Selection of Sealing Materials for Cryogenic Valves

In industrial production and energy transportation, valves serve as critical components for controlling fluid flow. Their sealing performance directly affects system safety and reliability. This is particularly true under cryogenic conditions, such as in the transport and storage of liquefied natural gas (LNG), liquid oxygen, and liquid nitrogen. Under such extreme temperatures, the selection of sealing materials becomes especially important. Cryogenic environments impose higher demands on the physical properties of materials. Any oversight can lead to sealing failure, resulting in leakage or even safety accidents. Therefore, understanding the effects of low temperatures on sealing materials and selecting and applying them scientifically and rationally is a core issue in cryogenic valve design.

At ambient temperatures, valves such as ball valves and butterfly valves typically use sealing pairs composed of both metallic and non-metallic materials. Non-metallic materials like rubber and polytetrafluoroethylene (PTFE) exhibit good elasticity and require relatively low sealing pressure, resulting in effective sealing performance. However, as temperatures drop to cryogenic or even ultra-low levels, the performance of these non-metallic materials changes significantly.

Firstly, non-metallic materials have much higher thermal expansion coefficients than metals. Under cryogenic conditions, they contract more significantly, leading to a sharp decrease in sealing pressure and a deterioration in sealing performance. Secondly, most non-metallic materials become hard and brittle at low temperatures, losing their original toughness and elasticity. For example, rubber completely loses its elasticity below its glass transition temperature, turning into a glassy state and failing to provide any sealing function. Additionally, rubber may swell in media such as LNG, further compromising its sealing ability. Therefore, in operating conditions below -70°C, non-metallic sealing materials are generally not used alone. Instead, they are combined with metals through special processes to enhance sealing reliability.

In cryogenic environments, the strength and hardness of metallic materials increase, but their ductility and toughness decrease significantly, exhibiting varying degrees of low-temperature brittleness. These changes pose challenges to valve sealing and safety. To prevent brittle fracture under low stress, cryogenic valve designs must select appropriate metallic materials based on operating temperatures.

Generally, when the operating temperature is above -100°C, ferritic stainless steel can be used. However, when the temperature falls below -100°C, critical components such as the valve body, bonnet, stem, and sealing seat should be made of austenitic stainless steel, copper alloys, or aluminum alloys with a face-centered cubic crystal structure. Among these, austenitic stainless steels like 304 and 316L are the most commonly used in cryogenic valves due to their high toughness at low temperatures.

However, austenitic stainless steels also have certain drawbacks. At room temperature, they are in a metastable state. When the temperature drops below the martensitic transformation point (Ms), austenite may transform into martensite. The martensitic crystal structure is less dense, and the resulting volume change can increase internal stress, causing deformation of the sealing surface and leading to sealing failure. Moreover, uneven contraction due to temperature differences between components or material property variations can generate thermal stress. If this stress exceeds the material's yield limit, irreversible deformation may occur, affecting sealing performance.

To minimize the impact of low temperatures on metallic sealing pairs, various design measures should be adopted to reduce the influence of sealing surface deformation on sealing performance. Firstly, materials with higher metallurgical stability, such as 316L stainless steel, should be prioritized. Although more expensive, their cryogenic performance is more reliable. Secondly, components made of austenitic materials should undergo cryogenic treatment to allow martensitic transformation and deformation to occur fully before final machining. The cryogenic treatment temperature should be below the material's transformation point and maintained for 2 to 4 hours. Multiple treatments or aging treatments may be necessary in some cases.

In terms of structural design, elastic sealing structures can be employed to partially compensate for cryogenic deformation. For example, elastic elements can be introduced in ball valves and butterfly valves, and conical sealing structures can be used in globe valves to reduce the impact of deformation on sealing surfaces. Additionally, special attention should be paid to the sealing design between the valve stem and the valve body. Since non-metallic sealing rings fail at low temperatures, packing box or bellows sealing structures are typically used. Bellows sealing is suitable for applications where micro-leakage is not acceptable, but it is more expensive and has a limited service life. Packing box structures, on the other hand, are more widely used due to their simple manufacturing and ease of maintenance.

