Sealing of Cryogenic Ball Valves

Many factors affect whether the cryogenic ball valve is ultimately sealed, although the ball valve has a simple structure. It is a valve sealed by the medium’s pressure and its sphere has a special structure.
 
1.1 Quality of sealing pairs
The quality of the ball valve’s sealing pair is mainly reflected in the roundness of the ball and the surface roughness of the sealing surface between the ball and the valve seat. The roundness of the sphere affects the fitness between the sphere and the valve seat. If the fitness is good, the resistance of the fluid moving along the sealing surface is increased, thereby improving the sealing. 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 seal. When the smoothness is low and the specific pressure is small, the amount of leakage increases. When the specific pressure is great, the effect of smoothness on leakages is significantly reduced. This is because the microscopic jagged peaks on the sealing surface are flattened. The smoothness of the soft sealing surface has a greater impact on sealing than that of the rigidity of metal to metal. According to the view that leakages can only be ensured 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 leakages must be less than 0.003μm. However, even if the metal surface is finely ground and the peak height still exceeds 0.1μm, which is 30 times greater than the diameter of a water molecule. It can be seen that it is difficult to improve the sealing only by improving the smoothness of the sealing surface. In addition to affecting the sealing, the quality of the sealing pair also directly affects the service life of the ball valve. Therefore, the quality of the sealing pair must be improved during manufacturing.
 
1.2 Seal specific pressure
Sealing-specific pressure refers to the pressure acting on the unit area of the sealing surface. The sealing-specific pressure is generated by the pressure difference between the front and back of the valve and the external 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. Tests have proven that, under other conditions being equal, the amount of leakage is inversely proportional to the square of the pressure difference, so the amount of leakage will decrease as the pressure difference increases. The pressure difference is an important factor in determining the sealing specific pressure, so the sealing specific pressure is crucial to the sealing of cryogenic ball valves. The sealing specific pressure applied to the ball cannot be too great. Excessive pressure is beneficial to sealing, but it will increase the valve operating torque. Therefore, a reasonable selection of sealing specific pressure is the prerequisite to ensure the sealing of cryogenic ball valves.
 
1.3 Physical properties of fluids
(1) Viscosity
A fluid's ability to penetrate is closely related to its viscosity. Under other conditions being the same, the greater the viscosity of the fluid is, the smaller its penetration capacity becomes. Gases and liquids have very different viscosities. 
① The viscosity of gas is dozens of times smaller than that of liquid, so its penetration ability is better than that of liquid. The exception is saturated steam, which can easily ensure sealing. 
② Compressed gas is more likely to leak than liquid.
 
(2) Temperatures
The fluid's ability to penetrate depends on the temperature that causes the viscosity to change. The viscosity of a gas increases with temperature and is proportional to the square root of the temperature of the gas. The viscosity of a liquid, on the other hand, decreases sharply with increasing temperature and is inversely proportional to the cube of temperature. In addition, changes in the part’s dimensions due to temperature changes will cause changes in sealing pressure in the sealing area and can damage the seal. The impact is particularly significant on the sealing of low-temperature fluids. Because the sealing pair in contact with the fluid is usually at a lower temperature than the force-bearing part, this causes the sealing pair components to shrink and relax. At low temperatures, the sealing is complicated, and most sealing materials fail at low temperatures. Therefore, the effect of temperature should be considered when selecting sealing materials.
 
(3) Surface hydrophilicity
The effect of surface hydrophilicity on leakages is caused by characteristics of capillary pores. When there is a thin oil film on the surface, it destroys the hydrophilicity of the contact surface and blocks the fluid channel, which requires a great pressure difference to pass fluid through capillary pores. Therefore, some ball valves use sealing grease to improve their sealing and service life. When using grease sealing, if the oil film is reduced during use. The grease used should be insoluble in the fluid medium and should not evaporate, harden or undergo other chemical changes. Low-temperature ball valves are not suitable for sealing grease. Under ultra-low temperature conditions, most grease will vitrify.
 
1.4 Structural dimensions
(1) Seal structure
Since the sealing pair is not rigid, its structural dimensions will inevitably change under the influence of sealing force or temperature changes and other factors, which will change the interaction force between the sealing pairs, resulting in reduced sealing. To compensate for this change, the seal should have a certain elastic deformation. At present, some ball valve seats adopt a structural form with elastic compensation or metal elastic support, and some balls also adopt an elastic ball. These are all positive forms of improving sealing.
 
(2) Sealing surface width
The width of the sealing surface determines the length of the capillary pores. When the width increases, the distance the fluid moves along the capillary pores increases proportionally, while the leakage decreases inversely proportionally. However, this is not the case. Because the contact surfaces of the sealing pairs cannot all match, When deformation occurs, the width of the sealing surface cannot all effectively play a sealing role. The increase in the width of the sealing surface will increase the required sealing force, so it is also important to choose the width of the sealing surface reasonably.
 
(3) Sealing ring sizes
Cryogenic ball valves generally adopt PCTFE sealing rings, and the linear expansion coefficient of PCTFE at low temperatures is much higher than that of metal. Therefore, at low temperatures, the PCTFE sealing ring will shrink due to shrinkage, resulting in a reduction in the sealing specific pressure with the ball and 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 cryogenic ball valves. The impact of dimensional shrinkage at low temperatures needs to be considered during design, and cold assembly technology must be used in the process.
 
2. Conclusion
In view of the common internal leakage in existing cryogenic ball valves in LNG receiving stations, the influences of the quality of the sealing pair, the specific pressure of the seal, the physical properties of the fluid, and the structure and size of the sealing pair on the sealing of cryogenic ball valves are analyzed based on the design criteria of cryogenic valves and the basic theory of valve sealing. Many factors affect the sealing of cryogenic ball valves, such as the stiffness of the ball and whether the ball center is concentric with the valve seat sealing surface during assembly. The sealing specific pressure and the structure and size of the sealing pair are important factors affecting the sealing of cryogenic ball valves and must be fully considered during design.