Cryogenics deals with the production of extremely low temperatures and their effect on different materials. The cryogenic temperature ranges from -150 °C to absolute zero. At such low temperatures, not all sealing solutions can perform effectively. When exposed to an extremely cold environment, the sealing material can become brittle or lose its flexibility, which is an essential part of an o-ring’s performance.
Consequences can be catastrophic if the seal used in cryogenic applications breaks or fails to prevent the leak, as they contain cryogenic liquids such as oxygen, nitrogen, hydrogen and argon. Flammability, asphyxiation, toxicity and extreme cold are the health hazards associated with cryogenic liquids. Seal failure can cause an explosion, impact the environment and affect the health of employees.
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Why Do Some O-Ring Seals Fail at Low Temperature?
In the ultra-cold environment, soft materials such as elastomers can lose their elasticity. Without its elasticity properties, an o-ring cannot deform, therefore failing to perform as a reliable seal.
Elastomers with a low-compression set, which is a measure of a material’s resistance to flow while in a compressed state, offer good resistance to changes in pressure and temperature. It ensures tight sealing, preventing cryogenic liquids from escaping. In a low-temperature environment, the compression set of some elastomers degrades, leading to seal leakage and failure of the o-ring seal.
When an o-ring is subjected to temperature below its glass transitions temperature (Tg), the temperature when a polymer becomes a hard, brittle material, it may crack or shatter due to turning brittle and resisting deformation.
Choosing the Right Material is Vital
When shopping for o-rings for cryogenic applications, choosing the right material is of paramount importance. It should perform effectively at cryogenic temperature. It is seen that traditional elastomers undergo a glass transition when exposed to low temperatures. Here are materials that have a lower service temperature limit.
Temperature range: -30°C to +150°C
Double bonds of nitrile’s butadiene segments are saturated with hydrogen to get the synthetic polymer HNBR, short for Hydrogenated Nitrile. It offers good resistance to heat, oils and chemicals.
Temperature range: -10°C to +260°C
Synthetic rubber FFKM, short for Perfluoroelastomer, has excellent resistance to extreme temperatures. As it contains more fluorine than the standard FKM, it offers the needed chemical resistance. Also, it is known for its almost universal chemical compatibilities, making FFKM o-rings suitable for cryogenic applications.
Temperature Range: -50°C to + 232°C
The central ingredient of silicone is VMQ, or vinyl-methyl-silicone. O-rings made of silicone are suitable for low temperatures in static applications.
Temperature Range: -50°C to +120°/150°C
High-density synthetic rubber EPDM, short for ethylene propylene diene monomers, offers good resistance to acids and alkalis.
Temperature Range: -200°C to +250°C.
The highly resilient PTFE, also called Teflon, maintains its strength and toughness at low temperatures. It has good chemical resistance but is not stretchable as it is hard plastic.
Factors to Consider When Choosing
When choosing the correct o-ring material for cryogenic temperatures, check its size, how much pressure it can handle, the industry standard it follows, chemical resistance, compatibility with properties of the application, mechanical properties and other aspects.
Also, before shopping for o-rings, it is essential to determine the temperature range of the application, in which the seal will be performing, and the allowable leakage rate. It will help select the most appropriate o-ring seal for cryogenic application.
Also read: Can I Refill A Liquid-filled Gauge?