Overview
Ball valves are widely used in various working conditions due to their simple structure, small installation space, and ball valves rely on medium force to seal and are not affected by external driving forces. At present, ultra-low temperature ball valves are generally used in LNG receiving stations. The number of ultra-low temperature ball valves accounts for 80% of the total number of valves in the entire LNG receiving station. There is a phenomenon of internal leakage of ultra-low temperature ball valves during use. 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 ultra-low temperature ball valves.
2 Design Guidelines
Due to the extremely low working temperature, the design and manufacture of ultra-low temperature valves face a series of technical difficulties, such as material selection, low temperature sealing, structural design, solution treatment, cryogenic treatment, heat insulation, quality inspection, maintenance, safety, etc. For this reason, there are a series of strict standards for the design of cryogenic valves. The international standard BS6364 "Cryogenic Valves" and MSSSP-134 "Requirements for Cryogenic Valves and their Valve Body/Bonnet Extensions" are mainly used in the world. The key points and rules for the design and manufacture of cryogenic valves are comprehensively specified. The standard JB/T7749 "Technical Conditions for Cryogenic Valves" is transformed from BS6364 "Cryogenic Valves".
When designing cryogenic valves, in addition to following the design principles of general valves, special requirements for cryogenic valve design should be followed according to the conditions of use.
① Valves should not be a significant source of heat in cryogenic systems. This is because the inflow of heat not only reduces the thermal efficiency, but if the inflow is too much, it will also cause the internal fluid to evaporate rapidly, resulting in an abnormal increase in pressure and causing danger.
②The low-temperature medium shall not have harmful effects on the handwheel operation and packing sealing performance.
③Valve assemblies directly in contact with low-temperature media should have explosion-proof and fire-proof structures.
④The valve assembly working at low temperature cannot be lubricated, so structural measures are required to prevent the friction parts from being scratched.
In the design process of cryogenic valves, in addition to considering the general requirements such as the flow capacity of cryogenic valves, some other indicators need to be considered in order to better evaluate the technical level of cryogenic valves. The technical level of cryogenic valves is usually evaluated by measuring whether the energy consumption is reasonable.
①Adiabatic performance of cryogenic valves.
② Cooling performance of cryogenic valves.
③Working performance of low temperature valve opening and closing seals.
④ The condition that the surface of the cryogenic valve does not freeze.
The working environment of cryogenic valves is very different from that of general valves. In the process of design, manufacture and inspection of cryogenic valves, in addition to following the general rules of valve design, manufacturing and inspection, attention should also be paid to the environment in which the cryogenic valves are located. Appropriate adjustments.
3 basic theory
The main factors affecting valve sealing are the structure of the sealing pair, the specific pressure of the sealing surface, the physical properties of the medium, and the quality of the sealing pair. But only when we really understand the principle of valve sealing and fully consider various factors that affect its sealing performance, can we prevent leakage and ensure sealing.
Taking the flat seal as an example, the sealing problem of the sealing surface connection is studied, and the sealing principle is briefly explained. The principle of sealed connection is shown in Figure 1, in which the container is filled with liquid and gas with a certain pressure and sealed with a cover plate. The static pressure of the medium in the container is: FJ=A×P
In the formula, FJ——medium force, N
A——the area of the medium acting on the cover plate, mm2
P——Static pressure of the medium in the container, MPa
In order to keep the cover plate in the position shown in the figure, an external force F=FJ must be applied in the vertical direction of the contact surface of the container and the cover plate, so that only the end faces can be guaranteed. Only when the sealing surface is an ideal plane, the medium will not pass through between the joint surfaces. In order to ensure the tightness of the contact surface, an interaction force must be generated between the sealing surfaces, that is, the cover is pressed against the container by force. When the force F>FJ, a certain specific pressure will be generated on the combined sealing surface, and the existing flatness on the plane will be deformed by relying on the specific pressure. If the deformation is within the elastic limit range of the material and there is little residual deformation, the sealing can be guaranteed when the force F is applied to the contact surface. In addition to the specific pressure of the seal, the factors to ensure the tightness of the connection also include the sealing structure and so on. But in this series of factors, the specific pressure value between the sealing surfaces plays a key role.
4 Sealing elements
Although the structure of the ball valve is simple, because it is a medium pressure self-sealing valve and the special structure of the ball, there are many factors that affect whether the ball valve is finally sealed.
4.1 Seal secondary quality
The quality of the ball valve sealing pair is mainly manifested in the roundness of the ball and the surface roughness of the sealing surface between the ball and the valve seat. The roundness of the ball affects how well the ball fits into the seat. If the degree of fit is high, the resistance of the fluid to move along the sealing surface will be increased, thereby improving the sealing performance. Generally, the roundness of the sphere is required to be grade 9.
The surface finish of the sealing surface has a great influence on the sealing. When the smoothness is low and the specific pressure is small, the leakage will increase. However, when the specific pressure is large, the effect of the finish on the leakage is significantly reduced, because the microscopic jagged peaks on the sealing surface are flattened, and the smoothness of the soft sealing surface has a greater effect on the sealing performance than the rigidity of metal to metal The seal is much smaller. From the point of view that only when the gap between the sealing pairs is smaller than the molecular diameter of the fluid can the fluid not leak, it can be considered that the gap to prevent fluid leakage must be less than 0.003 μm. However, the height of the peaks on even a finely ground metal surface is still over 0.1 μm, which is 30 times larger than the diameter of a water molecule. It can be seen that it is actually difficult to improve the sealing performance only by improving the smoothness of the sealing surface. The quality of the sealing pair not only affects the sealing performance, but also directly affects the service life of the ball valve. Therefore, the quality of the sealing pair must be improved during manufacture.
