1. Overview
Ball valves are widely used in various working conditions due to their simple structure, small installation space, and reliance on medium force for sealing without being affected by external driving forces. Currently, cryogenic ball valves are commonly adopted in LNG receiving terminals, accounting for 80% of the total number of valves in such terminals. However, internal leakage of cryogenic ball valves occurs during operation. Based on the design criteria of cryogenic valves and the basic theory of valve sealing performance, this paper analyzes the factors affecting the sealing of cryogenic ball valves.
2. Design Criteria
The extremely low operating temperature poses a series of technical challenges to the design and manufacturing of cryogenic valves, such as material selection, low-temperature sealing, structural design, solution treatment, deep cooling treatment, thermal insulation, quality inspection, maintenance, and safety. Therefore, there are a series of strict standards for the design of cryogenic valves. Internationally, the main standards used are
BS6364 "Cryogenic Valves" and MSSSP-134 "Requirements for Cryogenic Valves and Their
Extended Body/Bonnet". These two standards comprehensively specify the key points and rules for the design and manufacturing of cryogenic valves. The standard JB/T7749 "Technical Specifications for Cryogenic Valves" is transformed from BS6364 "Cryogenic Valves".
In the design of cryogenic valves, in addition to following the general valve design principles, special requirements for cryogenic valve design should be complied with according to the operating conditions.
① The valve should not be a significant heat source for the low-temperature system. This is because heat inflow not only reduces thermal efficiency but also causes rapid evaporation of internal fluid if excessive, leading to abnormal pressure rise and potential danger.
② The low-temperature medium should not have a harmful impact on handwheel operation and packing seal performance.
③ Valve assemblies in direct contact with low-temperature media should have explosion-proof and fire-resistant structures.
④ Valve assemblies operating at low temperatures cannot be lubricated, so structural measures need to be taken to prevent abrasion of friction parts.
In the design process of cryogenic valves, in addition to considering general requirements such as flow capacity, other indicators need to be taken into account to better evaluate the technical level of cryogenic valves. Usually, the technical level of cryogenic valves is evaluated by measuring the rationality of energy consumption.
① Thermal insulation performance of cryogenic valves.
② Cooling performance of cryogenic valves.
③ Working performance of opening and closing seals of cryogenic valves.
④ Conditions for no icing on the surface of cryogenic valves.
The working environment of cryogenic valves is quite different from that of general-purpose valves. In the process of design, manufacturing, and inspection of cryogenic valves, in addition to complying with the general rules for valve design, manufacturing, and inspection, appropriate adjustments should be made according to the environment of cryogenic valves.
3. Sealing Elements
Although the structure of ball valves is simple, as they are medium pressure self-sealing valves and have a special ball structure, there are many factors affecting the final sealing performance of ball valves.
3.1 Quality of Sealing Pair
The quality of the ball valve sealing pair is mainly reflected in the roundness of the ball and the surface roughness of the sealing surfaces of the ball and valve
seat. The roundness of the ball
affects the fit degree between the ball and the valve seat. A higher fit degree increases the resistance of fluid movement along the sealing surface, thereby improving sealing performance. Generally, the roundness of the ball is required to be grade 9.
The surface finish of the sealing surface has a significant impact on sealing. When the finish is low and the specific pressure is small, the leakage increases. When the specific pressure is large, the impact of finish on leakage decreases significantly because the microscopic serrated peaks on the sealing surface are flattened. The impact of the finish of the soft sealing surface on sealing performance is much smaller than that of metal-to-metal rigid sealing.
Based on the view that fluid leakage can be prevented only when the gap between the sealing pairs is smaller than the fluid molecule diameter, it can be considered that the gap to prevent fluid leakage must be less than 0.003 μm. However, even the peak height of a finely ground metal surface still exceeds 0.1 μm, which is 30 times larger than the diameter of water molecules. It can be seen that it is actually difficult to improve sealing performance only by increasing the surface finish of the sealing surface. In addition to affecting sealing performance, the quality of the sealing pair directly affects the service life of the ball valve. Therefore, the quality of the sealing pair must be improved during manufacturing.
