Sealing Elements Of Low-Temperature Bar Ball Valve
The ball valve is widely used in various working conditions due to its simple structure, small installation space, and relying on medium force to seal and not being affected by the external driving force.
Due to the extremely low working conditions, the design and manufacture of low temperature bar ball valves face a series of technical problems, such as material selection, low-temperature sealing, structural design, solution treatment, cryogenic treatment, heat insulation, quality inspection, maintenance, safety, etc.
When designing cryogenic valves, in addition to following general valve design principles, special requirements for cryogenic valve design should be followed according to the conditions of use.
① The valve should not become a significant heat source for the cryogenic system. This is because the influx of heat not only reduces the thermal efficiency, if the inflow is too much, it will also cause the internal fluid to evaporate rapidly, resulting in abnormal pressure rise, causing danger.
② The low-temperature medium should not have harmful effects on the operation of the handwheel and the sealing performance of the packing.
③The valve assembly directly in contact with the cryogenic medium should have an explosion-proof and fire-proof structure.
④ The valve assembly working at low temperatures cannot be lubricated, so structural measures need to be taken to prevent the friction parts from scratching.
In the cryogenic valve design process, in addition to considering the general requirements such as the flow capacity of the cryogenic valve, some other indicators need to be considered in order to better evaluate the technical level of the cryogenic valve. The technical level of cryogenic valves is usually evaluated by measuring whether energy consumption is reasonable.
① Thermal insulation performance of the low-temperature valve.
②The cooling performance of the cryogenic valve.
③The working performance of the low-temperature valve opening and closing seals.
④The condition that the surface of the cryogenic valve does not freeze.
There is a big difference between the working environment of cryogenic valves and general valves. In the process of cryogenic valve design, manufacturing and inspection, in addition to complying with the general rules of valve design, manufacturing, and inspection, attention should also be paid to the environment where the cryogenic valve is located. Appropriate adjustments.
Although the structure of the ball valve is simple, because it is a medium pressure self-sealing valve and the special structure of the ball, there are many factors that affect the final sealing of the ball valve.
The quality of the ball valve sealing pair is mainly manifested in the roundness of the ball and the surface roughness of the sealing surface of the ball and the valve seat. The roundness of the sphere affects the fit between the sphere and the valve seat. If the degree of the anastomosis is high, the resistance of the fluid to move along the sealing surface is increased, thereby improving the sealing performance. Generally, the roundness of the sphere is required to be level 9.
The surface finish of the sealing surface has a great influence on the sealing. When the finish is low and the specific pressure is small the leakage increases. When the specific pressure is large, the effect of smoothness on the leakage is significantly reduced. This is because the micro jagged peaks on the sealing surface are flattened, and the smoothness of the soft sealing surface has a greater effect on the sealing performance than the rigidity of metal to metal The seal is much smaller. According to the viewpoint that the fluid does not leak only when the gap between the sealing pairs is smaller than the diameter of the fluid molecules, it can be considered that the gap to prevent fluid leakage must be less than 0.003 μm. However, even after the finely polished metal surface, the height of the peaks still exceeds 0.1 μm, which is 30 times larger than the diameter of water molecules. It can be seen that it is in fact difficult to improve the sealing performance only by improving the smoothness of the sealing surface. The quality of the sealing pair not only affects the sealing performance but also directly affects the service life of the ball valve. Therefore, the quality of the sealing pair must be improved during manufacturing.
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