Sealing materials plays an important role in cryogenic trunnion ball valves, which can prevent the leakage of media and ensure safe operation of the valve. Their performance directly affects the sealing reliability, service life and operating stability of valve in ultra-low temperature environment. Their main functions and selection principles are highlighted below:
I. Key Functions functions of Sealing Materials
Cryogenic Sealing: At temperatures of -196°C (e.g., liquid nitrogen) or lower, sealing material must remain elastic and flexible to prevent leakage due to increased the sealing surface voids due to hardening or shrinkage of the material. Ordinary rubber, for example, can become brittle at low temperatures, while specialized cryogenic sealing materials (e.g., modified PTFE (PTFE)) can still maintain sealing pressure.
Media Corrosion resistance: Cryogenic media (e.g., liquefied natural gas, liquid oxygen) may be highly corrosive or chemically reactive. Sealing materials must be chemically stable to prevent reaction with the medium that causes the seal to fail. For example, in liquid oxygen applications, materials containing oils or readily oxidizing substances should be avoided to prevent combustion risks.
Abrasion and corrosion resistance: During valve opening and closing, friction between the sphere and the sealing cover can lead to wear and tear, especially under high pressure pressure differential, the erosion of media can accelerate damage to the sealing surface. Sealing materials must possess high hardness and self-lubrication to reduce wear and prolong service life.
Thermal Expansion and Contraction Compensation: At low temperatures, the valve body metal shrinks more than the sealing material, which may lead to uneven pressure distribution on the sealing surface. The sealing material must compensate for this shrinkage difference by elastic deformation to maintain the contact pressure of the seal.
Adhesion resistance: After a long period of inactivity, the sealing surface may be difficult to open and close due to crystallization or adhesion of the medium. The sealing material must have low surface energy to prevent media adhesion and ensure flexible operation of the valve.
II. Sealant Selection Principles
1.Temperature Adaptability
Cryogenic Limit: Select material according to minimum operating temperature to ensure that its glass transition temperature (Tg) is below working temperature.
For example:PTFE (PTFE): Applicable temperature drops to -200°C, but improvements are required to improve abrasion resistance.
PI (polyimide): Suitable for -269°C (near absolute zero) but costly.
Flexible Graphite: Applicable for high temperatures of -200°C, but must avoid contact with strong oxidizing media.
2.Media Compatibility
Chemical Properties: Corrosion resistant materials are selected according to media type (e.g. hydrocarbons, liquid oxygen, liquid nitrogen). For example, the use of grease-based materials in liquid oxygen applications is prohibited; PTFE or metal seals (e.g. stainless steel + flexible graphite) are preferred.
For particulate media, abrasionresistant materials (e.g., silicon carbide reinforced PTFE) are required.
3.Mechanical Properties
Compression rebound rate: The material must be quickly deformed after compression to compensate for relaxation during long-term use. Flexible graphite, for example, has a compression rebound of over 40%.
Stiffness and strength: High-pressure applications require high-hardness materials (e.g., cemented carbide), but flexibility must be balanced to avoid brittleness.
4.Seal Type Matching
Soft seal: suitable for low pressure, clean media, good sealing performance, but poor abrasion resistance
Hard seal: suitable for high pressure particle media; seal is achieved by contact with metal
Combined sealing: Combines the advantages of soft and hard seals such as a PTFE ring embedded in metal valve seat to balance abrasion resistance and sealing performance.
