Analysis of the Sealing Elements of Cryogenic Ball Valves

2026-10-10 09:27:15

1 Overview

      Ball valves are widely used in various working conditions because of their simple structure, small installation space, and the fact that they rely on the pressure of the medium itself for sealing and are not affected by external driving forces. At present, cryogenic ball valves are commonly used in LNG receiving terminals, and the number of cryogenic ball valves accounts for 80% of the total number of valves in an LNG receiving terminal. Internal leakage of cryogenic ball valves occurs in service. Based on the design criteria for cryogenic valves and the basic theory of valve sealing performance, this paper analyzes the elements that affect the sealing of cryogenic ball valves.

2 Design Criteria

Since the service temperature is extremely low, the design and manufacture of cryogenic valves face a series of technical challenges, such as material selection, low-temperature sealing, structural design, solution treatment, cryogenic treatment, thermal insulation, quality inspection, maintenance, and safety. For this reason, a series of strict standards govern the design of cryogenic valves. Internationally, the main standards adopted are BS 6364 "Specification for valves for cryogenic service" and MSS SP-134 "Requirements for cryogenic valves and body/bonnet extensions", which comprehensively specify the key points and rules for the design and manufacture of cryogenic valves. The Chinese standard JB/T 7749 "Technical specification for cryogenic valves" was converted from BS 6364.

When designing cryogenic valves, in addition to following the general design principles for valves, the special requirements for cryogenic valve design should be followed according to the service conditions. (1) A valve should not act as a significant heat source in the cryogenic system. This is because an influx of heat, apart from reducing thermal efficiency, will, if excessive, cause the internal fluid to evaporate rapidly, producing an abnormal pressure rise and creating a hazard. (2) The cryogenic medium should not have a harmful effect on handwheel operation or on the packing sealing performance. (3) Valve subassemblies in direct contact with the cryogenic medium should have explosion-proof and fire-safe structures. (4) Valve subassemblies working at low temperatures cannot be lubricated, so structural measures must be taken to prevent galling of the friction parts. In the design process of cryogenic valves, besides general requirements such as flow capacity, several other indicators need to be considered so as to better evaluate the technical level of cryogenic valves. Usually, the technical level of a cryogenic valve is evaluated by measuring whether its energy consumption is reasonable: (1) the thermal insulation performance of the cryogenic valve; (2) the cool-down performance of the cryogenic valve; (3) the working performance of the opening/closing sealing elements of the cryogenic valve; (4) the conditions for the absence of ice formation on the valve surface.

      The working environment of cryogenic valves differs greatly from that of general-purpose valves. In the design, manufacture and inspection of cryogenic valves, in addition to observing the general rules for valve design, manufacture and inspection, appropriate adjustments should be made according to the environment in which the cryogenic valves operate.

3 Basic Theory

      The main factors affecting valve sealing are the structure of the sealing pair, the specific pressure on the sealing faces, the physical properties of the medium, and the quality of the sealing pair. Only by truly understanding the sealing principle of the valve and fully considering the various factors that affect its sealing performance can leakage be prevented and tightness be ensured.

      Taking flat-face sealing as an example, the tightness of a sealing-face connection is studied here to briefly explain the sealing principle. The principle of the sealing connection is shown in Figure 1, where a vessel is filled with liquid and gas at a certain pressure and closed with a cover plate. The static pressure of the medium in the vessel acts as

F_J=A×P

where  FJ ——— force exerted by the medium, N

A ——— area of the cover plate on which the medium pressure acts, mm2

P ——— static pressure of the medium in the vessel, MPa

1. Vessel   2. Cover plate

Figure 1  Principle of the sealing connection

      In order to keep the cover plate in the position shown in the figure, an external force F = FJ must be applied in the direction perpendicular to the contact surface between the vessel and the cover plate, but this alone can only ensure that the end faces are brought into contact. Only when the sealing faces are ideal planes will the medium be prevented from passing through the joint. To ensure the tightness of the contact surfaces, an interactive force must be produced between the sealing faces, that is, the cover plate must be pressed onto the vessel by force. When the acting force F > FJ, a certain specific pressure is produced on the jointed sealing faces, and this specific pressure deforms the existing flatness of the planes. If the deformation stays within the elastic limit of the material and produces only a small residual deformation, tightness can be ensured when the force F is applied to the contact surfaces. In addition to the sealing specific pressure, the factors ensuring the tightness of the connection also include the structure of the sealing pair, and so on. Among this series of factors, the value of the specific pressure between the sealing faces plays the key role.

4 Sealing Elements

      Although the structure of a ball valve is simple, because it is a valve sealed by the pressure of the medium itself and because of the special spherical structure of the ball, there are many elements that determine whether the ball valve is finally sealed.

4.1 Quality of the sealing pair

      The quality of the sealing pair of a ball valve is mainly reflected in the roundness of the ball and the surface roughness of the sealing faces of the ball and the seat. The roundness of the ball affects the degree of conformity between the ball and the seat. A high degree of conformity increases the resistance to fluid movement along the sealing faces, thereby improving the sealing performance. Generally, the roundness of the ball is required to be of grade 9.

      The surface finish of the sealing faces has a great influence on sealing. When the finish is poor and the specific pressure is small, the leakage increases. When the specific pressure is large, the influence of the finish on leakage decreases significantly, because the microscopic sawtooth-like peaks on the sealing surfaces are flattened. The influence of the surface finish of soft sealing faces on sealing performance is much smaller than that of rigid metal-to-metal seals. According to the view that fluid leakage can be prevented only when the gap between the sealing pair is smaller than the diameter of the fluid molecules, it can be considered that the gap required to prevent fluid leakage must be smaller than 0.003 μm. However, even on a finely lapped metal surface, the height of the asperities still exceeds 0.1 μm, i.e. 30 times larger than the diameter of a water molecule. It can thus be seen that improving the sealing performance solely by improving the surface finish of the sealing faces is in fact difficult to achieve. Besides sealing performance, 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.

