The ball valve is simple in structure, small in installation space, and sealed by medium force, not affected by external driving force, so it is widely used in various working conditions. At present, ultra cryogenic ball valves are widely used in LNG terminals, and the number of ultra cryogenic ball valves accounts for 80% of the total number of valves in LNG terminals. There is an internal leakage of ultra cryogenic ball valves in use. Based on the design criteria of cryogenic ball valve and the basic theory of valve sealing performance, this article analyzes the factors affecting the sealing of ultra cryogenic ball valve.
Design of Cryogenic Ball Valve Sealing
Due to the extremely low working temperature, the design and manufacture of ultra cryogenic ball valve are faced with a series of technical problems, such as material selection, low temperature sealing, structural design, solution treatment, cryogenic treatment, heat insulation, quality inspection, maintenance, safety, etc.
Therefore, there are a series of strict standards for the design of low-temperature valves. Internationally, standards BS6364 “Cryogenic Valve” and MSSP-134 “Valves for Cryogenic Service, Including Requirements for Body/bonnet Extensions” are mainly adopted. These two standards stipulate the key points and rules of design and manufacture of cryogenic ball valve. The standard JB / t7749 “Cryogenic Valve Technical Specifications”is transformed according to BS6364 “Cryogenic Valve”.
In the design of cryogenic ball valve, in addition to the general valve design principles, the special requirements of cryogenic ball valve design shall be followed according to the use conditions.
- The valve shall not become a significant heat source of the low temperature system, because the heat inflow will not only reduce the thermal efficiency, but also cause the internal fluid to evaporate rapidly if the inflow is too much, resulting in abnormal pressure rise and danger.
- Low temperature medium shall not have harmful effect on handwheel operation and packing sealing performance.
- The valve assembly in direct contact with low-temperature medium shall have explosion-proof and fire-proof structure.
- The valve assembly working at low temperature cannot be lubricated, so structural measures shall be taken to prevent abrasion of friction parts.
In the design process of cryogenic ball valve, in addition to the general requirements such as the circulation capacity of cryogenic ball valve, some other indicators need to be considered in order to better evaluate the technical level of cryogenic ball valve.
Generally, the technical level of cryogenic ball valve is evaluated by measuring whether the energy consumption is reasonable.
- Insulation performance of cryogenic ball valve.
- Cooling performance of cryogenic ball valve.
- Working performance of cryogenic ball valve seal.
- The condition that the surface of cryogenic ball valve is not frozen.
The working environment of cryogenic ball valve is quite different from that of general valve. In the process of design, manufacture and inspection of low-temperature valve, in addition to the general rules of valve design, manufacture and inspection, it is also necessary to pay attention to make appropriate adjustment according to the environment of low-temperature valve.
Basic Theory of Cryogenic Ball Valve Sealing
The main factors affecting the 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, etc. However, only when the sealing principle of the valve and various factors affecting its sealing performance are fully considered can the leakage be prevented to ensure sealing.
Taking the plane seal as an example, study the sealing problem of the connection of the sealing surface, and the sealing principle is simply explained. The principle of sealing connection is shown in Figure 1. The container is filled with liquid and gas with a certain pressure and it is sealed with a cover plate. The static pressure of the medium in the container is FJ = a × P.
FJ — medium force, N
A — area of medium acting on cover plate, mm2
P — static pressure of medium in the vessel, 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 between the container and the cover plate, so as to ensure only the end face fit. Only when the sealing surface is an ideal plane, the medium can not pass through the joint surface. In order to ensure the sealing of the contact surface, mutual force must be generated between the sealing surfaces, that is, the cover plate shall be tightly pressed on the container. When the force F > FJ, a certain specific pressure will be produced on the combined sealing surface, and the existing flatness on the plane will be deformed by the specific pressure. If the deformation is within the elastic limit of the material, and there is little residual deformation, when the force F is applied to the contact surface, the sealing property can be guaranteed. In addition to the specific pressure of the seal, the factors to ensure the tightness of the connection also include the structure of the seal pair and so on. But in this series of factors, the specific pressure between sealing surfaces plays a key role.
Sealing Elements of Cryogenic Ball Valve
Although the structure of ball valve is simple, it is a self sealing valve with medium pressure and the special structure of the ball, so there are many factors that affect the final sealing of the ball valve.
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 surface between the ball and the valve seat. The roundness of the ball affects the fit between the ball and the valve seat. If the degree of fit is high, the resistance of the fluid moving along the sealing surface will be increased, so as to improve the sealing performance. Generally, the roundness of the sphere is required to be grade 9.
The surface finish of sealing surface has a great influence on sealing. When the surface finish is low and the specific pressure is small, the leakage increases. When the specific pressure is large, the influence of the finish on the leakage is significantly reduced, because the micro serrated peak on the sealing surface is flattened, and the influence of the finish of the soft sealing surface on the sealing performance is much smaller than that of the metal to metal rigid sealing.
