The term cryogenic temperatures typically applies to temperatures from below freezing all the way down to absolute zero, which is -460° F (or -273° C). There are many fluids that are shipped and used at cryogenic temperatures. Such fluids include liquified gases such as argon, hydrogen, oxygen, nitrogen, and helium, as well as liquified fuels such as LOX, LNG, cryogenic ethylene, and LPG. Designing seals to work with fluids in cryogenic conditions results in a set of serious challenges.
1. Lubrication
One of the major issues involved in cryogenic seal design is how to approach seal lubrication. At temperatures this low, most standard lubricants simply will not function and the liquids or gases that are being sealed cannot typically provide the necessary properties to reduce friction. In such cases, lubricants may be simply impossible, and the only alternatives are options such as these modified or unfilled polymer materials:
- PTFE
- PCTFE
- TFM
- UHMW PE
One of the reasons why lubrication may prove impossible lies in the fact that it may freeze, causing the equipment to lock up and the seal itself to shatter.
2. Dimensional Stability
Another serious challenge in seal design lies in the coefficient of thermal expansion. Just going from room temperature to the seals operating temperature will likely involve a significant change in temperature that can result in contraction. Changes in dimension, in turn, affect critical clearances and sealing ability. The specification of critical seal dimensions, including clearances, must be developed with the understanding that the size of the seal and its components will be different at cryogenic temperatures than at room temperature when it is manufactured. This particular aspect is dependent on the coefficient of thermal expansion of the seal materials.
3. Material Selection
Materials will behave differently at such low temperatures, with most of them (elastomers and metals included) behaving as a brittle material rather than an elastic material. The correct choice of a material based on its behavior at cryogenic temperatures, and not at room temperature, is critical and challenging. Many times, the seal material needs to be spring-energized in order to effectively function. In addition, it must also possess the following characteristics:
- friction
- excellent wear properties
- a small coefficient of thermal expansion
- chemical compatibility with the fluids to which it is exposed
Add to that the potential issues with lubrication, and the choice of a suitable material becomes rather difficult — but not impossible.
4. Potential Safety and Environmental Concerns
The performance and reliability of a seal design are extremely important when the potential safety and environmental hazards involved are taken into consideration. Such dangers include:
- toxicity
- flammability
- extreme cold
- asphyxiation
- explosion resulting from rapid expansion of the cryogenic materials
Because of the potential danger to humans, it is vital that cryogenic seals be extremely reliable and offer the best possible performance.
Conclusion
Whether you are transporting LNG or using liquid nitrogen in a laboratory setting, the choice of reliable seals is extremely important. Issues such as lubrication, dimensional stability, material selection, and safety concerns make seal design for cryogenic applications a challenging undertaking.
In part 2 of this series on cryogenic seals, we will focus on seal options for cryogenic applications as we look more closely at solutions to the challenges discussed in this post.
You may also enjoy these articles:
- The 3 Most Popular Spring Energized Seals Materials for Cryogenic Sealing Applications
- Cryogenic Seals for LNG Applications
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