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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 equipment. Therefore, it is necessary to strengthen the maintenance of pneumatic control valves. 1. The key inspection parts during maintenance are the inner wall of the inspection room: In situations where there is a 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, so it is necessary to focus on checking the pressure resistance and corrosion resistance. ； Check valve seat: Due to the penetration of medium during operation, the inner surface of the thread used to fix the valve seat is easily corroded, causing the valve seat to loosen. ； Check the spool: The valve core is one of the movable parts of the control valve. It is seriously eroded by the medium. During maintenance, it is necessary to carefully check whether all parts of the valve core are corroded and worn. Especially in the case of high pressure difference, the wear of the valve core is more serious due to cavitation and cavitation. Severely damaged valve cores should be replaced ； Check seal packing: Check whether the packing asbestos rope is dry. If polytetrafluoroethylene filler is used, check whether it is aged and whether its mating surface is damaged. ； Check whether the rubber film in the actuator is aged and cracked. 2. Routine maintenance of pneumatic control valves. When the control valve uses graphite-asbestos as filler, lubricating oil should be added to the filler about once every three months to ensure that the control valve is flexible and easy to use. If it is found that the packing pressure cap is very low, the packing should be replenished. If the PTFE dry packing is found to be hardened, it should be replaced in time. ； Pay attention to the operation of the regulating valve during patrol inspection, and check whether the valve position indicator and the regulator output match. ； For regulating valves with positioners, the air source should be checked frequently and any problems found should be dealt with promptly. ； The control valve should always be kept hygienic and all parts intact and functional. 3. Common faults and causes (1) The regulating valve does not operate. The fault symptoms and causes are as follows: 1． No signal, no air source. ①The air source is not turned on. ② Because the water in the air source freezes in winter, the air duct is blocked or the filter and pressure reducing valve are clogged and malfunctioned. ③ The compressor fails. ； ④The air supply main pipe is leaking. 2． There is air source but no signal. ①Regulator failure, ②signal tube leakage ； ③Positioner bellows leaking ； ④The adjusting diaphragm is damaged. 3． The positioner has no air source. ①filter clogged ； ②Pressure reducing valve failure I③ pipeline leakage or blockage. 4． The positioner has air source but no output. The positioner's orifice is clogged. 5. There is a signal, but no action. ①The valve core falls off, ② the valve core is stuck with the society or the valve seat ； ③Valve stem bent or broken ； ④The valve seat and valve core are frozen or charred and dirty. ； ⑤The actuator spring is rusted due to long-term disuse. (2) The action of the regulating valve is unstable. The fault symptoms and causes are as follows: 1． Air source pressure is unstable. ①Compressor capacity is too small ； ②Pressure reducing valve failure. 2． Signal pressure is unstable. ①The time constant of the control system (T=RC) is inappropriate ； ②The regulator output is unstable. 3． The air source pressure is stable and the signal pressure is also stable, but the action of the regulating valve is still unstable. ①The ball valve of the amplifier in the positioner is not tightly closed due to wear and tear caused by dirt. When the air consumption increases particularly, output oscillation will occur. ； ②The nozzle baffle of the amplifier in the positioner is not parallel and the baffle cannot cover the nozzle. ； ③Air leakage in output pipes and lines ； ④The rigidity of the actuator is too small ； ⑤The friction resistance is large during the movement of the valve stem, and there is blockage at the contact parts. (3) Regulating valve vibration. The fault symptoms and causes are as follows: 1． The regulating valve vibrates at any opening. ①Unstable support ； ②There is a vibration source nearby ； ③The valve core and bushing are severely worn. 2． The regulating valve vibrates when approaching the fully closed position. ①The regulating valve is larger and is often used at a small opening. ； ②The medium flow direction of the single-seat valve is opposite to the closing direction. (4) The regulating valve is slow in action. The phenomena and causes of sluggishness are as follows:: 1． The valve stem is only dull when moving in one direction. ①Diaphragm damage and leakage in pneumatic diaphragm actuator ； ②The "O" seal in the actuator is leaking. 2． The valve stem is sluggish when reciprocating. ①There is sticky matter blocking the valve body ； ②Polytetrafluoroethylene filler deteriorates and hardens or graphite-asbestos filler lubricating oil dries ； ③If the packing is added too tightly, the friction resistance will increase. ； ④The frictional resistance is large due to the valve stem not being straight. ； ⑤Pneumatic control valves without positioners can also cause sluggish action. (5) The leakage of the regulating valve increases. The reasons for the leak are as follows: 1． The leakage is large when the valve is fully closed. ①The valve core is worn and 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. ①The medium pressure difference is too large, the rigidity of the actuator is small, and the valve is not closed tightly. ； ②There is foreign matter in the valve ； ③Bushing sintered. (6) The adjustable range of flow rate becomes smaller. The main reason is that the valve core is corroded and becomes smaller, thereby increasing the adjustable minimum flow rate. Understand the failure phenomena and causes of pneumatic control valves and take appropriate measures to solve them. This post was last edited by Captain Yang on 2008-5-10 10:10 ]