Sealing Elements of Cryogenic Ball Valves
Ball valves are widely used in various working conditions due to their simple structure, small installation space, and reliance on medium force for sealing, which are not affected by external driving forces.
Sealing Elements
1. Quality of the sealing pair
Its main performance is the roundness of the ball and the surface roughness of the sealing surface between the ball and the valve seat. The surface smoothness of the sealing surface has a significant impact on the sealing. When the smoothness is low and the specific pressure is small, the leakage increases. When the specific pressure is high, the effect of smoothness on leakage is significantly reduced.
2. Sealing specific pressure
Sealing pressure refers to the pressure acting on a unit area of the sealing surface. The size of the specific pressure directly affects the sealing, reliability, and service life of ball valves. The amount of leakage is inversely proportional to the pressure difference.
3. Physical properties of fluids
(1) Viscosity
The permeability of a fluid is closely related to its viscosity. Under the same conditions, the higher the viscosity of the fluid is, the smaller its permeability becomes.
(2) Temperature
The permeability of a fluid depends on the temperature that causes the viscosity to change. The viscosity of a gas increases with the increase in temperature, which is proportional to the square root of the gas's temperature. The viscosity of a liquid, on the other hand, decreases sharply with increasing temperature, which is inversely proportional to the cube of temperature.
(3) Surface hydrophilicity
The influence of surface hydrophilicity on leakage is caused by the characteristics of pores. When there is a thin oil film on the surface, it damages the hydrophilicity of the contact surface and blocks the fluid channel, requiring a great pressure difference to allow the fluid to pass through the pores.
4. Structural dimensions
(1) Structure of the sealing pair
The sealing pair is not absolutely rigid, so its structural dimensions will inevitably change under the influence of sealing force or temperature changes, which will change the mutual force between the sealing pairs, resulting in a decrease in sealing performance. To compensate for this change, the sealing element should have a certain degree of elastic deformation.
(2) Sealing surface width
The width of the sealing surface determines the length of the capillary hole. As the width increases, the distance of fluid movement along the pores increases proportionally, while the leakage decreases inversely. However, this is not the case, because the contact surfaces of the sealing pair cannot all match, and when deformation occurs, the width of the sealing surface cannot all effectively play a sealing role. As the width of the sealing surface increases, the required sealing force needs to be increased, so it is also important to choose the sealing surface width reasonably.
(3) Sizes of the sealing ring
PCTFE sealing rings are commonly used for ultra-low temperature ball valves, and the linear expansion coefficient of PCTFE is much higher than that of metals at low temperatures. Therefore, at low temperatures, the PCTFE sealing ring will shrink and reduce its size, resulting in a decrease in the sealing pressure to 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 ultra-low temperature ball valves. When designing, it is necessary to consider the impact of size shrinkage at low temperatures, and the cold assembly process should also be used in the process.
Sealing Elements
1. Quality of the sealing pair
Its main performance is the roundness of the ball and the surface roughness of the sealing surface between the ball and the valve seat. The surface smoothness of the sealing surface has a significant impact on the sealing. When the smoothness is low and the specific pressure is small, the leakage increases. When the specific pressure is high, the effect of smoothness on leakage is significantly reduced.
2. Sealing specific pressure
Sealing pressure refers to the pressure acting on a unit area of the sealing surface. The size of the specific pressure directly affects the sealing, reliability, and service life of ball valves. The amount of leakage is inversely proportional to the pressure difference.
3. Physical properties of fluids
(1) Viscosity
The permeability of a fluid is closely related to its viscosity. Under the same conditions, the higher the viscosity of the fluid is, the smaller its permeability becomes.
(2) Temperature
The permeability of a fluid depends on the temperature that causes the viscosity to change. The viscosity of a gas increases with the increase in temperature, which is proportional to the square root of the gas's temperature. The viscosity of a liquid, on the other hand, decreases sharply with increasing temperature, which is inversely proportional to the cube of temperature.
(3) Surface hydrophilicity
The influence of surface hydrophilicity on leakage is caused by the characteristics of pores. When there is a thin oil film on the surface, it damages the hydrophilicity of the contact surface and blocks the fluid channel, requiring a great pressure difference to allow the fluid to pass through the pores.
4. Structural dimensions
(1) Structure of the sealing pair
The sealing pair is not absolutely rigid, so its structural dimensions will inevitably change under the influence of sealing force or temperature changes, which will change the mutual force between the sealing pairs, resulting in a decrease in sealing performance. To compensate for this change, the sealing element should have a certain degree of elastic deformation.
(2) Sealing surface width
The width of the sealing surface determines the length of the capillary hole. As the width increases, the distance of fluid movement along the pores increases proportionally, while the leakage decreases inversely. However, this is not the case, because the contact surfaces of the sealing pair cannot all match, and when deformation occurs, the width of the sealing surface cannot all effectively play a sealing role. As the width of the sealing surface increases, the required sealing force needs to be increased, so it is also important to choose the sealing surface width reasonably.
(3) Sizes of the sealing ring
PCTFE sealing rings are commonly used for ultra-low temperature ball valves, and the linear expansion coefficient of PCTFE is much higher than that of metals at low temperatures. Therefore, at low temperatures, the PCTFE sealing ring will shrink and reduce its size, resulting in a decrease in the sealing pressure to 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 ultra-low temperature ball valves. When designing, it is necessary to consider the impact of size shrinkage at low temperatures, and the cold assembly process should also be used in the process.
