Wide use of pneumatic control valve and daily maintenance

Pneumatic control valves are one of the widely used instruments in petrochemical companies. Its accurate and normal operation is of great significance for ensuring the normal operation and safe production of the process device. Therefore, it is necessary to strengthen the maintenance of pneumatic control valve.

A, between the inner wall of focus site when a maintenance inspection checking <br> <br>: where high differential pressure and corrosive media, body wall, diaphragm membrane often medium impact and corrosion resistance must focus on checking Pressure corrosion resistance;

Check the valve seat: Because the medium penetrates during work, the inner surface of the thread used to fix the valve seat is susceptible to corrosion and causes the valve seat to loosen;

Checking the valve plug: The valve plug is one of the movable parts of the regulating valve. It is seriously eroded by the medium. When checking, it is necessary to check whether the various parts of the valve plug are corroded and worn. Especially in the case of high pressure difference, the valve plug The cavitation caused by cavitation is even more serious. Severely damaged valve plug should be replaced; check the seal packing: check the packing asbestos rope is dry, such as the use of PTFE packing, should pay attention to check whether the aging and its mating surface is damaged;

Check whether the rubber film in the actuator is aged and cracked.

Second, the pneumatic control valve used daily maintenance <br> <br> when regulating valve uses a graphite asbestos packing when, about three months should be added to a lubricating oil in the filler to ensure the control valve and flexible to use. If the filler pressure cap is found to be very low, fillers should be added. If it is found that the polytetrafluoroethylene dry filler is hardened, it should be replaced in time; attention should be paid to the operation of the regulating valve in the inspection tour, check the valve position indicator and regulator Whether the output is consistent or not; for the regulating valve with positioner, the gas source should be checked frequently, and the problem should be solved in time; the health of the regulating valve and the completeness and ease of use of the components should be maintained.

Third, common faults and the reasons for the (a) regulating valve does not move. The symptoms and causes are as follows:

1. No signal, no air source. 1 The air source is not open, 2 Because the air source contains water in the winter, freezing, leading to blockage of the air duct or filter, pressure relief valve plug failure, 3 compressor failure; 4 gas source mains leakage.

2. There is a source of gas, no signal. 1 regulator failure, 2 signal tube leakage; 3 positioner bellows leak; 4 adjust the mesh diaphragm damage.

3. Locator without air source. 1 Filter clogged; 2 Pressure reducing valve fault I3 Pipe leaking or blocked.

4. The positioner has a gas source and no output. The retainer throttle block.

5. There is a signal, no action. 1 spool off, 2 spool and society or with the valve seat stuck; 3 stem bent or broken; 4 seated spool frozen or coke block dirt; 5 actuator spring due to long-term use without rusting.

(b) The operation of the regulating valve is not stable. The symptoms and causes are as follows:

1. The source pressure is not stable. 1 compressor capacity is too small; 2 pressure relief valve failure.

2. The signal pressure is not stable. 1 The time constant of the control system (T=RC) is inappropriate; 2 the regulator output is not stable.

3. The source pressure is stable and the signal pressure is also stable, but the operation of the regulating valve is still unstable. 1 The ball valve of the amplifier in the positioner is loosely shut off by the dirt. When the air consumption increases, the output oscillation will occur. 2 The nozzle baffle of the amplifier in the positioner is not parallel, and the baffle cannot cover the nozzle. 3 Output pipe , Line leakage; 4 The rigidity of the actuator is too small; 5 The frictional resistance in the valve stem movement is large, and there is a blocking phenomenon with the contact parts.

(c) Adjust valve vibration. The symptoms and causes are as follows:

1. The regulator valve vibrates at any opening. 1 The support is unstable; 2 There is a vibration source nearby; 3 The valve plug and the bush wear badly.

2. The regulator valve vibrates near the fully closed position. 1 Larger control valve, often used in a small opening; 2 single seat valve medium flow and closed in the opposite direction.

(d) The motion of the regulating valve is dull. The phenomenon of dullness and causes are as follows:

1. The stem is only dull when operated in one direction. 1 Diaphragm breakage and leakage in the pneumatic diaphragm actuator; 2 "O" seal leakage in the actuator.

2. The valve stem is dull when it is reciprocated. 1There is sticky blockage in the valve body; 2The PTFE packing is hardened or the graphite-asbestos packing oil is dry; 3The packing is too tight, the frictional resistance is increased; 4The frictional resistance is large because the valve stem is not straight; Locating the pneumatic control valve of the positioner can also cause dullness.

(five) The leakage of the regulating valve increases. The reason for the leak is as follows:

1. The amount of leakage is large when the valve is fully closed. 1 The valve core is worn, the internal leakage is serious, and the 2 valve is not adjusted properly.

2. The valve does not reach the fully closed position. 1 medium pressure difference is too large, the actuator stiffness is small, the valve is not strict; 2 foreign objects in the valve; 3 bushing sintering.

(6) The flow adjustable range becomes smaller. The main reason is that the spool is corroded and the adjustable minimum flow rate is increased.

Understand the symptoms and causes of the malfunction of the pneumatic control valve and take measures to resolve it.

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