Cryogenic Valve Sealing Challenges and Their Countermeasures

In industrial applications, cryogenic valves are used extensively, especially in the transmission and storage of liquefied natural gas (LNG), liquid oxygen, and liquid nitrogen. Whether cryogenic valves can achieve reliable sealing is directly related to the safety and efficiency of the system. However, the requirements for valve sealing in low-temperature environments are extremely high, and previous sealing designs are prone to fail under such conditions. This article focuses on the sealing challenges faced by cryogenic valves, as well as the solutions to these problems, so that readers can clearly understand the key technologies in this field.

The impact of low temperatures on valve sealing performance is quite complex, mainly reflected in changes in material properties and whether the sealing structure can adapt. At low temperatures, both non-metallic and metallic materials undergo significant changes in physical and mechanical properties, and these changes directly affect sealing performance and service life.

At room temperature, non-metallic materials such as rubber and asbestos, which have good elasticity and reliable sealing properties, are widely used for valve sealing. However, these materials become unsuitable for cryogenic environments. For example, when rubber is exposed to temperatures below its glass transition temperature, it becomes brittle like glass, losing its elasticity completely, and therefore loses its sealing ability. In addition, rubber in liquefied natural gas (LNG) may swell and cause greater risk of sealing failure. Although asbestos is resistant to high temperatures, it still cannot avoid permeation leakage at low temperatures. Its high coefficient of thermal expansion also leads to gaps caused by temperature changes, resulting in sealing failure.

Metallic materials also change in properties at low temperatures. When the temperature decreases, the strength and hardness of metals increase, while plasticity and toughness drop sharply. This phenomenon is known as cold brittleness. It causes metal components to become susceptible to brittle fracture at low temperatures, greatly affecting valve performance and safety. For instance, ferritic stainless steel exhibits obvious cold brittleness below −100℃, therefore it is generally not used in cryogenic valve design.

Due to the changes in material properties, traditional sealing ring structures become ineffective in low-temperature environments. For example, although bellows sealing can prevent leakage, a single-layer structure has a short service life, while a multi-layer structure incurs high cost and manufacturing difficulty, so it is not commonly used in cryogenic valve production. Packing sealing structures, though simple to manufacture and easy to maintain or replace, will gradually lose elasticity at low temperatures, resulting in reduced sealing performance. Moreover, friction between the packing and the valve stem increases due to freezing caused by medium leakage, leading to packing scratches and serious leakage.

To cope with the various challenges that cryogenic environments pose to valve sealing, scientific and reasonable countermeasures are essential. These countermeasures must consider material selection along with structural and process improvements, so that cryogenic valves can remain reliable and safe under extreme conditions.

Materials form the foundation of cryogenic valve sealing. Proper material selection effectively addresses challenges posed by low-temperature environments. For cryogenic valves, material selection must consider physical properties, chemical stability, and mechanical performance at low temperatures.

In cryogenic valves, sealing materials are particularly critical. Common sealing materials include non-metallic and metallic types. Non-metallic materials such as flexible graphite and PTFE are widely used because of their excellent sealing performance and cryogenic resistance. Flexible graphite is impermeable to gases and liquids, features a high compression rate (greater than 40 percent), good resilience (greater than 15 percent), and low stress relaxation (less than 5 percent). This means it can maintain good sealing performance even with low tightening pressure. Moreover, flexible graphite has self-lubricating properties, preventing wear between packing and valve shafts, making it superior to traditional asbestos materials.

For metallic materials, austenitic stainless steels such as 0Cr18Ni9 and 00Cr17Ni12Mo2 (304, 316L) are preferred because they have no cold brittleness transition temperature and retain high toughness at low temperatures. These materials do not suffer from cold brittleness and can effectively prevent sealing failure caused by material fracture.

In addition to single-material solutions, composite materials also have great potential in cryogenic valve sealing. By combining metallic and non-metallic components, the advantages of both can be utilized to improve sealing performance. Such composite structures maintain good sealing at low temperatures and avoid the shortcomings of single materials.

