4. Sealing elements
Although the structure of the ball valve is simple, there are many factors that affect the final sealing of the ball valve due to the pressure self-sealing of media and the special structure of the ball.
4.1 Quality of sealing pairs
The quality of sealing pairs of ball valves mainly manifests in the roundness of the ball and surface roughness of sealing surfaces of the ball and valve seat. The roundness of the sphere affects the fit between the sphere and valve seat. If they fit well, the resistance of the fluid to move along the sealing surface will be increased, thereby improving the sealing performance. Generally, the roundness of the sphere is required to be level 9.
The surface finish quality of the sealing surface has a great influence on the sealing. When the surface finish quality and specific pressure are low, the leakage increases. When specific pressure is high, 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 less effect on the sealing performance than rigidity sealing of metal to metal. 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, and it can be considered that the gap to prevent fluid leakages must be less than 0.003 μm. However, raised peaks of metal surfaces still exceed 0.1 μm, even if they are finely ground, which are 30 times bigger 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.
4.2 Sealing specific pressure
The specific sealing pressure refers to the pressure acting on the unit area of the sealing surface. The specific sealing pressure is produced by the pressure difference between the front and rear of valves and the applied sealing force. The specific pressure directly affects the sealing performance, reliability and service life of the ball valve. The amount of leakage is inversely proportional to the pressure difference. Tests have proved that under the same conditions, the leakage is inversely proportional to the square of the pressure difference. Therefore, the leakage will decrease as the pressure difference increases. The pressure difference is an important factor that determines the specific sealing pressure, so it is very important to the sealing performance of cryogenic ball valves. The sealing specific pressure applied on the sphere should not be too great. Too great pressure is good for sealing, but will increase the valve operating torque. Therefore, a reasonable selection of the sealing specific pressure is a prerequisite to ensure the sealing of cryogenic ball valves.
4.3 Physical properties of fluids
(1) Viscosity
The permeability of a fluid is closely related to its viscosity. Under the same other conditions, the greater viscosity of the fluid is, the smaller its permeability becomes. The viscosity of gas and liquid differ greatly. The viscosity of gas is dozens of times smaller than that of liquid, so its permeability is better than that of liquid. The exception is saturated steam, which is easy to ensure sealing. Compressed gas leaks more easily than liquid.
(2) Temperature
The permeability of a fluid depends on the temperature that causes the viscosity to change. The viscosity of a gas increases as the temperature rises, and it is proportional to the square root of the temperature of the gas. The viscosity of the liquid is the opposite, it decreases sharply with the increase in temperature, and it is inversely proportional to the cube of the temperature. In addition, changes in the size of parts caused by temperature changes will cause changes in the sealing pressure in the sealing zone and can damage the seal. The impact on the sealing of cryogenic fluids is particularly significant, because temperature of the sealing pair which is in contact with the fluid is usually lower than that of the force-receiving part, causing the sealing pair to shrink and be loose. 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.
(3) Surface hydrophilicity
The effect of surface hydrophilicity on leakage is caused by the characteristics of capillary pores. When there is a thin oil film on the surface, hydrophilicity of the contact surface is destroyed and the fluid channel is blocked, so that a great pressure difference is required to make the fluid pass through the capillaries. Therefore, some ball valves adopt sealing grease to improve the sealing and service life. When grease is used to seal, it should be noted that if the oil film is reduced during use, grease should be added. The grease used should not dissolve in the fluid medium, nor should it evaporate, harden or undergo other chemical changes. Sealing grease are not suitable for cryogenic ball valves. Under ultra-low temperature conditions, most of the grease will vitrify.
4.4 Structural dimensions
(1) Sealing structure
Since the sealing pair is not absolutely rigid, its structural size will inevitably change under the influence of the sealing force or temperature changes, which will change the interaction force between the sealing pairs, and the result is a reduction in the sealing performance. The seal should have certain elastic deformation to compensate for this change. At present, some ball valve seats adopt structure with elastic compensation or metal elastic support, and some balls also adopt elastic ball structure. These are all positive forms of improving sealing performance.
(2) Width of sealing surface
The width of the sealing surface determines the length of the capillary. When the width increases, the fluid movement along the capillary pores increases proportionally, while the leakage decreases inversely. However, this is not the case in fact. Because contact surfaces of sealing pairs cannot be completely matched, the width of the sealing surface cannot be fully effective in sealing when deformation occurs. The increase in the width of the sealing surface will increase the required sealing force, so it is also important to select the width of the sealing surface reasonably.
(3) Sizes of seal rings
Cryogenic ball valves generally adopt PCTFE seal rings, and the linear expansion coefficient of PCTFE at low temperatures is much higher than that of metal. Therefore, sizes of PCTFE seal rings will become smaller due to shrinkage at low temperatures, resulting in a decrease in the specific pressure of the seal with the ball. There will be a leakage channel between the seal and valve seat. Therefore, the size of the PCTFE seal ring is also an important factor affecting sealing of the cryogenic ball valve. The impact of size shrinkage at low temperatures needs to be considered for the design, and the cold assembly process must be used in the process.
5. Conclusion
In view of the widespread internal leakage of cryogenic ball valves in existing LNG receiving stations, factors affecting sealing of cryogenic valves, such as quality of sealing pairs, specific pressure of seals, physical properties of fluid as well as structure and sizes of sealing pairs are analyzed based on the design criteria of cryogenic valves and basic theory of valve sealing. There are many other factors that affect sealing of cryogenic ball valves, such as stiffness of the ball and the center of the ball being concentric with the sealing surface of the valve seat during assembly. The specific pressure of sealing as well as structure and sizes of the seal pair are important elements that affect the sealing of the cryogenic ball valve, and must be fully considered in the design.