4.2 Sealing specific pressure

      The sealing specific pressure refers to the pressure acting on a unit area of the sealing face. It is produced by the pressure difference between the upstream and downstream sides of the valve and by the externally applied sealing force. The magnitude of the specific pressure directly affects the sealing performance, reliability and service life of the ball valve. Leakage is inversely related to the pressure difference. Tests have proved that, with other conditions being equal, the leakage is inversely proportional to the square of the pressure difference, so the leakage decreases as the pressure difference increases. Since the pressure difference is a decisive factor for the sealing specific pressure, the sealing specific pressure is of vital importance to the sealing performance of cryogenic ball valves. On the other hand, the sealing specific pressure applied to the ball must not be too high: a higher pressure is beneficial to sealing, but it increases the operating torque of the valve. Therefore, a reasonable selection of the sealing specific pressure is the prerequisite for ensuring the sealing of cryogenic ball valves.

4.3 Physical properties of the fluid

(1) Viscosity

      The permeation capability of a fluid is closely related to its viscosity. With other conditions being equal, the higher the viscosity of the fluid, the lower its permeation capability. The viscosity of gases differs greatly from that of liquids. (1) The viscosity of a gas is tens of times lower than that of a liquid, so its permeation capability is stronger than that of a liquid. Saturated steam, however, is an exception: saturated steam can be sealed easily. (2) A compressed gas leaks more easily than a liquid.

(2) Temperature

      The permeation capability of a fluid depends on the temperature, which causes the viscosity to change. The viscosity of a gas increases with rising temperature and is proportional to the square root of the gas temperature. The viscosity of a liquid behaves in the opposite way: it decreases sharply with rising temperature and is inversely proportional to the cube of the temperature. In addition, changes in the dimensions of parts caused by temperature variations will change the sealing pressure within the sealing zone and can destroy the seal. This influence is especially significant for the sealing of low-temperature fluids. Since the sealing pair in contact with the fluid is usually at a lower temperature than the force-bearing parts, the components of the sealing pair contract and become slack. Sealing at low temperatures is complicated, and most sealing materials fail at low temperatures. Therefore, the influence of temperature must be taken into account when selecting sealing materials.

(3) Surface hydrophilicity

      The influence of surface hydrophilicity on leakage arises from the capillary characteristics. When the surface is covered with a very thin oil film, the hydrophilicity of the contact surfaces is destroyed and the fluid passages are blocked, so that a larger pressure difference is required for the fluid to pass through the capillaries. For this reason, some ball valves use sealing grease to improve the sealing performance and service life. When grease sealing is adopted, attention should be paid to replenishing the grease if the oil film diminishes during service. The grease used should be insoluble in the fluid medium and should not evaporate, harden or undergo any other chemical change. Sealing grease is not suitable for cryogenic ball valves, because under cryogenic service conditions most greases vitrify.

4.4 Structural dimensions

(1) Structure of the sealing pair

      Since a sealing pair is not absolutely rigid, its structural dimensions inevitably change under the action of the sealing force, temperature variations and other factors. This changes the interactive force between the components of the sealing pair, and the result is reduced sealing performance. To compensate for such changes, the sealing elements should be capable of a certain amount of elastic deformation. At present, some ball valve seats adopt structural forms with elastic compensation or metal elastic support, and some balls adopt an elastic-ball structure. All of these are active measures for improving the sealing performance.

(2) Width of the sealing faces

      The width of the sealing faces determines the length of the capillaries. When the width increases, the travel path of the fluid along the capillaries increases proportionally, while the leakage decreases proportionally. In practice this is not entirely true, because the contact surfaces of the sealing pair cannot conform to each other completely, and after deformation occurs the width of the sealing faces cannot play its sealing role over its full extent. On the other hand, an increase in the width of the sealing faces increases the required sealing force. Therefore, the reasonable selection of the sealing face width is also of importance.

(3) Dimensions of the sealing ring

      Cryogenic ball valves generally use PCTFE sealing rings. However, the linear expansion coefficient of PCTFE at low temperatures is far higher than that of metals, so at low temperatures a PCTFE sealing ring shrinks and becomes smaller in size. The result is a reduced sealing specific pressure against the ball and the formation of leakage passages between the ring and the seat. Therefore, the dimensions of the PCTFE sealing ring are also an important element affecting the sealing of cryogenic ball valves: the influence of dimensional shrinkage at low temperatures must be taken into account in the design, and cold-assembly techniques should also be adopted in manufacture.

5 Conclusion

      In view of the widespread internal leakage of cryogenic ball valves in existing LNG receiving terminals, and based on the design criteria for cryogenic valves and the basic theory of valve sealing, this paper has analyzed the elements affecting the sealing of cryogenic ball valves, namely the quality of the sealing pair, the sealing specific pressure, the physical properties of the fluid, and the structure and dimensions of the sealing pair. There are many other elements that affect the sealing of cryogenic ball valves, such as the rigidity of the ball and whether the ball center is concentric with the seat sealing face during assembly. The sealing specific pressure and the structure and dimensions of the sealing pair are the important elements affecting the sealing of cryogenic ball valves, and they must be given full consideration in the design.