Failure causes and common maintenance methods of pneumatic control valve

Failure causes and common maintenance methods of pneumatic control valve

The maintenance method of the sealing part between the valve stem and the valve body of the pneumatic control valve is as follows: first disconnect the valve actuator from the valve body, then open the upper gland, take out the gasket under the gland, hook out the "V" packing in the stuffing box, replace it with a new packing, and then install the gland to complete the maintenance. This maintenance can only last for two to six months. Some even leak after maintenance, and the gland cannot be pressed too tightly, otherwise it will affect the automatic adjustment of the valve. At the initial stage of maintenance, 4 ~ 6 circles of "V" packing are generally added. Later, due to the gradual weakening of spring tension, the added "V" packing is gradually increased, and the fluorine lined butterfly valve has been increased to more than ten circles. From the above analysis, it can be concluded that the main reason for poor sealing is that the spring in the stuffing box loses elasticity due to rust, while the PTFE sealing packing we use is hard and pressed too tightly, which will lock the valve stem, resulting in inflexible valve action and too loose pressure, which can not play a sealing role.

There are five common maintenance methods for pneumatic control valves:

① Increase sealing grease method: for valves without sealing grease, it can be considered to increase sealing grease to improve the sealing performance of valve stem.

② Adding packing method: in order to improve the sealing performance of packing to valve stem, the method of adding packing can be adopted. It is usually in the form of double-layer and multi-layer mixed fillers, simply increasing the number from 3 to 5, but the effect is not obvious.

③ Replacement of graphite packing method: for the tetrafluoro packing widely used, because its working temperature is in the range of - 20 ~ 200 ℃, when the temperature changes greatly at the upper and lower limits, its sealing performance will decline obviously, aging is fast and its service life is short. Flexible graphite filler can overcome these shortcomings and has long service life. Therefore, some factories change all the PTFE packing to graphite packing, and even the newly purchased control valve will replace the PTFE packing with graphite packing. However, the action of the regulating valve with graphite packing is large return difference, and some of them still have crawling phenomenon at the beginning, which must be considered.

④ Change the flow direction and place P2 at the valve stem end: when △ P is large and P1 is large, it is obviously more difficult to seal P1 than P2. Therefore, the flow direction can be changed from P1 at the valve stem end to P2 at the valve stem end, which is more effective for valves with high pressure and large differential pressure. For example, the sealing P2 shall be considered for bellows.

⑤ Adopt lens gasket sealing method: seal the upper and lower covers, and seal the valve seat with the upper and lower valve bodies. In case of plane seal, the sealing performance is poor under high temperature and high pressure, resulting in leakage. At this time, the lens gasket can be used for sealing, and satisfactory results can be obtained.