Although the structure of the ball valve is simple, there are many factors that affect the final sealing of the ball valve due to the pressure self-sealing of media and the special structure of the ball.
4.1 Quality of sealing pairs
The quality of sealing pairs of ball valves mainly manifests in the roundness of the ball and surface roughness of sealing surfaces of the ball and valve seat. The roundness of the sphere affects the fit between the sphere and valve seat. If they fit well, the resistance of the fluid to move along the sealing surface will be increased, thereby improving the sealing performance. Generally, the roundness of the sphere is required to be level 9.
The surface finish quality of the sealing surface has a great influence on the sealing. When the surface finish quality and specific pressure are low, the leakage increases. When specific pressure is high, 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 less effect on the sealing performance than rigidity sealing of metal to metal. 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, and it can be considered that the gap to prevent fluid leakages must be less than 0.003 μm. However, raised peaks of metal surfaces still exceed 0.1 μm, even if they are finely ground, which are 30 times bigger 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.
4.2 Sealing specific pressure
The specific sealing pressure refers to the pressure acting on the unit area of the sealing surface. The specific sealing pressure is produced by the pressure difference between the front and rear of valves and the applied sealing force. The specific pressure directly affects the sealing performance, reliability and service life of the ball valve. The amount of leakage is inversely proportional to the pressure difference. Tests have proved that under the same conditions, the leakage is inversely proportional to the square of the pressure difference. Therefore, the leakage will decrease as the pressure difference increases. The pressure difference is an important factor that determines the specific sealing pressure, so it is very important to the sealing performance of cryogenic ball valves. The sealing specific pressure applied on the sphere should not be too great. Too great pressure is good for sealing, but will increase the valve operating torque. Therefore, a reasonable selection of the sealing specific pressure is a prerequisite to ensure the sealing of cryogenic ball valves.
4.3 Physical properties of fluids
(1) Viscosity
The permeability of a fluid is closely related to its viscosity. Under the same other conditions, the greater viscosity of the fluid is, the smaller its permeability becomes. The viscosity of gas and liquid differ greatly. The viscosity of gas is dozens of times smaller than that of liquid, so its permeability is better than that of liquid. The exception is saturated steam, which is easy to ensure sealing. Compressed gas leaks more easily than liquid.
(2) Temperature
The permeability of a fluid depends on the temperature that causes the viscosity to change. The viscosity of a gas increases as the temperature rises, and it is proportional to the square root of the temperature of the gas. The viscosity of the liquid is the opposite, it decreases sharply with the increase in temperature, and it is inversely proportional to the cube of the temperature. In addition, changes in the size of parts caused by temperature changes will cause changes in the sealing pressure in the sealing zone and can damage the seal. The impact on the sealing of cryogenic fluids is particularly significant, because temperature of the sealing pair which is in contact with the fluid is usually lower than that of the force-receiving part, causing the sealing pair to shrink and be loose. 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.
(3) Surface hydrophilicity
The effect of surface hydrophilicity on leakage is caused by the characteristics of capillary pores. When there is a thin oil film on the surface, hydrophilicity of the contact surface is destroyed and the fluid channel is blocked, so that a great pressure difference is required to make the fluid pass through the capillaries. Therefore, some ball valves adopt sealing grease to improve the sealing and service life. When grease is used to seal, it should be noted that if the oil film is reduced during use, grease should be added. The grease used should not dissolve in the fluid medium, nor should it evaporate, harden or undergo other chemical changes. Sealing grease are not suitable for cryogenic ball valves. Under ultra-low temperature conditions, most of the grease will vitrify.
4.4 Structural dimensions
(1) Sealing structure
Since the sealing pair is not absolutely rigid, its structural size will inevitably change under the influence of the sealing force or temperature changes, which will change the interaction force between the sealing pairs, and the result is a reduction in the sealing performance. The seal should have certain elastic deformation to compensate for this change. At present, some ball valve seats adopt structure with elastic compensation or metal elastic support, and some balls also adopt elastic ball structure. These are all positive forms of improving sealing performance.
(2) Width of sealing surface
The width of the sealing surface determines the length of the capillary. When the width increases, the fluid movement along the capillary pores increases proportionally, while the leakage decreases inversely. However, this is not the case in fact. Because contact surfaces of sealing pairs cannot be completely matched, the width of the sealing surface cannot be fully effective in sealing when deformation occurs. The increase in the width of the sealing surface will increase the required sealing force, so it is also important to select the width of the sealing surface reasonably.
(3) Sizes of seal rings
Cryogenic ball valves generally adopt PCTFE seal rings, and the linear expansion coefficient of PCTFE at low temperatures is much higher than that of metal. Therefore, sizes of PCTFE seal rings will become smaller due to shrinkage at low temperatures, resulting in a decrease in the specific pressure of the seal with the ball. There will be a leakage channel between the seal and valve seat. Therefore, the size of the PCTFE seal ring is also an important factor affecting sealing of the cryogenic ball valve. The impact of size shrinkage at low temperatures needs to be considered for the design, and the cold assembly process must be used in the process.
5. Conclusion
In view of the widespread internal leakage of cryogenic ball valves in existing LNG receiving stations, factors affecting sealing of cryogenic valves, such as quality of sealing pairs, specific pressure of seals, physical properties of fluid as well as structure and sizes of sealing pairs are analyzed based on the design criteria of cryogenic valves and basic theory of valve sealing. There are many other factors that affect sealing of cryogenic ball valves, such as stiffness of the ball and the center of the ball being concentric with the sealing surface of the valve seat during assembly. The specific pressure of sealing as well as structure and sizes of the seal pair are important elements that affect the sealing of the cryogenic ball valve, and must be fully considered in the design.
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