Structural design is a key factor in ensuring sealing performance in cryogenic valves. A reasonable structure can effectively compensate for changes in material performance under low temperatures, making valves more reliable and durable.

Packing sealing structures are widely used in cryogenic valves due to simple manufacturing and convenient maintenance. To ensure sealing reliability of packing at low temperatures, a long-neck bonnet design can be adopted to keep the packing away from the cryogenic medium so that it operates closer to ambient temperature. Since the thermal expansion coefficient of packing is much greater than that of metal packing chambers and valve stems, packing installed at room temperature may lose preload after cooling due to different shrinkage, which leads to leakage. Therefore, multiple sets of disc spring washers can be used on the gland bolts to provide continuous preload compensation at low temperatures, ensuring sealing effectiveness.

For metal sealing pairs, the structural design must consider the effects of cryogenic environments. Elastic sealing structures can be used for gate valves, ball valves, and butterfly valves to allow partial compensation for cryogenic deformation. For cryogenic globe valves, tapered sealing structures are adopted to reduce the impact of low-temperature deformation on sealing surfaces. These structural design strategies effectively minimize the negative influence of low-temperature deformation and ensure reliable sealing performance.

Process treatment is an important guarantee for sealing performance of cryogenic valves. Reasonable processing helps optimize material properties and enhance sealing performance.

For materials such as austenitic stainless steel, cryogenic treatment is key to ensuring stable performance at low temperatures. Cryogenic treatment allows martensitic transformation and deformation to be completed before final machining. The treatment temperature must be lower than the material's transformation temperature (Ms) and lower than the actual operating temperature of the valve. A treatment duration of 2 to 4 hours is suitable, and multiple treatments or appropriate aging treatments may be used if necessary. This process effectively reduces deformation of sealing surfaces caused by phase transformation at low temperatures and improves sealing reliability.

Surface treatment is also an important processing method to enhance sealing performance in cryogenic valves. For example, grinding and polishing of the sealing surface improve smoothness and eliminate micro gaps, enhancing sealing effectiveness. In addition, applying cryogenic-resistant and corrosion-resistant coatings can further improve the performance of sealing surfaces.

In the practical application of cryogenic valve sealing technology, many cases demonstrate the effectiveness and innovation of different solutions. The following are specific case studies involving sealing problems and countermeasures in cryogenic media such as liquefied natural gas (LNG) and liquid oxygen.

The storage and transportation of liquefied natural gas require extremely high sealing performance. LNG is stored at approximately −162℃, making traditional non-metallic sealing materials unsuitable. In practical applications, LNG valves are mostly made of austenitic stainless steel and receive cryogenic treatment to ensure stable performance. Flexible graphite is commonly used as packing, combined with long-neck bonnet structures and multiple sets of disc spring washers to maintain preload under low temperatures. This design has proven highly effective in preventing LNG leakage and ensuring safe system operation.

Liquid oxygen is stored at approximately −183℃, placing even stricter requirements on material performance. Austenitic stainless steel and rigorous cryogenic treatment are typically used in liquid oxygen valve design. In addition, due to the strong oxidizing nature of liquid oxygen, careful selection of packing materials is essential. Flexible graphite, with excellent sealing properties and oxidation resistance, is the preferred choice. With rational structural design and material treatment, liquid oxygen valves can operate reliably in practical conditions, ensuring the safe delivery and storage of liquid oxygen.

Cryogenic valve sealing technology is particularly important in the industrial sector. By gaining an in-depth understanding of material property changes in low-temperature environments and employing proper material selection, structural optimization, and processing methods, the sealing problems of cryogenic valves can be effectively resolved, improving system safety and reliability. With continuous advancements in material science and engineering, cryogenic valve sealing technology will continue to evolve, providing strong assurance for the safe and efficient operation of industrial processes.