The sealing material of valve seat is generally polytetrafluoroethylene. Among the existing plastics, polytetrafluoroethylene has the strongest chemical stability. Its chemical stability is even more stable than that of precious metal gold and platinum, glass, ceramics, enamel, special steel and alloy. The substances with strong chemical corrosiveness - concentrated acid, dilute acid, concentrated lye, and the strongest oxidant can not have any effect on PTFE; It is called. Except that some halogenated amines or aromatic hydrocarbons cause slight swelling of polytetrafluoroethylene plastics, ketones, acids, cools and other organic solvents have no effect on polytetrafluoroethylene, and will not cause weight change and swelling. Only molten alkali metals, chlorine trifluoride and elemental fluorine can act on it, but only under high temperature and pressure. In addition, polytetrafluoroethylene is not affected by oxygen or ultraviolet rays, does not absorb water, and is incombustible. It has high heat resistance and cold resistance. The temperature range of use is - 180~250 ℃. After being treated at 250 ℃ for 240h, the mechanical properties do not decrease, and the weight loss per hour at 390 ℃ is 0.006%. Its dielectric properties are excellent, and its electrical properties are not affected by frequency and temperature. It can work at 250 ℃ for a long time. It can be seen from the above that it is ideal to use PTFE as the seal of pneumatic ball valve. The elastic modulus of polytetrafluoroethylene is very large. Therefore, the performance of the material determines that its elastic deformation is very small. Once it is used for a long time, the elasticity of the material is not enough to make up for the wear of the sealing surface. At this time, the seal of the pneumatic ball valve will be damaged. Therefore, when designing the valve seat, we consider how to increase its elasticity from the structure, and we choose the elastic structure.

Maintenance of packing
1. The packing is directly related to whether the key sealing element leaks when the valve is opened and closed. If the packing fails, causing leakage, the valve is also invalid. Strengthening maintenance can extend the service life of the packing
2. When the valve leaves the factory, in order to ensure the elasticity of the packing, it is generally subject to the static pressure test without leakage. After the valve is installed into the pipeline, due to temperature and other factors, leakage may occur. At this time, the nuts on both sides of the packing gland should be tightened in time as long as there is no leakage. In the future, leakage should occur again, and tightening should not be done once, so as to avoid the loss of elasticity and sealing performance of the packing.
3. Some valve packing contains molybdenum disulfide lubricating paste. After several months of use, corresponding lubricating grease should be added in time. When it is found that the packing needs to be supplemented, corresponding packing should be added in time to ensure its sealing performance.