In modern industry, valves act like the faucets of fluid systems, controlling the flow of fluids. Their performance and reliability are directly related to the safety and efficiency of the entire system. Especially in places like Liquefied Natural Gas (LNG) receiving stations, where the requirements for temperature and sealing are extremely high, cryogenic ball valves are the most critical among all. Today, let's talk in detail about the key elements affecting the sealing performance of cryogenic ball valves and how to optimize them, hoping to provide some reference for technical improvement in related industries.
Because of their simple structure, small installation space, and self-sealing capability by the medium pressure, ball valves are widely used in many industrial scenarios. In LNG receiving stations, cryogenic ball valves account for 80% of the total number of valves. However, with prolonged use, internal leakage of cryogenic ball valves gradually emerges, which not only affects the normal operation of the system but may also cause safety hazards. Therefore, understanding the sealing performance of cryogenic ball valves, identifying the key factors affecting sealing, and taking effective optimization measures have become urgent tasks.
To understand the sealing performance of cryogenic ball valves, it is necessary to comprehensively analyze the factors that influence it. These factors are interrelated and jointly determine the sealing effect of the ball valve under extremely low-temperature conditions. Only by clarifying these key elements can we carry out targeted optimization and improvement.
The quality of the sealing pair is the fundamental factor affecting the sealing performance of cryogenic ball valves. The roundness of the ball and the surface roughness of the sealing surface between the ball and the valve seat are key indicators for evaluating sealing pair quality. The roundness of the ball directly affects the fit between the ball and the valve seat. A high degree of fit can increase the resistance of fluid movement along the sealing surface, thereby improving sealing performance. Usually, the roundness of the ball is required to reach grade 9. The surface finish of the sealing surface also has a significant impact on sealing performance. When the surface finish is low and the specific pressure is small, the leakage rate will increase significantly. However, even finely polished metal surfaces may have peak heights exceeding 0.1 μm, which is far greater than the diameter of a water molecule. This indicates that it is unrealistic to rely solely on improving the surface finish to enhance sealing performance. In addition, the quality of the sealing pair not only affects the sealing performance but is also directly related to the service life of the ball valve. Therefore, during the manufacturing process, the quality of the sealing pair must be strictly controlled.
Sealing specific pressure refers to the pressure acting on the unit area of the sealing surface, which is generated by the pressure difference between the front and rear of the valve and the externally applied sealing force. The magnitude of the sealing specific pressure directly affects the sealing performance, reliability, and service life of the ball valve. The leakage rate is inversely proportional to the pressure difference, that is, under the same conditions, the leakage rate is inversely proportional to the square of the pressure difference. This means that as the pressure difference increases, the leakage rate decreases accordingly. Therefore, selecting an appropriate sealing specific pressure is the key to ensuring the sealing performance of cryogenic ball valves. It should be noted that although higher sealing specific pressure is beneficial for sealing, it also increases the operating torque of the valve, thus affecting normal operation. Therefore, during design and use, it is necessary to find a balance between sealing performance and operational convenience.
The permeability of a fluid is closely related to its viscosity. Under the same conditions, the greater the viscosity of the fluid, the lower its permeability. The viscosity of gases is usually dozens of times smaller than that of liquids, so gases have stronger permeability. However, saturated steam is an exception, and it is relatively easy to seal. In addition, compressed gases are more prone to leakage than liquids. The permeability of a fluid is also affected by temperature. The viscosity of gases increases with temperature, being proportional to the square root of the gas temperature, while the viscosity of liquids sharply decreases with temperature, being inversely proportional to the cube of the temperature. Temperature changes can also cause dimensional changes in parts, thereby affecting the sealing pressure within the sealing zone and even destroying the seal. This effect is particularly significant for the sealing of cryogenic fluids. Therefore, when selecting sealing materials, the influence of temperature must be fully considered.
The structure and dimensions of the sealing pair have an important impact on the sealing performance of cryogenic ball valves. The width of the sealing surface determines the length of the capillary pores. Theoretically, the wider the sealing surface, the longer the distance the fluid travels along the capillary pores, and the smaller the leakage should be. However, in practice, this is not entirely the case, because the contact surfaces of the sealing pair cannot fully match, and the width of the deformed sealing surface cannot all effectively contribute to sealing. Furthermore, increasing the sealing surface width also increases the required sealing force. Therefore, it is crucial to select an appropriate sealing surface width. At present, some ball valve seats adopt structural forms with elastic compensation or metal elastic support, and even the ball body adopts an elastic ball structure. These designs all help improve sealing performance.
