by Sara McCaslin, PhD Sara McCaslin, PhD No Comments

PTFE Spring Energized Seals for Cryogenic Applications

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.

by Sara McCaslin, PhD Sara McCaslin, PhD No Comments

Spring Energized Sealing Solutions For Cryogenic Services in LNG Plants

There are a limited number of reliable sealing solutions for cryogenic services in LNG plants, two leading polymers in use are spring-energized PTFE or UHMW seals.

Challenges of Working with LNG

Leaks involving LNG (Liquified Natural Gas) at cryogenic temperatures are dangerous to the health and safety of workers and to plant operation. Issues such as toxicity, extreme cold, asphyxiation, flammability, and explosions resulting from rapid expansion of LNG all point to the need for a reliable, leak-proof seal.

Finding an effective sealing solution for use in the cryogenic work environment of LNG plants can be extremely challenging. Keep in mind that nitrogen exists in liquid form under normal atmospheric pressure between -346°F and -320.44°F. It’s liquid to gas expansion ratio is very high at 1:694, which means as it boils (starting at its boiling point of -320.44°F) it will expand 694x its original volume. This can lead to an extremely high-pressure change if it occurs in a sealed environment, and most LNG seals must remain functional at either vacuum pressures or extremely high pressures.

Cryogenic Seals for LNG Plants

The temperatures involved with LNG happen to lie where many elastomeric and polymeric materials lose their elasticity and begin to behave as brittle materials. Some seals will also experience dimensional fluctuations related to temperature changes, further increasing the probability of failure. If temperature fluctuations are cyclical, there are going to be problems related to cyclic stress. Yet another issue related to dynamic cryogenic seals is lubrication: at such low temperatures, standard lubrication solutions simply will not work.

The Options For Sealing are Limited two either UHMW or PTFE Polymers and a Full Contact- Anti-Shrink Spring is Essential.

Both seal jacket materials can be specified PTFE, often known by its trade names Teflon or Flourolon 1000. The Ultra High Molecular Weight PE or UHMW, Fluorolon 5000 can handle the low temperatures involved in cryogenic service without becoming brittle (some grades can handle temperatures as low as -350°F) or succumb quickly to the effects of cyclical stress. In addition, both UHMW and  PTFE are self-lubricating, low friction supports dry running, and is a nonstick/slip material. In addition, both products are compatible with a wide range of chemicals, including those it would encounter in an LNG plant.

A spring-energized seal is a seal assembly that includes an energizing spring that forces the seal lip against the mating surface to achieve a highly leak-proof seal. This seal design, when combined with a PTFE lip, has been found ideal for cryogenic applications involving LNG. The spring energizer adds permanent resilience to the seal and can compensate for lip wear, eccentricity, hardware misalignment, and (perhaps most importantly when working with LNG) extreme pressures and dimensional changes. 

The recommended geometry for the spring energizer is a simple helical spring when cryogenic temperatures and either static, reciprocating, or rotary motion is involved. However, oscillatory or static motion may require the use of a solid spring. Recommended spring materials include  17-7 precipitation hardening stainless steel, 301/304 stainless steel, or, in some applications, Hastelloy, 316 stainless steel, Inconel, or Elgiloy.

Conclusion

The design of cryogenic seals for use in LNG plants can be challenging and must meet extremely high standards for reliability and safety, but PTFE spring-energized seals are an excellent starting point.

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Factors Influencing PTFE Seal Behavior: The Mating Surface – Part 1 in a 3 Part Series

The useful life of a PTFE seal is heavily influenced by the surface over which the seal slides or rotates. This includes both the material choice, surface finish, and hardness of the mating surface. In this article, we will look at how the mating surface influences the life and behavior of a PTFE seal and how to account for that effect during the design process.

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