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PTFE cyrogenic seals

Cryogenic seals provide a mechanical containment mechanism for materials held at cryogenic 
temperatures, such as cryogenic fluids. Various techniques, including soldering and welding are available for creating seals, however specialized materials and processes are necessary to hermetically entrap cryogenic constituents under vacuum-tight conditions. Most commonly used are liquid helium and liquid nitrogen, which boil at very low temperatures, below -153 °C (120 K), as well as hydrocarbons with low freezing points and refrigerating mixtures. – information from Wikipedia.com.

The demand for seals for use in cryogenics applications is increasing, and there are certain facts about them that all engineers should now.  Here are six things you should know about cryogenic seals…

 

6 Things You Should Know about Cryogenic Seals

 

Number 1:  What temperatures are typically classified as cryogenic?

Typically, applications where temperatures fall below -238° F (or -150° C)  to absolute zero which is -460° F (or -273° C) are considered cryogenic.

 

Number 2:  What kind of applications are cryogenic seals used with?

Cryogenic seals are used in a variety of applications.  These include LNG fueling systems and compressors, rocket propulsion filling systems, specialty gas manufacturing, scientific instrumentation, magnetic resonance imaging, infrared telescopes, radio astronomy, biosystems, satellite tracking systems, pharmaceutical research, and more.

 

Number 3:  What kind of fluids do cryogenic seals come in contact with?

Cryogenic fluids include liquid oxygen, liquid hydrogen, liquid helium, hydrocarbons that have very low freezing points, refrigerants, and coolants.  In fact, cryogenics can be traced back to the 1800s and the first use of refrigerants.

 

Number 4:  What is an example of an issue that makes the design of cryogenic seals different from that of regular seals?

For extreme cryogenic applications, the seal faces and components may need to be completely dried before installation or you run the risk of the faces freezing together.  As a result, lubricants may be prohibited, which means that an unfilled polymer like PTFE, PCTFE and some applications UHMW might be preferred. Also, any moisture near the area of the fluid will freeze, which can even cause problems for the space within the seal, or between the clamp ring and the gland. 

 

Number 5:  What kind of issues are encountered in using polymer materials with cryogenic seals?

While materials such as PTFE are commonly used in cryogenic seals, there can be some drawbacks.  Many polymers materials typically have high coefficients of thermal expansion, which means their dimensions can change significantly when exposed to large changes in temperature.  At very low temperatures, their behavior can change from elastic to brittle.  However, this can be accounted for by using the behavior of the polymer at low temperatures are the design variables, rather than its behavior at room temperature.  Another approach to counter these issues is to energize the seal with a spring.

 

Number 6:  What are the benefits to using polymers with cryogenic seals?

There are many benefits to using polymers such as PTFE materials for cryogenic seals.  In applications such as aerospace, their high strength-to-weight ratios mean significant reductions in weight compared to using all metal seals.  Their low thermal conductivities acts as insulators.  Polymers are corrosion resistant, and many (such as PTFE) has self-lubricating properties.  In addition, polymers have a reputation for being durable and fatigue resistant.  All of these characteristics are extremely useful for cryogenic seals.

 

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