Sealing Elements of Cryogenic Ball Valves (Part One)

1. Overview
A ball valve is widely used in various working conditions due to its simple structure, small installation space and relying on medium force to seal and not being affected by external driving force. At present, cryogenic ball valves are commonly used in LNG receiving stations. The cryogenic ball valve accounts for 80% of the number of valves in the entire LNG receiving station. There is a phenomenon of internal leakages for cryogenic ball valves in use. The factors that affect the sealing of cryogenic ball valves are analyzed in this article based on the design criteria of cryogenic valves and the basic theory of valve sealing performance.
 
2. Design criteria
The design and manufacture of ultra-low temperature valves faces a series of technical problems due to the extremely low working conditions, 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. Internationally, the BS6364 standard Cryogenic Valves and MSSSP-134 Requirements for Cryogenic Valves and Valve Body and Bonnet Extensions are mainly adopted. The key points and rules for the design and manufacture of cryogenic valves are fully specified. The JB/T7749 standard of Technical Conditions for Cryogenic Valves is edited based on the BS6364 Cryogenic Valves.
 
When designing cryogenic valves, in addition to following general valve design principles, the following special requirements for cryogenic valve design should be met according to the conditions of use:
 
① The valve should not become a significant heat source for the cryogenic system. This is because the inflow of heat not only reduces the thermal efficiency, but also make the internal fluid evaporate rapidly if the inflow is too much, causing abnormal pressure rises and danger.
② The low temperature medium shouldn’t cause harmful effects on the operation of hand wheels and sealing performance of the packing.
③ The valve assembly which is directly in contact with cryogenic media should have structure of explosion-proof and fire prevention.
④ The valve assembly working at low temperature cannot be lubricated, so structural measures need to be taken to prevent the friction parts from scratching.
 
In the cryogenic valve design process, in addition to considering the general requirements, for example, the flow capacity of the cryogenic valve, some other indicators also need to be considered in order to better evaluate the technical level of the cryogenic valve. The technical level of cryogenic valves is usually evaluated by measuring whether energy consumption is reasonable.
 
① Thermal insulation performance of cryogenic valves
② The cooling performance of cryogenic valves
③ The working performance of opening and closing seals of cryogenic valves
④ The condition that the surface of cryogenic valves does not freeze.
 
There is a big difference between the working environment of cryogenic valves and general valves. In the process of design, manufacturing and inspection of cryogenic valves, in addition to complying with the general rules of valve design, manufacturing and inspection, attention should also be paid to the environment in which the cryogenic valve is located and make appropriate adjustments.
 
3. Basic theories
The main factors affecting valve sealing are the structure of sealing pairs, specific pressure of sealing surfaces, physical properties of media and quality of sealing pairs. However, only by fully understanding the sealing principle of the valve and fully considering various factors that affect the sealing performance, can leakages be prevented and ensure good sealing.
 
Take the plane seal as an example to study the sealing problem of the sealing surface connection, and briefly explain the sealing principle. The principle of the sealing connection is shown in the Figure 1. The container is filled with liquid and gas with certain pressure and sealed with a cover plate. The acting force of static pressure of the medium in the container is: FJ=A×P
 
In the formula, FJ is the medium force (N). A represents the area of the medium acting on the cover plate, (mm²). P means the 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 being equal to FJ must be applied in the vertical direction of the contact surface between the container and  cover plate, so that only the end faces can be sealed. Only when the sealing surface is an ideal plane, won’t the medium pass through the bonding surfaces. In order to ensure sealing of the contact surface, an interaction force must be generated between the sealing surfaces, that is, the cover plate must be pressed tightly on the container. When the acting force F is greater than FJ, certain specific pressure will be generated on the combined sealing surface, and the existing flatness on the plane will be deformed by the specific pressure. If the deformation is within elastic limits of materials and there is not much residual deformation, when the force F is applied to the contact surface, the sealing performance can be guaranteed. In addition to the specific pressure of the seal, the factors that ensure sealing of the connection also include the structure of the seal, but in this series of factors, the specific pressure between the sealing surfaces plays a key role.