Cryogenic seals must not only handle unbelievable low temperatures, but also perform under high pressure conditions. If you arent familiar with cryogenic seals, a cryogenic seal is a special type polymer seal used with media at temperatures below -238°F and down to -460°F (absolute zero). Cryogenic seals can be found in a variety of industries, from LNG compressors and loading arms in the petroleum industry to rocket propulsion systems in the aerospace industry.
Other important articles on Cryogenic Seals from Advanced EMC Technologies:
- 6 Things You Should Know about Cryogenic Seals
- Dummys Guide to Cryogenic Loading Arm Seals
- Six Kinds of Polymer Jacket Materials for Spring-Energized Seals
Why do we need cryogenic seals?
A long time ago, the petroleum industry discovered that if natural gas is cooled to a liquid (LNG), it is much easier to transport. In fact, many chemicals can be transported or more easily used if they are cooled to cryogenic temperatures. These include liquid natural gas, liquid petroleum gas (LPG), helium, hydrogen, nitrogen, and oxygen, just to name a few. When dealing with such super-cooled media, a different approach is required for achieving a tight seal.
Spring-energized PTFE Cryogenic Seal
A popular type of cryogenic seal is a PTFE spring-energized seal. A spring-energized seal is a polymer seal assembly with a spring included that forces the seal against the mating surface to achieve as leak-proof a seal as possible. The spring lip is made of PTFE, or some variant of it, and the energizing spring is made of metal. The energizing spring is typically made of 17-7 precipitation hardening stainless steel or 301/304 stainless steel, although in some special applications you may see Elgiloy, Hastelloy, Inconel, or 316 stainless steel.
PTFE is popular because it is thermally stable across a temperature range that includes cryogenic temperatures, possesses an extremely low coefficient of friction (0.09 for virgin PTFE) allowing it to be used in dry running environments, has reduced slip-stick behavior, is not subject to explosive decompression, and is chemically non-reactive.
Forms of PTFE
The variants of PTFE used for these applications include PTFE, PCTFE (polychlorotri-fluoroethylene), and TFM. Many times virgin (unfilled) PTFE wont have the required mechanical properties for a seal, so additives or filler materials can be added.
For cryogenic applications, the typical filler materials are graphite, carbon, glass, and bronze. Bronze actually reduces the friction from 0.09 to 0.08, but is not always recommended for rotary applications. Glass-filled PTFE is well adapted to use with helium, and graphite+carbon can improve the properties of PTFE without compromising its chemical compatibility.
If you are designing a cryogenic seal, whether its for an LNG loading arm or a superconducting application, keep in mind the usefulness of PTFE spring-energized cryogenic seals.
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