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Seal Elements Of Cryogenic Ball Valve

1 Overview

Due to its simple structure and small installation space, the ball valve is widely used in all working conditions because it is sealed by the medium force without external driving force. At present, LNG receiving stations generally adopt ultra-low temperature ball valves. The number of ultra-low temperature ball valves accounts for 80% of the total number of valves of the LNG receiving station. There is leakage of the ultra-low temperature ball valve during use. Based on the design criteria of cryogenic valve and the basic theory of valve sealing performance, the paper analyzes the factors that affect the sealing of cryogenic valve.

cryogenic ball valves.jpg

2 design guidelines

Due to the extremely low operating temperature, the design and manufacture of ultra-low temperature valves face a series of technical problems, such as material selection, low temperature sealing, structural design, solution treatment, cryogenic treatment, thermal insulation, quality testing, maintenance and safety. To this end, there are a series of strict standards for the design of cryogenic valves. The international standard mainly adopts the standard BS6364 "cryogenic valve" and MSSSP-134 "requirements for the cryogenic valve and its body / bonnet extension body" Comprehensive provisions of the design and manufacture of cryogenic valve points and rules. Standard JB / T7749 "cryogenic valve technical conditions" is based on BS6364 "cryogenic valve" into.


In the design of cryogenic valves, in addition to the general design principles of the valve should be followed, should be based on the conditions of use, to follow the special requirements of cryogenic valve design.


① Valves should not be a significant source of heat in cryogenic systems. This is because the inflow of heat in addition to reducing thermal efficiency, such as excessive influx, but also the rapid evaporation of the internal fluid, resulting in abnormal pressure, causing danger.

② low temperature medium should not be hand wheel operation and packing sealing performance have a detrimental effect.

③ direct contact with the cryogenic medium valve assembly should have explosion-proof and fireproof structure.

④ working at low temperature valve assembly can not be lubricated, so the need to take structural measures to prevent abrasion friction parts.


In the design of low-temperature valve, in addition to consider the general ability to circulate cryogenic valves and other general requirements, but also need to consider some other indicators in order to better evaluate the technical level of low-temperature valve. Usually by measuring the energy consumption is reasonable on the technical level of low-temperature valve evaluation.


① cryogenic valve insulation performance.

② cryogenic valve cooling performance.

③ cryogenic valve opening and closing seal performance.

④ cryogenic valve surface conditions.


Cryogenic valve and general valve of the working environment there is a big difference in the low temperature valve design, manufacturing and testing process in addition to comply with the valve design, manufacture and testing of the general rules, but also should pay attention to the environment in which the cryogenic valve Appropriate adjustment.

cryogenic ball valve.jpeg

3 basic theory

Affect the valve sealing factors are the main sealing structure, the sealing surface pressure, the physical properties of the media and the quality of the seal and so on. But only in the true understanding of the principle of the valve seal case, give full consideration to various factors that affect the sealing performance, in order to prevent leakage and ensure sealing.


Taking plane seal as an example, we study the sealing problem of sealing face connection and simply explain the sealing principle. Seal connection principle shown in Figure 1, wherein the container is filled with a certain pressure of liquid and gas, and sealed with a cover, the container static pressure medium as: FJ = A × P


Where FJ --- medium force, N.

          A --- medium acting on the cover area, mm2

          P --- media static pressure container, MPa


In order to keep the cover in the position shown in the figure, the external force F = FJ must be applied in the vertical direction of the contact surface between the container and the cover so that only the end faces can be fitted. Only when the sealing surface is the ideal plane, the medium will not be through the junction between the surface. In order to ensure the contact surface of the seal, the sealing surface must be generated between the interaction force, which is forced to cover pressed on the container. When the force F> FJ, in the combined sealing surface will produce a certain pressure, relying on the pressure to flat on the plane has been deformed. If the deformation is within the elastic limit of the material, and produces little residual deformation, the contact surface exerts force F to ensure its sealing. In addition to the seal than the pressure, to ensure the connection of sealing factors include sealing the secondary structure and so on. But in this series of factors, the pressure between the sealing surface has a key role.


4 sealing elements

Although the ball valve structure is simple, but because of its self-sealing valve for medium pressure, coupled with the special structure of the ball, so the final ball valve seal a lot of factors.


4.1 Seal Vice quality


The quality of the ball valve seal is mainly manifested as the roundness of the ball and the surface roughness of the ball and valve seat sealing surface. The roundness of the sphere affects the fit of the sphere to the seat. If the fit is high, then the resistance of the fluid moving along the sealing surface is increased, thereby improving the sealing. The roundness of the ball is generally required to be 9 levels.


