Valve Sealing Performance in Low-Temperature Conditions

The sealing performance of an industrial valve is one of the key indicators used to evaluate its quality. It primarily involves internal and external sealing performance. Sealing performance directly impacts the valve's operational efficiency and service life, particularly under harsh conditions such as high pressure, low temperatures, and corrosive media. Therefore, understanding the characteristics of different valve sealing elements, the selection of materials, and their applicable ranges is essential for designing and using efficient and safe valves.

Types of Valve Sealing Performance

Valve sealing performance can be divided into two main categories: internal sealing and external sealing.

1. Internal Sealing Performance

Internal sealing refers to the sealing effect between the valve seat and the closing component. For example, in ball valves, it is the seal between the ball and the valve seat; in butterfly valves, it is the seal between the butterfly plate and the valve seat; in globe valves, it is the seal between the valve disc and valve seat; and in gate valves, it is the seal between the gate and valve seat. The effectiveness of internal sealing directly affects fluid leakage during operation and the reliability of valve operation. Common forms of internal sealing include flat sealing, spherical sealing, and conical sealing.

2. External Sealing Performance

External sealing refers to the sealing between the valve stem and the valve body. This sealing is especially important for preventing the leakage of external media, particularly for systems handling high-pressure or toxic media. Good external sealing performance is critical in these cases.

Selection and Characteristics of Sealing Materials

The choice of sealing material directly affects the sealing performance and durability of the valve. Sealing materials are mainly classified into two types: metal-to-nonmetal seals and metal-to-metal seals. Selecting the appropriate sealing material is crucial based on the valve's working environment and the properties of the medium.

1. Non-Metallic Seals

For valves such as ball valves and butterfly valves that work at normal temperatures, metal-to-nonmetal seals are commonly used. Non-metallic materials have good elasticity, excellent sealing ability, and require low sealing pressure, making them effective for sealing fluids. However, in low-temperature environments, the coefficient of expansion of non-metallic materials is usually larger than that of metals, causing uneven contraction at low temperatures. This can reduce the sealing pressure and even lead to sealing failure. Additionally, many non-metallic materials lose their elasticity at extremely low temperatures and become brittle, compromising sealing performance. For example, rubber loses its elasticity below its glass transition temperature, becoming glass-like and losing its sealing ability.
To address this issue, low-temperature valve designs typically avoid using traditional non-metallic sealing materials, especially in environments where temperatures fall below -70°C. Non-metallic materials can be combined with metals through special processes to form composite structures to improve low-temperature sealing performance. However, some non-metallic materials still perform well at low temperatures. For example, polyethylene developed by Irish Alloy Ltd. retains excellent toughness at -269°C, and Mylar plastic developed in France remains elastic at liquid hydrogen temperatures (-253°C), while certain polycarbonate materials perform well at the temperature of liquid nitrogen (-196°C).

2. Metal Seals

The performance of metal sealing materials at low temperatures is influenced by the material's physical properties. At low temperatures, metals experience an increase in strength and hardness, while plasticity and toughness decrease, making them prone to low-temperature brittleness. This can affect the sealing performance and safety of the valve. Therefore, low-temperature valve designs typically use metals with good low-temperature properties. For temperatures above -100°C, ferritic stainless steel is commonly used; for temperatures below -100°C, austenitic stainless steel, copper, and copper alloys are often selected for sealing elements.

Austenitic stainless steels (such as 304 and 316L) are widely used for low-temperature valve seals because they maintain good toughness and sealing ability under low-temperature conditions. However, austenitic stainless steel may undergo phase transformation at low temperatures, where austenite changes into martensite, causing the sealing surface to warp and deform, thereby affecting the sealing effect. Additionally, temperature differences between the valve body, valve bonnet, and valve stem, along with the different physical properties of the materials, can lead to thermal stresses that cause the sealing surface to elastically distort or permanently deform.

Low-Temperature Valve Design Optimization

The impact of low temperatures on valve sealing performance is an important consideration when designing low-temperature valves. To minimize the negative effects of low temperatures on sealing surfaces, the following measures can effectively improve the sealing performance of low-temperature valves.

1. Material Selection

Choosing materials with high metallurgical stability is critical for low-temperature valve designs. For example, 316L stainless steel is widely used for low-temperature valve seals due to its high low-temperature stability. Selecting these materials helps reduce the impact of low temperatures on sealing surfaces, maintaining good toughness and sealing ability.

2. Low-Temperature Treatment

For materials like austenitic stainless steel, low-temperature treatment is an important method to reduce the effects of phase transformation. By treating the material below its phase transformation temperature (MS point) and performing appropriate finishing at low temperatures, the negative impact of low temperatures on sealing performance can be effectively eliminated. The low-temperature treatment temperature should be below the material's phase transition point and the valve's actual working temperature. The treatment time is generally between 2 to 4 hours, and multiple treatments may be required as needed.

3. Structural Design

Low temperatures affect not only the materials but also the structural design of the valve. For example, when designing gate valves, ball valves, and butterfly valves, elastic sealing structures can be incorporated to compensate for sealing surface deformation caused by low temperatures. For globe valves, conical sealing structures can be used to reduce the impact of low-temperature deformation on the sealing surface, thus improving sealing performance.

Conclusion

The sealing performance of a valve is a critical characteristic, especially under low-temperature conditions. The selection and design of sealing elements are even more important in these environments. Appropriate material selection, low-temperature treatment, and structural design can effectively enhance the sealing performance of low-temperature valves, ensuring their reliability and safety under extreme conditions. By optimizing materials and designs, the durability and sealing efficiency of low-temperature valves can be significantly improved, meeting the demands of operations in extreme temperatures.