According to the viewpoint that only when the gap between the sealing pairs is smaller than the diameter of the fluid molecule can the fluid be kept from leaking, it can be considered that the gap to prevent the fluid from leaking must be less than 0.003 μ M. However, even after fine grinding, the height of metal surface crest is still more than 0.1 μ m, which is 30 times larger than the diameter of water molecule.
Therefore, it is difficult to improve the sealing performance only by improving the sealing surface finish. The quality of sealing pair not only affects the sealing performance, but also directly affects the service life of ball valve. Therefore, the quality of sealing pair must be improved in manufacturing.
2. Seal Specific Pressure
Sealing specific pressure refers to the pressure acting on unit area of sealing surface. The specific pressure of the seal is caused by the pressure difference between the front and the back of the valve and the external sealing force. The specific pressure directly affects the sealing, reliability and service life of the ball valve. The leakage is inversely proportional to the pressure difference. The test shows that under the same other conditions, the leakage is inversely proportional to the square of the pressure difference, so the leakage will decrease with the increase of the pressure difference.
The pressure difference is an important factor to determine the specific pressure of the seal. Therefore, the specific pressure of sealing is very important for the sealing performance of ultra cryogenic ball valve. The specific pressure of the seal applied to the sphere shall not be too large. If it is too large, it is good for sealing, but it will increase the operating torque of the valve, so the reasonable selection of the sealing pressure is the premise to ensure the sealing of ultra cryogenic ball valve.
3. Physical Properties of Fluid
(1) Viscosity
The permeability of fluid is closely related to its viscosity. Under the same conditions, the higher the viscosity of the fluid, the smaller its permeability. The viscosity of gas and liquid is very different. ① The viscosity of gas is dozens of times smaller than that of liquid, so its permeability is stronger than that of liquid except for saturated steam. Saturated steam is easy to ensure sealing. ② Compressed gases leak more easily than liquids.
(2) Temperature
The permeability of a fluid depends on the temperature at which the viscosity changes. The viscosity of gas increases with the increase of temperature, which is proportional to the square root of gas temperature. On the contrary, the viscosity of liquid decreases sharply with the increase of temperature, which is inversely proportional to the cube of temperature. In addition, the change of part size caused by the change of temperature will lead to the change of sealing pressure in the sealing area and damage the sealing. For the sealing of low temperature fluid, the effect is particularly significant. The temperature of the seal pair in contact with the fluid is usually lower than that of the stressed parts, so that the seal pair parts can contract and it can be loose. At low temperature, the sealing is complex, and most sealing materials fail at low temperature. Therefore, the influence of temperature should be considered when choosing sealing materials.
(3) Surface hydrophilicity
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 large pressure difference is needed to make the fluid pass through the capillary.
Therefore, some ball valves use sealing grease to improve sealing performance and service life. When using grease seal, pay attention to supplement grease if the oil film is reduced during the use. The grease used shall be insoluble in the fluid medium and shall not evaporate, harden or have other chemical changes. For low-temperature ball valve, sealing grease is not suitable. Under ultra-low temperature condition, most of the grease will be vitrified.
4. Structural Dimension
(1) Seal Sub Structure
The sealing pair is not absolutely rigid. Under the influence of sealing force or temperature change and other factors, the structure size will inevitably change, which will change the interaction between the sealing pairs, and the result is the decrease of sealing performance. In order to compensate for this change, the seal is of some elastic deformation. At present, some ball valve seats adopt the structural form with elastic compensation or metal elastic support, and some ball bodies also adopt the elastic ball structure. These are all positive ways to improve sealing performance.
(2) Width of Sealing Surface
The width of the sealing surface determines the length of the pores. When the width increases, the flow path along the pores increases in direct proportion, while the leakage decreases in inverse proportion. But this is not the fact. The contact surface of the sealing pair can not be completely consistent. When the deformation occurs, the width of the sealing surface can not play an effective role in sealing. On the other hand, with the increase of sealing surface width, the sealing force needed should be increased, so it is important to choose the sealing surface width reasonably.
(3) Seal Ring Size
PCTFE seal ring is widely used in ultra cryogenic ball valve. However, the linear expansion coefficient of PCTFE is much higher than that of metal at low temperature, so the seal ring of PCTFE will shrink and its size will be smaller at low temperature. As a result, the specific pressure of the ball decreases and a leakage channel is generated between the ball and the valve seat. Therefore, the size of PCTFE seal ring is also an important factor affecting the sealing of ultra cryogenic ball valve. The influence of size shrinkage at low temperature should be considered in the design, and the cold assembly process should be adopted in the process.
Conclusion of Cryogenic Ball Valve Sealing
Based on the design criteria of cryogenic valve and the basic theory of valve sealing, this paper analyzes the factors affecting the sealing of cryogenic ball valve, such as the quality of sealing pair, the specific pressure of sealing, the physical properties of fluid and the structure and size of sealing pair.
English 
German 
Spanish 
French 
Russian 
Portuguese 
Turkish