When designing a cryogenic seal, the first step is to determine the operating temperature range of the seal.

Typically, cryogenic seals operate below -65 degrees Fahrenheit.

When designing seals of this level (high temperatures are around 300 degrees Fahrenheit), it is necessary to understand what degree of leakage control is required on the low temp end.

However, there may be an allowable leakage rate which allows for reduced drag. When requiring zero leak, the drag in the system is often increased to support some elastomeric contact with a dynamic surface. In the case of static seals, elastomers span this range although increased squeeze may be necessary.

JST typically designs within the above range.

While -65°F is extremely cold, it is not considered cryogenic. Liquid nitrogen at -320°F (-195°C) requires special hardware and seal material considerations.

First, many projects and applications don't utilize lubricant in dynamic applications. To improve seal ability, a better-than-average surface finish is required.

Surface finish often holds lubricity. But without this, a smooth finish reduces friction, improves life, lowers drag, and improves sealability.

Static seals often need to have a near-zero leak rate; this means hardware considerations and surfaces can be even more important. This can mean polishing grooves, which can be very challenging in some applications.

Cryogenic Seal Materials

Elastomeric materials lose their flexibility at these extreme temperatures, and when we encounter temperatures below -180°F (-195°C), modified fluoropolymers can be used, such as PCTFE, which is known to operate at temperatures as low as -460 °F.

The goal is to keep fillers from very little to none. (Fillers extend wear life of these polymers). These materials require some kind of spring force to activate them, as they have very poor memory. They also require an energizer to force them in contact with the mating surfaces.

Cryogenic Seal Types

JST designs with a variety of spring types such as cantilever, canted coil, and helical. There are also a variety of materials such as various grades of stainless steel, Hastelloy, Elgiloy, and Inconel.

These materials and spring types allow a great deal of flexibility in conforming to a customer's hardware envelope.

Cryogenic Seal Jacket Material

A final consideration is the machined surface finish of the seal jacket material.

Many of these cryogenic applications typically don't have high amount of dynamic motion, so break in may not be possible. Super finish allows the seals to have the best chance of sealing right out of the box.

Some typical applications in cryogenic applications include liquid oxygen or nitrogen valving. If meter flow is needed, the effects of friction become a very relevant factor, as customers often have limited space and limited force to open and close valves.

JST has the ability to estimate friction forces knowing the spring rates and the coefficient of friction of the materials we seal with. Having all these pieces in place ensures seals perform well in extreme conditions.

Normally, it's required to seal from 70°Fahrenheit down to these cryogenic temperatures, but sometimes this must be extended to ranges of 100s of degrees Fahrenheit. Special consideration must be given to how much spring force can be applied, as too much can cause cold flow of the polymer, rendering the seal inoperative when it returns to cold temperatures.

Sealing cryogenics is a balancing act of many factors such as spring force, surface finish, friction and allowable leakage.