤ The actuator spring rusted due to long-term use.

（2） The action of the regulating valve is unstable. The fault symptoms and causes are as follows:

1. The air supply pressure is unstable. ① The compressor capacity is too small; ② The pressure reducing valve is faulty.

2. The signal pressure is unstable. ① The time constant (T=RC) of the control system is inappropriate; ② The regulator output is unstable.

3. The air supply pressure and signal pressure are stable, but the action of the regulating valve is still unstable. ① The ball valve of the amplifier in the positioner is not tightly closed due to the wear of dirt, and the output shock will occur when the air consumption is particularly increased; ② The nozzle baffle of the amplifier in the positioner is not parallel, and the baffle cannot cover the nozzle; ③ Air leakage of output pipe and line; ④ The rigidity of the actuator is too small; ⑤ The friction resistance of the valve rod is large during the movement, and the contact part is blocked.

（3） The regulating valve vibrates. The fault symptoms and causes are as follows:

1. The regulating valve vibrates at any opening. ① Unstable support; ② There are vibration sources nearby; ③ The valve core and bushing are seriously worn.

2. The regulating valve vibrates when it is close to the fully closed position. ① The regulating valve is selected too large and is often used at a small opening; ② The medium flow direction of the single-seat valve is opposite to the closing direction.

（4） The regulating valve acts slowly. The phenomena and causes of dullness are as follows:

1. The valve rod is only sluggish when acting in one direction. ① The diaphragm in the pneumatic diaphragm actuator is damaged and leaked; ② The "O" seal in the actuator leaks.

2. The valve rod is sluggish when it reciprocates. ① There is a blockage in the valve body; ② Polytetrafluoroethylene filler is deteriorated and hardened or graphite asbestos filler lubricating oil is dried; ③ If the packing is too tight, the friction resistance will increase; ④ The friction resistance is large because the valve rod is not straight; ⑤ Pneumatic control valve without positioner will also cause sluggish action.

（5） The leakage of the regulating valve increases. The causes of leakage are as follows:

1. Large leakage when the valve is fully closed. ① The valve core is worn and the internal leakage is serious. ② The valve is not adjusted properly and cannot be closed tightly.

2. The valve cannot reach the fully closed position. ① The medium pressure difference is too large, the actuator rigidity is small, and the valve cannot be closed tightly; ② There is foreign matter in the valve; ③ The bushing is sintered.

（6） The adjustable range of flow is reduced. The main reason is that the valve core is corroded and becomes smaller, thus increasing the adjustable small flow.

Understand the failure phenomenon and cause of pneumatic control valve, and take measures to solve the problem.