Maintenance of pneumatic control valves and causes of failure

pneumatic control valve is one of the instruments widely used in petrochemical enterprises. It works accurately and normally, which is of great significance to ensure the normal operation and safe production of the process device. Therefore, it is necessary to strengthen the maintenance of pneumatic control valves.

 

First, the key inspection parts during maintenance

 

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

 

Check the seat: due to the infiltration of medium during work, the inner surface of the thread used to fix the seat is susceptible to corrosion and relaxes the seat

 

Check the valve core: the valve core is one of the movable parts of the regulating valve, the erosion of the medium is more serious, and the parts of the valve core should be carefully checked whether they are corroded and worn during maintenance, especially in the case of high pressure difference, the wear of the valve core is more serious due to cavitation. Seriously damaged valve core should 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 whether the rubber film in the actuator is aged and whether there is cracking.

 

2. Daily maintenance of pneumatic control valves

 

When the control valve is filled with graphite-asbestos, lubricating oil should be added to the packing once in about three months to ensure that the control valve is flexible and easy to use. If it is found that the pressure cap of the packing is very low, the packing should be supplemented, and if the PTFE dry packing is found to be hardened, it should be replaced in time; attention should be paid to the operation of the control valve in the patrol inspection, and the output of the valve position indicator and the regulator should be checked consistently; the control valve with a positioner should be checked frequently, and the problem should be dealt with in time; the hygiene of the control valve and the integrity of each component should be kept intact and easy to use.

 

3. Common faults and their causes

 

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

 

1. No signal, no air source. (1) The air source is not opened, (2) The air duct is blocked or the filter and pressure reducing valve are blocked and malfunctioning due to the water content of the air source freezing in winter, (3) The compressor is faulty, and (4) The air source main pipe is leaking.

 

2. There is a gas source, no signal. (1) Regulator failure, (2) Signal tube leakage, (3) Positioner bellows leakage, (4) Regulator diaphragm damage.

 

3. The positioner has no air source. (1) Filter blockage, (2) Pressure reducing valve failure I, (3) Pipeline leakage or blockage.

 

4. The positioner has an air source, no output. The throttle hole of the positioner is blocked.

 

5. There is a signal, no action. (1) The valve core falls off, (2) the valve core is stuck with the society or with the valve seat, (3) the valve stem is bent or broken, (4) the valve seat and the valve core is frozen or coke, and (5) the actuator spring is 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 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 of the control system (T=RC) is inappropriate, and (2) the output of the regulator 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. (1) The ball valve of the amplifier in the positioner is not tightly closed by dirt wear, and the output oscillation 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) The output pipe and line leakage; (4) The rigidity of the actuator is too small; (5) The friction resistance is large in the movement of the valve stem, and there is a blockage phenomenon with the contact part.

 

(3) Vibration of the regulating valve. The fault symptoms and causes are as follows:

 

1. The control valve vibrates at any opening. (1) The support is unstable, (2) There is a vibration source nearby, (3) The valve core and bushing are seriously worn.

 

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

 

(4) The action of the regulating valve is sluggish. The phenomenon of sluggishness and its causes are as follows:

 

1. The stem is sluggish only when acting in one direction. (1) Diaphragm damage and leakage in the pneumatic film actuator;(2) O" type seal leakage in the actuator.

 

2. The valve stem is dull when it reciprocates. (1) There is sticky blockage in the valve body; (2) The PTFE packing deteriorates and hardens or the graphite-asbestos packing lubricating oil is dry; (3) The packing is added too tightly, and the friction resistance increases; (4) The friction resistance is large due to the valve stem is not straight; (5) The pneumatic control valve without a positioner will also lead to sluggish action.

 

(5) The leakage of the regulating valve increases. The reasons for the leak are as follows:

 

1. Large leakage when the valve is fully closed. (1) The valve core is worn out, and the internal leakage is serious, (2) The valve is not adjusted and closed tightly.

 

2. The valve cannot reach the fully closed position. (1) The pressure difference of the medium is too large, the rigidity of the actuator is small, and the valve is not tightly closed; (2) There are foreign bodies in the valve; (3) The bushing is sintered.

 

(6) The adjustable range of flow becomes smaller. The main reason is that the spool is corroded and made smaller, resulting in a higher adjustable minimum flow rate.

 

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