4.2 Seal specific pressure
Seal specific pressure refers to the pressure acting on the unit area of the sealing surface. The sealing specific pressure is generated by the pressure difference between the front and back of the valve and the external sealing force. The specific pressure directly affects the sealing performance, reliability and service life of the ball valve. The amount of leakage is inversely proportional to the pressure difference. Experiments have proved that, under other conditions being the same, the amount of leakage is inversely proportional to the square of the pressure difference, therefore, the amount of leakage will decrease with the increase of the pressure difference. The pressure difference is an important factor to determine the sealing specific pressure, so the sealing specific pressure is very important for the sealing performance of the cryogenic ball valve. The sealing specific pressure applied to the ball should not be too large, too large is beneficial to sealing, but will increase the operating torque of the valve, so a reasonable selection of sealing specific pressure is the premise to ensure the sealing of the ultra-low temperature ball valve.
4.3 Physical properties of fluids
1) Viscosity
The penetrability of a fluid is closely related to its viscosity. Under other conditions being the same, the greater the viscosity of the fluid, the smaller its penetrating ability. Gases and liquids have very different viscosities. ①The viscosity of gas is dozens of times smaller than that of liquid, so its penetration ability is stronger than that of liquid. But saturated steam is an exception, and saturated steam is easy to ensure sealing. ②Compressed gas is easier to leak than liquid.
(2) temperature
The penetrability of a fluid depends on the temperature causing the change in viscosity. The viscosity of a gas increases with the increase of temperature, which is proportional to the root of the temperature of the gas. The viscosity of the liquid is the opposite, it decreases sharply with the increase of temperature, and it is inversely proportional to the cube of the temperature. In addition, changes in part dimensions due to temperature changes will cause changes in seal pressure in the seal area and can damage the seal. Its influence is particularly significant for the sealing of cryogenic fluids. Because the sealing pair in contact with the fluid is usually cooler than the stress member, this causes the sealing pair to shrink and relax. In the low temperature state, its sealing is complicated, and most sealing materials fail at low temperature. Therefore, the influence of temperature should be considered when selecting the sealing material.
3) Surface hydrophilicity
The effect of surface hydrophilicity on leakage is caused by the characteristics of capillary pores. When there is a thin layer of oil film on the surface, it will destroy the hydrophilicity of the contact surface and block the fluid channel, which requires a large pressure difference to Pass fluid through capillary pores. Therefore, some ball valves use sealing grease to improve sealing and service life. When using grease seal, it should be noted that if the oil film decreases during use, the grease should be replenished. The grease used should be insoluble in the fluid medium, and should not evaporate, harden or undergo other chemical changes. Cryogenic ball valves are not suitable for sealing grease, and most of the grease will vitrify under ultra-low temperature conditions.
4.4 Structural dimensions
(1) Sealing structure
Since the sealing pair is not absolutely rigid, its structural size will inevitably change under the influence of sealing force or temperature change, which will change the interaction force between the sealing pairs, resulting in reduced sealing performance. In order to compensate for this change, the seal should have a certain elastic deformation. At present, some ball valve seats adopt the structural form with elastic compensation or metal elastic support, and some balls also adopt elastic ball structure. These are an aggressive form of improved sealing performance.
(2) Width of sealing surface
The width of the sealing surface determines the length of the capillary pores. When the width increases, the movement distance of the fluid along the capillary increases proportionally, while the leakage decreases inversely. But this is not the case in fact, because the contact surfaces of the sealing pair cannot be completely matched, and when deformation occurs, the width of the sealing surface cannot fully effectively play a sealing role. On the other hand, the increase in the width of the sealing surface will increase the required sealing force, so it is also important to choose the width of the sealing surface reasonably.
(3) Dimensions of sealing ring
Ultra-low temperature ball valves generally use PCTFE sealing rings, and the linear expansion coefficient of PCTFE at low temperatures is much higher than that of metals. Therefore, at low temperatures, PCTFE sealing rings will shrink in size, resulting in a decrease in the sealing specific pressure with the ball and A leakage path is created between it and the valve seat. Therefore, the size of the PCTFE sealing ring is also an important factor affecting the sealing of the ultra-low temperature ball valve. The impact of dimensional shrinkage at low temperature should be considered in the design, and the cold assembly process should be adopted in the process.
5 Conclusion
In view of the widespread internal leakage of ultra-low temperature ball valves in existing LNG receiving stations, based on the design principles of cryogenic valves and the basic theory of valve sealing, the quality of the sealing pair, the specific pressure of the seal, the physical properties of the fluid, and the structure and size of the sealing pair affect the sealing of the ultra-low temperature ball valve. elements are analyzed. There are many other factors that affect the sealing of ultra-low temperature ball valves, such as the rigidity of the ball and whether the center of the ball is concentric with the sealing surface of the valve seat during assembly. The sealing specific pressure and the structure and size of the sealing pair are important factors affecting the sealing of the cryogenic ball valve, which must be fully considered in the design.