by Sara McCaslin, PhD Sara McCaslin, PhD No Comments

When cryogenic temperatures are involved, a failed seal can have extremely serious repercussions that can include personal safety, explosions, damage to local ecosystems, and highly expensive downtime. One of the most dependable solutions to date for sealing in cryogenic environments is PTFE spring-energized seals. In this week’s blog post, we will discuss PTFE spring energized seals for cryogenic applications!

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Cryogenic Applications of Spring-Energized Seals

There are a host of cryogenic applications that depend on spring-energized seals. In the medical field, they are indispensable for MRI (Magnetic Resonance Imaging) equipment. In space applications, spring-energized seals can be found in equipment for radio astronomy and infrared telescopes as well as rocket propulsion systems. LNG fueling systems and compressors depend on them, as well as speciality gas manufacturing. Spring-energized seals are also needed in both pharmaceutical and medical research and can be found in scientific instrumentation for a wide range of disciplines. They are also critical for many food, dairy, and pharmaceutical applications.

But why do so many cryogenic environments require the use of a spring-energized seal?

Sealing Issues at Cryogenic Temperatures

The temperature range for cryogenic applications ranges from below freezing at -32°F down to absolute zero at -460°F. At these cryogenic temperatures, many seal materials begin to behave unpredictably, often exhibiting stiff or even brittle behavior. And changes in temperatures will cause dimensional changes in the seal, often compromising the integrity of the seal. To complicate things further, media at cryogenic temperatures may be chemically aggressive toward certain seal jacket materials. Finally, lubricants are usually prohibited at cryogenic temperatures because of issues with freezing, which means that a suitable material should be low friction and dry running.

Using the right sealing solution, however, can provide a reliable, gas-tight sealing system. And that, in turn, supports compliance with applicable safety and environmental regulations. 

Spring-Energized Seals

Unlike traditional seals, spring-energized seals include an energizer that applies a near-constant load throughout the circumference of the seal. This allows the lip of the seal to remain in contact with the mating surface in a variety of situations, including …

  • Eccentricity
  • Out of round 
  • Misalignment
  • Wear
  • Pressure fluctuations
  • Temperature fluctuations

In the context of cryogenic applications, spring-energized seals are used to maintain contact with the surface during the dimensional variations that result from temperature changes. In addition, spring-energized seals can be used in both static and dynamic applications, including rotating and/or oscillating movement.

Spring-Energizers Suitable for Cryogenic Temperatures

Spring energizers come in many different geometries, but for cryogenic applications, metal V ribbon springs are typically used. V springs, also known as cantilever springs, are used in extremely harsh operating environments and work extremely well in both cryogenic and vacuum pressure applications. 

A key feature of metal V springs as an energizer is their ability to provide a moderate yet very consistent load over a wide range of deflection. This aids in securing the lip of the seal against the mating surface even during dimensional changes due to wide temperature variations. For cryogenic environments, the spring-energizer is typically manufactured from either stainless steel or Inconel, Elgiloy, or Hastelloy.

However, in some instances, elastomeric o-rings can be used as the energizer as opposed to using a metal spring. O-ring energizers are durable and work well under a wide range of temperatures, but are best used when metal must be avoided in an application. 

Media Involved in Cryogenic Applications

As discussed earlier, there are a wide range of applications that require highly reliable sealing solutions. Spring-energized seals are excellent at maintaining seal integrity under such conditions, but thought must also be given to the seal jacket material, which will be in direct contact with media at cryogenic temperatures.

The most typical media of concern include …

  • LOx (Liquid Oxygen)
  • LHE (Liquid Helium)
  • LH2 (Liquid Hydrogen)
  • LAR (Liquid Argon)
  • LN2 (Liquid Nitrogen)
  • Liquid Xenon
  • LCO2 (Liquid Carbon Dioxide)
  • LNG (Liquid Natural Gas)
  • LPG (Liquid Petroleum Gas)
  • LMG (Liquid Methane Gas)
  • Various refrigerants and coolants

When a spring-energized seal is being specified, it is extremely important to select a material that not only has excellent properties at cryogenic temperatures but is compatible with the chemicals involved.

PTFE Spring-Energized Seals

One of the most widely used seal jackets for cryogenic applications is PTFE, better known by the trade name Teflon. PTFE provides excellent performance at a range of operating temperatures, including cryogenic, as well as pressure fluctuations. Its wide operating temperature range is complemented by a wide operating pressure range that includes vacuum pressures.

Virgin PTFE has the lowest coefficient of friction of any solid material, and even with the addition of filler materials it still remains extremely low. Lubricants will not be needed when a PTFE sealing jacket is used because it is self-lubricating, dry running, and exhibits no start and stop behavior. PTFE is also the most chemically compatible polymer available, solving the problem of chemical resistance issues. And for food, dairy, and pharmaceutical applications, PTFE is available in FDA-approved grades.

Conclusion

Where reliable sealing is critical in the presence of cryogenic temperatures, PTFE spring-energized seals are a proven solution in applications ranging from the rocket propulsion systems to MRIs. If you are looking for the right seal that offers superior performance in a cryogenic operating environment, contact Advanced EMC today. Our team of sealing experts can guide you in the process of specifying the right kind of cryogenic PTFE spring-energized seal.

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