LNG applications can be challenging, with cryogenic temperatures and extreme temperatures combining to create a nightmare for your typical seal. However, there are excellent, reliable sealing solutions for LNG applications.
Facts About LNG
Nitrogen exists in liquid form under normal atmospheric pressure between -346°F and -320.44°F (the temperature at which it begins to boil). The liquid to gas expansion ratio of liquid nitrogen is very high: 1:694, which means as it boils it will expand 694 times its original volume and cause a very significant change in pressure if it is in a sealed environment. It is commercially produced by the fractional distillation of air.
Liquid nitrogen is obviously used as a coolant for superconductors, infrared homing missiles, vacuum
pump traps, and CCD cameras. It is commonly used in the medical industry for dermatological
applications as well as the preservation of biological samples. Liquid nitrogen is used in a variety of
laboratory settings involving cryogenic research. It also finds applications in the food and drug
industries for refrigeration. Liquid nitrogen is also used to protect chemicals from being contaminated
by moisture in the atmosphere, as is the case with loading arms.
Sealing Challenges Faced with Cryogenic Seals
There are several issues involved with cryogenic seals that make their design and implementation
especially challenging.
- First, there will most likely be high pressures or vacuum pressures involved.
- Next, dimensional changes are to be expected after installation and must be addressed in the design and specification of the seal.
- Another potential problem is the behavior of the seal material at cryogenic temperatures. Most materials will exhibit brittle behavior at extremely low temperatures. However, that can be addressed by specifying the seal material based on its performance at the operating temperature.
- Lubrication becomes especially challenging at cryogenic temperatures, beginning with stick slip. Any moisture or lubrication present in the seal environment can freeze, which can result in a shattered seal.
A spring-energized seal is a seal assembly with a spring included that forces the polymer seal lip against the mating surface to achieve as leak-proof a seal as possible. This seal design has been found ideal for cryogenic applications, including those involving liquid nitrogen.
Sealing Solutions for Liquid Nitrogen
A variety of sealing solutions exist for liquid nitrogen applications, including PTFE spring-energized seals.
As a sealing material, PTFE offers:
- Outstanding performance at the temperatures involved with liquid nitrogen
- Extremely low friction
- Low thermal conductivity
- Corrosion resistant
- Dry running capabilities so that lubricants aren’t necessary
- Outstanding chemical compatibility
- Excellent wear characteristics
Other polymeric sealing materials include PCTFE, TFM and UHMW PE materials. These also offer
chemical compatibility, thermal stability, corrosion resistance, and very low friction.
A spring-energized seal is a seal assembly with a spring included that forces the polymer seal lip against the mating surface to achieve as leak-proof a seal as possible. This seal design has been found ideal for cryogenic applications, including those involving liquid nitrogen.
Conclusion
Cryogenic sealing involving liquid nitrogen can be quite challenging, but certainly not impossible thanks to spring-energized seals and lip materials such as PTFE. PTFE and materials such as TFM, PCTFE, and UHMW PE offer ultra-low friction, chemical compatibility, useful thermal properties, and dimensional stability needed in cryogenic seals for LNG applications.