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.

PTFE Rotary Shaft Seals
by Jackie Johnson Jackie Johnson No Comments

PTFE Rotary Shaft Seals in High Speed Applications

The design and specification of rotary shaft seals is challenging enough, but things get even more complicated for high-speed seals. High-speed rotary shaft seals pose their own set of wear and heat generation problems that can make it difficult to select an appropriate lip material, but PTFE is up to the challenge.

Rotary Shaft Seals in High-Speed Applications

In the context of rotary shaft seals, high speeds are often defined as those above 3,600 rpms. Such seals can be found in industries such as pulp and paper, wind energy, pumps, gearboxes, steel and aluminum processing, electric motors, medical devices, etc.

High-speed applications, such as those found in turbomachinery, can cause a seal to wear out faster and generate more heat because speed and friction do not get along well together. If the heat generated is sufficient, it can result in higher operating temperatures and changes to the geometry of the seal. And not all high-speed applications are compatible with lubricants, so in some cases, the seal may need to be capable of dry running. It is also key that these seals do not exhibit stick and slip behavior at startup.

Requirements for High-Speed Rotary Shaft Seals

PTFE Rotary Shaft Seals

PTFE Rotary Shaft Seals

High speed rotary shaft seal materials, in addition to the normal requirements for seals, must be …

  • abrasion and wear-resistant (to reduce wear)
  • Dimensionally stable (to prevent changes in geometry due to high temperatures)
  • Thermally conductive (to dissipate heat generation)
  • High operating temperature (to account for heat generated during use)
  • Possess an extremely low coefficient of friction (to reduce heat generation and wear)
  • Reduced stick-slip and breakout friction
  • Self-lubricating (for when lubricants cannot be used)

While there are several options available for seals that meet these requirements, one in particular stands-out: PTFE, or polytetrafluoroethylene.

PTFE High-Speed Rotary Shaft Seals

PTFE exhibits several key qualities necessary for high-speed rotary shaft seals. It has good abrasion and wear-resistant properties, is dimensionally stable, and has good thermal conductivity. PTFE also has an operating temperature of up to 500°F and a melting point of almost 650°F. It also has the lowest coefficient of friction of any solid currently known to mankind, exhibits reduced stick-slip, has extremely low breakout friction, is self-lubricating, and can continuously operate as a dry-running material.

PTFE also comes in various grades beyond virgin PTFE. It is available fillers such as Molybdenum Disulfide (MoS2) for increasing wear resistance, carbon for increasing wear resistance while keeping friction low, glass for better hardness and wear resistance, or various combinations of these. Keep in mind that there are also FDA approved seals for use in connection with pharmaceuticals and medical applications as well as food and beverage production.

PTFE rotary shaft seals are available in hydrodynamic, plain and multi-lip configuration and for situations where the production volume is low, they can be constructed from machined shells so there are no tooling charges.  At the same time, high production volumes can be manufactured from pressed shells to reduce unit costs.

Conclusion

For applications that demand reliable, long-lasting high-speed rotary shaft seals, PTFE is the engineer’s choice for reliable performance. It combines low friction, high operating temperatures, good wear properties, and dry running capabilities that can handle the rigors of high-speed applications.