Seal surface finish has a significant impact on the seal. When the finish is low, the pressure is small, the leakage increased. When the specific pressure is greater, the effect of finish on leakage is significantly reduced because the microscopic jagged peaks on the sealing surface are flattened, and the effect of the soft sealing finish on the sealing performance is better than that of the metal to metal Sealing a lot smaller. From the point of view that the fluid does not leak only when the gap between the seal pairs is less than the diameter of the fluid molecules, it is believed that the gap to prevent fluid leakage must be less than 0.003 μm. However, even the finely ground metal surface peak height still exceeds 0.1 μm, which is 30 times larger than the water molecule diameter. This shows that only by improving the sealing surface finish to improve the sealing, in fact, is difficult to achieve. In addition to sealing the quality of the seal seal, but also a direct impact on the life of the ball valve, therefore, manufacturing must improve the quality of seal.

4.2 seal than the pressure


Sealing pressure is the pressure acting on the sealing surface unit area. Sealing than the pressure is the pressure difference between the valve before and after the valve and the external sealing force generated. The pressure than the size of a direct impact on the ball valve seal, reliability and service life. Leakage is inversely proportional to pressure difference. Experiments show that under other conditions the same, the leakage is inversely proportional to the square of the pressure drop, so the leakage will decrease with the increase of the pressure drop. The pressure differential is an important factor in determining the specific pressure of the seal, so the seal pressure is critical to the sealing performance of the cryogenic ball valve. The pressure exerted on the sphere should not be too large. Too large is beneficial to the sealing, but it will increase the operating torque of the valve. Therefore, it is a prerequisite to ensure the seal of the cryogenic valve.


4.3 Physical properties of fluids


(1) Viscosity

Fluid penetration is closely related to its viscosity. Under other conditions the same, the greater the fluid viscosity, the smaller its permeability. Gas and liquid viscosity vary widely. ① gas viscosity than the viscosity of liquid small times, so its ability to penetrate stronger than liquid. However, with the exception of saturated steam, saturated steam easily ensures sealing. Compressed gas is more likely to leak than liquid.


(2) temperature

The fluid's ability to penetrate depends on the temperature at which the viscosity changes. Gas viscosity increases with increasing temperature, which is proportional to the square root of the gas temperature. The viscosity of a liquid is the opposite, it decreases sharply with increasing temperature, which is inversely proportional to the cube of temperature. In addition, changes in part size due to temperature changes will cause changes in the seal pressure within the seal area and damage the seal. This is particularly noticeable for the sealing of cryogenic fluids. Because the seal in contact with the fluid is usually lower than the temperature of the force member, which causes the seal assembly to shrink and relax. At low temperatures, the seal is complex and most seals fail at low temperatures. Therefore, the choice of sealing material should take into account the impact of temperature.


(3) Surface hydrophilicity

Hydrophilicity of the surface of the leakage is caused by capillary properties, when the surface has a thin film, the destruction of the hydrophilic surface and block the fluid channel, which requires a larger pressure difference in order to Pass the fluid through the pores. Therefore, some ball valve sealing grease, to improve the sealing and service life. In the use of grease seal, it should be noted that during use, such as oil film reduction, should be added grease. The grease used should be insoluble in the fluid medium and should not evaporate, harden, or otherwise chemically change. Cryogenic ball valve is not suitable for sealing grease, in the ultra-low temperature conditions, most of the oil will be vitrified.


4.4 structure size


(1) Seal Vice structure

As the seal is not absolutely rigid, it under the influence of the sealing force or temperature changes and other factors, the structure will inevitably change the size, which will change the interaction between the sealing pair, the result is reduced sealing performance. To compensate for this change, the seal should be a certain degree of elastic deformation. At present, some ball valve seat with elastic compensation or metal elastic support structure, some ball also uses a flexible ball structure. These are all positive forms of improving the sealing performance.


(2) sealing surface width

The width of the sealing surface determines the length of the capillary. As the width increases, the fluid flow increases proportionally with the capillary movement and the amount of leakage decreases inversely. But in fact is not the case, because the sealing surface of the contact surface can not all match, when the deformation occurs, the width of the sealing surface can not effectively play a role in sealing. On the other hand, the increase of the width of the sealing surface, to increase the required sealing force, so a reasonable choice of the width of the sealing surface is more important.


(3) ring size

PCTFE sealing ring is widely used in ultra-low temperature ball valves. However, PCTFE has much lower linear expansion coefficient than metal at low temperature. Therefore, PCTFE sealing ring shrinks in size at a low temperature. As a result, Leakage channel between it and the valve seat. Therefore, the size of the PCTFE sealing ring is also an important factor that affects the seal of ultra-low temperature ball valves. The design needs to consider the effect of shrinkage under low temperature and the cold assembly process on the process.

5 Conclusion


In view of the ubiquitous internal leakage phenomenon of cryogenic ball valve of existing LNG receiving station, based on the design principles of cryogenic valve and the basic theory of valve sealing, the influence of cryogenic ball valve seal from the quality of seal, the pressure ratio of seal, the physical properties of fluid and the structure and size of seal pair Analysis of the elements. There are many sealing elements that affect the cryogenic ball valve, such as the rigidity of the ball and whether the center of the ball is concentric with the seat sealing surface during assembly. Seal pressure and seal the structure and size of the seal is an important factor affecting the cryogenic valve, the design must be fully considered.


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