In cryogenic temperatures, most O-rings become brittle and fail. Many times cryogenic applications can also involve media that is not chemically compatible with traditional elastomeric materials. However, for axial sealing applications, there is an effective, dependable solution: encapsulated O-rings. 

What is an Encapsulated O-Ring?

In short, encapsulated o-rings combine the chemical resistance and extreme-temperature performance of FEP or PFA with the elasticity and resilience of silicone, FKM, or stainless steel energizers. Stainless steel springs or rugged elastomers are encapsulated within a durable, chemically resistant jacket made from FEP (fluorinated ethylene propylene) orPFA (perfluoro alkoxy copolymer). These o-rings are used with valve stems, flanges, joints, swivels, pumps, turbo expanders, and waterless fracking.

Materials Used With Encapsulated O-Rings

For the outside of the O-ring, the most popular materials currently in use are FEP, PFA, and PTFE. FEP is highly resistant to chemical attack, offers a low compression set, and has a low coefficient of friction. Its operating temperature range is -420°F through 400°F and is less expensive than PFA. In addition, FEP is available in FDA-approved grades. 

PFA is resistant to a wide range of corrosive chemicals, including naphtha, acid, aromatic solvents, petroleum, and alcohol. It has a wider operating temperature range than FEP, from -420°F through 500°F. It also possesses a low compression set and resistance to cracking and stress in addition to a higher overall mechanical strength when compared to FEP. It is also available in grades that have the following approvals: USP IV, FDA-compliant, EU Reg. 1935/2004, ADI-free, and 3-A Sanitary Standards.

The materials used for the interior energizer of encapsulated o-rings include 302 stainless steel for spring energized as well as silicone, FKM (trade name Viton), or EPDM. Whether its a spring-energized approach or a core of silicone, FKM, or EPDM, the interior of the encapsulated o-ring provides the additional resiliency that makes these o-rings so effective for cryogenic applications. Note that silicone and FKM ensure an even pretensioning at the sealing point.

Silicone provides a softer core than FKM and offers very good cold flexibility. Using FKM as the core material means that the o-ring will be able assume its original shape very quickly after installation/deformation due to its outstanding compression set characteristics. It is not, however, as temperature resistant as silicone. While EPDM can be used as a core material, it is not recommended because of how it reacts to the heat involved in manufacturing the encapsulated o-rings.

Solid-Core vs Hollow-Core Encapsulated O-Rings

The two basic types of encapsulated O-rings are solid core and hollow core. Solid core o-rings have an energizer made from either silicone or FKM (fluoroelastomer, trade name Viton). Both types of cores provide good elasticity and low compression set, but when used in cryogenic applications silicone is usually the better choice because it remains more flexible at lower temperatures. Hollow-core encapsulated o-rings, on the other hand, are used when there is a need for extreme elasticity or for fragile applications. 

Advantages of Encapsulated O-Rings

There are several advantages to using encapsulated O-ring, besides the fact that they  can outperform traditional seals in harsh environments that can include extreme temperatures and corrosive media. These o-rings are …

• Chemically resistant
• Available in non-contaminating and FDA-approved materials
• Low coefficient of friction that prevents issues with stick-slip behavior
• Low permeation
• Excellent corrosion resistance
• Low compression set
• Excellent service life
• Reliable sealing
• Cost effective

Encapsulated O-Rings for Cryogenic Operating Environments

For cryogenic environments, the best approach to encapsulated O-rings is the use of FEP or PFA exterior with a steel flat wound ribbon spring at the core. This configuration can handle cryogenic temperatures all the way down to -420°F and pressures up to 3000 psi as long as vented holes are placed in the FEP jacket to prevent dangerous blowouts. These encapsulated o-rings work best in static and slow dynamic applications and are ideal for applications that involve cryogenic media such as liquid oxygen, liquid nitrogen, hydrogen. These encapsulated o-rings are readily available in both metric and US cross-sections and a wide range of diameters.

Conclusion

For cryogenic applications where traditional o-rings have failed, encapsulated o-rings with an FEP/PFA exterior and a 302 stainless flat wound ribbon at the core is an excellent option. It has already been successfully used by NASA in not one but several successful rocket launches and has been incorporated into designs by Lockheed and Boeing.