Sealing Elements of Cryogenic Ball Valves (Part 2)

4 Sealing elements

Although the structure of the low temperature ball valve is simple, because it is a medium pressure self-sealing valve, combined with the special structure of the ball, there are many factors that affect the final sealing of the ball valve.

4.1 Seal quality

The quality of the ball valve sealing pair is mainly represented by 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 match 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. The roundness of the sphere is generally required to be 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 the smoothness on the leakage is significantly reduced, because the micro-saw-shaped peaks on the sealing surface are flattened, and the smoothness of the soft sealing surface has a greater impact on the sealing performance than the rigidity of the metal on the metal The seal is much smaller. According to the viewpoint that the fluid can not be leaked only when the gap between the sealing pairs is smaller than the diameter of the fluid molecule, it can be considered that the gap to prevent fluid leakage must be less than 0.003 μm. However, even the peak height of the finely ground metal surface still exceeds 0.1 μm, which is 30 times larger than the diameter of water molecules. It can be seen from this that it is actually difficult to improve the sealing performance by relying only on improving the smoothness of the sealing surface. In addition to affecting the sealability, the quality of the sealing pair directly affects the service life of the ball valve. Therefore, the quality of the sealing pair must be improved during manufacturing.

4.2 Sealing pressure

Seal specific pressure refers to the pressure acting on the unit area of the sealing surface. The specific pressure of the seal is caused by the pressure difference between the front and back of the valve and the applied sealing force. The specific pressure directly affects the sealing, reliability, and service life of the ball valve. The amount of leakage is inversely proportional to the pressure difference. The test proves that under the same conditions, the amount of leakage is inversely proportional to the square of the pressure difference. Therefore, the amount of leakage will decrease as the pressure difference increases. The pressure difference is an important factor in determining the specific pressure of the seal, so the specific pressure of the seal is very important for the sealing performance of the ultra-low temperature ball valve. The sealing specific pressure applied to the ball can not be too large, too large is conducive to sealing, but it will increase the operating torque of the valve, so a reasonable choice of sealing specific pressure is the prerequisite for ensuring ultra-low temperature ball valve sealing.

4.3 Physical properties of fluids

(1) Viscosity

The permeability of fluid is closely related to its viscosity. Under other conditions being the same, the greater the viscosity of the fluid, the lower its permeability. The viscosity of gas and liquid differ greatly. ① The viscosity of gas is dozens of times lower than that of liquid, so its permeability is stronger than liquid. The exception is saturated steam, which is easy to ensure sealing. ②Compressed gas leaks more easily than liquid.

(2) Temperature

The fluid's permeability depends on the temperature that causes the viscosity to change. The viscosity of the gas increases with increasing temperature, which is proportional to the square of the temperature of the gas. The viscosity of a liquid is the opposite. It decreases sharply with increasing temperature, and it is inversely proportional to the cube of temperature. In addition, changes in the size of the parts caused by temperature changes will cause changes in the sealing pressure in the sealing area and can break the seal. The effect on the sealing of cryogenic fluids is particularly significant. Because the sealing pair that is in contact with the fluid is usually cooler than the forced member, this causes the sealing pair to shrink and relax. At low temperatures, the sealing is complicated, and most sealing materials fail at low temperatures. Therefore, the influence of temperature should be considered when selecting the sealing material.

Low Temperature Ball Valve

(3) Surface hydrophilicity

The effect of surface hydrophilicity on leakage is caused by the characteristics of the pores. When there is a thin oil film on the surface, the hydrophilicity of the contact surface is destroyed and the fluid channel is blocked, which requires a large pressure difference Pass fluid through capillary holes. Therefore, some ball valves use sealing grease to improve sealing performance and service life. When using grease seals, it should be noted that if the oil film decreases during use, grease should be added. The grease used should not dissolve in the fluid medium, nor should it evaporate, harden, or other chemical changes. Low-temperature ball valves are not suitable for sealing grease. Under ultra-low temperature conditions, most greases will vitrify.

4.4 Structure size

(1) Sealed auxiliary structure

Since the sealing pair is not absolutely rigid, its size will inevitably change under the influence of the sealing force or temperature changes, which will change the interaction force between the sealing pair, and the result is reduced sealing performance. To compensate for this change, the seal should be elastically deformed. At present, some ball valve seats adopt a structure with elastic compensation or metal elastic support, and some balls also use an elastic ball structure. These are all a positive form of improving sealing performance.

(2) Width of sealing surface

The width of the sealing surface determines the length of the capillary hole. When the width is increased, the flow of fluid along the capillary hole increases proportionally, while the amount of leakage decreases inversely. But in fact it is not the case, because the contact surfaces of the sealing pair cannot all coincide, and when the deformation occurs, the width of the sealing surface cannot fully effectively seal. On the other hand, the increase in the width of the sealing surface increases the required sealing force, so it is also important to choose the width of the sealing surface reasonably.

(3) Seal ring size

Ultra-low temperature ball valves generally use PCTFE sealing rings, and the coefficient of linear expansion of PCTFE at low temperatures is much higher than that of metals. Therefore, at low temperatures, PCTFE sealing rings will shrink due to shrinkage and reduce the size. As a result, the sealing pressure with the ball will decrease and there is a leakage channel between it and the valve seat. Therefore, the size of the PCTFE sealing ring is also an important factor affecting the sealing of the ultra-low temperature ball valve. The effect of dimensional shrinkage at low temperature needs to be considered in the design, and the cold assembly process is also used in the process.

5 Conclusion

In view of the general leakage phenomenon of ultra-low temperature ball valves in existing LNG receiving stations, based on the design principles of the low temperature steel series and the basic theory of valve sealing, the quality of the sealing pair, the specific pressure of the seal, the physical properties of the fluid and the structure of the sealing pair And the size and other factors that affect the sealing of ultra-low temperature ball valves. There are many sealing factors that affect the ultra-low temperature ball valve, such as the stiffness of the ball and whether the center of the ball is concentric with the sealing surface of the valve seat during assembly and so on. The specific pressure of the seal and the structure and size of the sealing pair are important factors affecting the sealing of the ultra-low temperature ball valve, and must be fully considered in the design.