Cryogenic ball valves generally use PCTFE sealing rings. However, the linear expansion coefficient of PCTFE at low temperatures is much higher than that of metal, resulting in dimensional shrinkage at low temperatures, which reduces the sealing specific pressure with the ball and may create leakage channels between the valve seats. Therefore, the size of the PCTFE sealing ring is an important factor affecting the sealing performance of cryogenic ball valves. During design, the effect of dimensional shrinkage at low temperatures must be considered, and cold assembly processes should be adopted to ensure sealing performance under cryogenic conditions.
After an in-depth analysis of the factors affecting the sealing performance of cryogenic ball valves, let's discuss how to enhance their sealing performance through effective optimization strategies. These strategies are developed from multiple perspectives to address sealing issues encountered in practical applications, ensuring stable and reliable operation under extreme conditions.
During manufacturing, the roundness of the ball and the surface roughness of the sealing face must be strictly controlled. High-precision machining equipment and advanced testing technology should be used to ensure that the roundness of the ball meets the grade 9 standard. At the same time, by optimizing machining processes, the smoothness of the sealing surface can be improved, reducing microscopic serrated peaks. In addition, advanced surface treatment technologies such as coating technology can be used to further improve the wear resistance and corrosion resistance of the sealing surface, thereby extending the service life of the ball valve.
When designing cryogenic ball valves, sealing specific pressure should be calculated and selected reasonably according to actual operating conditions. Through precise mechanical analysis and simulation experiments, the optimal range of sealing specific pressure can be determined to ensure that sealing requirements are met without increasing operating torque excessively. During actual use, changes in sealing specific pressure should be regularly monitored, and the external sealing force should be adjusted in time to cope with possible changes in sealing performance.
Adopting sealing pair structures with elastic compensation functions, such as elastic valve seats or elastic ball structures, can effectively compensate for dimensional changes caused by temperature variations or sealing force, thereby maintaining good sealing performance. In addition, the contact area and shape of the sealing pair can be optimized to maintain optimal sealing under various working conditions. For example, asymmetric sealing surface designs can automatically adjust the contact state under different pressures, enhancing sealing performance.
When selecting sealing materials, the influence of low-temperature environments must be fully considered. In addition to PCTFE sealing rings, other materials with low thermal expansion coefficients and high cryogenic resistance, such as polytetrafluoroethylene (PTFE) or certain special rubber materials, can also be considered. These materials have better dimensional stability and sealing performance at low temperatures. At the same time, the selected sealing materials must be compatible with the fluid medium to avoid dissolution, evaporation, hardening, or other chemical changes. In practical applications, the performance of sealing materials should be tested and optimized according to specific working conditions and sealing requirements.
In the manufacturing process of cryogenic ball valves, a strict quality control system should be established to ensure that each production step meets design requirements and that the product's quality and performance are guaranteed. During operation, ball valves should be regularly maintained and inspected to promptly identify and address potential sealing problems. For example, regularly inspect the wear condition of the sealing surface and replace severely worn sealing parts in time; monitor changes in sealing specific pressure and adjust the external sealing force as needed; inspect the aging condition of sealing materials and replace aged materials promptly. Through enhanced quality control and maintenance, the service life of cryogenic ball valves can be effectively extended, and their sealing performance and reliability improved.
Cryogenic ball valves are widely used in low-temperature conditions such as LNG receiving stations, but optimizing their sealing performance remains an ongoing challenge for the industry. By thoroughly analyzing key factors affecting sealing performance, such as sealing pair quality, sealing specific pressure, fluid physical properties, sealing pair structure and dimensions, and sealing material selection, and implementing corresponding optimization strategies, the sealing performance and reliability of cryogenic ball valves can be significantly improved. In practical applications, various factors should be comprehensively considered in combination with specific working conditions to formulate reasonable optimization plans while strengthening quality control and maintenance to ensure the long-term stable operation of cryogenic ball valves in low-temperature environments. With continuous technological advancement and innovation, the sealing performance of cryogenic ball valves is expected to improve further, providing more reliable support for the development of cryogenic industries.