Packing box sealing is a common sealing method in cryogenic valves, but it imposes strict requirements on material selection and operating environment. Generally, packing materials cannot operate below -40°C, as their elasticity decreases and sealing performance deteriorates. Therefore, during design, a long-neck bonnet structure should be used to keep the packing box away from the cryogenic medium and maintain its operating temperature above 0°C.

Common packing materials include PTFE, asbestos, PTFE-impregnated asbestos rope, and flexible graphite. Among these, asbestos has issues with permeability leakage, and PTFE is unsuitable for cryogenic environments due to its high linear expansion coefficient and severe cold flow. Flexible graphite, with its excellent overall performance, is currently one of the best sealing materials. It is impermeable to both gases and liquids, has a compression rate of over 40%, a recovery rate of over 15%, and a stress relaxation rate of less than 5%. It can achieve sealing under relatively low tightening pressure and has good self-lubricating properties, effectively reducing wear between the valve stem and the packing.

Moreover, since the linear expansion coefficient of non-metallic packing is much greater than that of the metallic valve stem and packing box, the packing contracts more significantly at low temperatures, potentially reducing preload and causing leakage. To address this issue, disc spring washers can be used on the packing gland bolts to provide preload compensation, ensuring that the packing maintains sufficient sealing pressure under cryogenic conditions.

As a critical component connecting the actuator and the valve plug, the valve stem's material selection and surface treatment also affect sealing performance. Austenitic stainless steel valve stems should undergo cryogenic treatment before final machining to minimize low-temperature deformation. Additionally, due to their relatively low surface hardness, which makes them prone to galling with the packing, they should be hard chrome-plated or nitrided to improve surface hardness and wear resistance.

In flange connections and external sealing, gaskets are commonly used sealing elements. At low temperatures, gasket materials tend to harden and lose plasticity, so higher requirements are placed on their sealing and recovery properties. During design, it is essential to ensure that gaskets maintain reliable sealing at ambient, cryogenic, and fluctuating temperatures. Specific measures include selecting gasket materials with lower linear expansion coefficients, reducing gasket thickness, and increasing bolt elongation.

For cryogenic valves operating below -100°C, the valve body and bolts are typically made of austenitic stainless steel with consistent linear expansion coefficients. Therefore, greater emphasis should be placed on gasket material selection and bolt preload design. Ideal cryogenic gaskets should have low hardness at room temperature, good recovery performance at low temperatures, a small linear expansion coefficient, and high mechanical strength. In practical applications, spiral-wound gaskets made of flexible graphite and stainless steel strips offer the best sealing performance. To further improve sealing reliability, disc spring washers can be installed on the bolts to compensate for preload changes caused by temperature variations.

This article discusses the selection and application of sealing materials for cryogenic valves, focusing on the effects of cryogenic environments on the performance of metallic and non-metallic materials and proposing corresponding solutions. It is noted that non-metallic materials tend to become hard and brittle at low temperatures, leading to a decline in sealing performance. Therefore, they should not be used alone in operating conditions below -70°C. Among metallic materials, austenitic stainless steel is widely used due to its excellent low-temperature toughness. However, it carries the risk of deformation caused by phase transformation, which must be controlled through cryogenic treatment and structural optimization.

For valve stem sealing, a packing box structure is recommended, using high-performance packing materials such as flexible graphite, combined with disc spring washers to achieve preload compensation. Valve stem materials should also undergo cryogenic treatment and surface hardening to improve wear resistance and sealing performance. For flange connections, gasket materials should be carefully selected, and bolt preload should be controlled to ensure reliable sealing at low temperatures.

In summary, the sealing design of cryogenic valves must comprehensively consider material properties, structural configurations, and process treatments. Through scientific approaches, sealing reliability can be enhanced to ensure safe operation of systems under extreme low-temperature conditions.