3.2 Sealing Specific Pressure
Sealing specific pressure refers to the pressure acting on the unit area of the sealing surface. It is generated by the pressure difference between the front and rear of the valve and the external sealing force. The magnitude of specific pressure directly affects the sealing performance, reliability, and service life of the ball valve. Leakage is inversely proportional to the pressure difference. Tests have shown that under the same other conditions, leakage is inversely proportional to the square of the pressure difference, so leakage decreases with the increase of pressure difference. Since the pressure difference is an important factor determining the sealing specific pressure, the sealing specific pressure is crucial for the sealing performance of cryogenic ball valves. The sealing specific pressure applied to the ball should not be too large. Although a larger specific pressure is beneficial to sealing, it will increase the valve operating torque. Therefore, the reasonable selection of sealing specific pressure is the premise to ensure the sealing of cryogenic ball valves.
3.3 Physical Properties of Fluid
3.3.1 Viscosity
The permeability of fluid is closely related to its viscosity. Under the same other conditions, the higher the fluid viscosity, the lower its permeability. The viscosity of gas and liquid differs greatly. ① The viscosity of gas is dozens of times smaller than that of liquid, so its permeability is stronger than that of liquid. However, saturated steam is an exception, which is easy to seal. ② Compressed gas is more prone to leakage than liquid.
3.3.2 Temperature
The permeability of fluid depends on the temperature that causes viscosity change. The viscosity of gas increases with the rise of temperature and is proportional to the square root of the gas temperature. On the contrary, the viscosity of liquid decreases sharply with the rise of temperature and is inversely proportional to the cube of temperature. In addition, the change in part dimensions caused by temperature change will lead to changes in the sealing pressure in the sealing area and may damage the seal. Its impact is particularly significant for the sealing of low-temperature fluids. Because the sealing pair in contact with the fluid is usually lower in temperature than the force-bearing parts, this causes the sealing pair components to shrink and relax. Sealing at low temperatures is complex, and most sealing materials fail at low temperatures. Therefore, the influence of temperature should be considered when selecting sealing materials.
3.3.3 Surface Hydrophilicity
The impact of surface hydrophilicity on leakage is caused by the characteristics of capillary pores. When there is a thin oil film on the surface, the hydrophilicity of the contact surface is destroyed and the fluid channel is blocked, so a larger pressure difference is required to make the fluid pass through the capillary pores. Therefore, some ball valves use sealing grease to improve sealing performance and service life. When using grease for sealing, attention should be paid to supplementing grease if the oil film decreases during use. The grease used should not be soluble in the fluid medium, nor should it evaporate, harden, or undergo other chemical changes. Cryogenic ball valves are not suitable for using sealing grease, as most greases will vitrify under ultra-low temperature conditions.
3.4 Structural Dimensions
3.4.1 Structure of Sealing Pair
Since the sealing pair is not absolutely rigid, its structural dimensions will inevitably change under the action of sealing force or temperature change. This will change the interaction force between the sealing pairs, resulting in reduced sealing performance. 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 structure. These are positive forms to improve sealing performance.
3.4.2 Width of Sealing Surface
The width of the sealing surface determines the length of the capillary pores. When the width increases, the path of fluid movement along the capillary pores increases proportionally, while the leakage decreases inversely. But in fact, this is not the case because the contact surfaces of the sealing pairs cannot be fully fitted. After deformation, the width of the sealing surface cannot fully play an effective sealing role. On the other hand, the increase in the width of the sealing surface requires an increase in the required sealing force. Therefore, the reasonable selection of the width of the sealing surface is also important.
3.4.3 Size of Sealing Ring
Cryogenic ball valves generally use PCTFE sealing rings. The linear expansion coefficient of PCTFE at low temperatures is much higher than that of metal. Therefore, the PCTFE sealing ring will shrink at low temperatures, resulting in reduced sealing specific pressure with the ball and the formation of leakage channels with the valve seat. Therefore, the size of the PCTFE sealing ring is also an important factor affecting the sealing of cryogenic ball valves. The influence of size shrinkage at low temperatures should be considered in the design, and the cold assembly process should be adopted in the process.
4. Conclusion
Aiming at the common internal leakage phenomenon of cryogenic ball valves in existing LNG receiving terminals, based on the design criteria of cryogenic valves and the basic theory of valve sealing, this paper analyzes the factors affecting the sealing of cryogenic ball valves, including the quality of the sealing pair, sealing specific pressure, physical properties of the fluid, and the structure and size of the sealing pair. There are many other factors affecting the sealing of cryogenic ball valves, such as the stiffness
of the ball and whether the ball center is
concentric with the sealing surface of the valve seat during assembly. Sealing specific pressure and the structure and size of the sealing pair are important factors affecting the sealing of cryogenic ball valves, which must be fully considered in the design.
