Orings are used in various industries. Some industries require seals to be highly performance-resistant in harsh, low-temperature conditions. Cryogenic applications most often concern a working environment where the temperature drops below -150°C. Cryogenic seals should ensure safety and reliable operation without the risk of leakage. O-ring seals designed for low temperatures are of great importance in the pharmaceutical, aerospace, medical, food (including dairy), oil and gas, petrochemical industries.
Cryogenika – What is it?
Cryogenic investigates the behaviour of materials at extremely low temperatures, which alter its chemical properties. Cryogenic liquids, which include m.in. oxygen, nitrogen, helium, methane and argon can be very dangerous. Contact with cryogenic liquid is a potential life-threatening, toxic or suffocation hazard. Liquids are flammable and leakage can cause explosion due to rapid spread. Orings in cryogenic applications should take these critical concerns into account. The tolerance to low temperatures of seals used in cryogenic applications must be significantly lower than that of standard orings. The use of liquid nitrogen is used in medicine (cryosurgery, cryotherapy), metal working, freezing food, while liquid oxygen in the petrochemical industry. Liquid helium (NMR tomographs) or liquid methane (drive fuel of motor vehicles) are also used among cryogenic fluids. Please note that the use of liquid nitrogen and liquid helium applies to boiling at very low temperatures (below -153°C). Cryogenic applications include applications where the temperature drops below -150° C to – 460° C (absolute zero). For the storage of liquefied m.in (nitrogen and helium), special vessels are needed to provide excellent thermal insulation. All information and recommendations for the storage of cryogenic liquids should be strictly observed.
Effect of low temperature on orings
Round-section orings that can be degraded in low-temperature applications. Low temperatures can reduce sealing performance. Elastomeric products in contact with low temperatures may shrink, which in turn reduces their compression capacity and can lead to leakage. When the low temperature limit is exceeded, rubber products harden, becoming less flexible and more fragile. This will cause the performance of the orings to be interrupted and they will start leaking. In order to prevent failures resulting from operation in a low temperature environment, various tests are carried out to test how specific rubber mixtures behave at low temperatures. Chemical compatibility in combination with appropriate physical properties and resistance of materials to extremely low temperatures is the key to producing efficient o-rings.
Seal performance test
The factor that has the greatest impact on the flexibility of elastomeric products is low temperature operation. As the temperature drops, the elasticity of the rubber (depending on the mixture) gradually decreases until it completely loses this property. Orings that are flexible at low temperatures ensure proper sealing performance. The performance of orings intended for applications at low temperatures is measured by standard tests to measure physical properties and material performance. Tests are performed on: brittleness (ASTM D2137), temperature recall (ASTM D1329), compression kit (ASTM D395). Performance tests shall be carried out to ensure adequate performance of orings operating in a low temperature environment.
As a result of working at low temperatures, the fragility of rubber products increases. Increased brittleness can lead to rupture, rupture or damage to the oring. For this purpose, a brittleness test (ASTM D2137) is carried out, which measures the resistance to cracking of orings, which after bending is subjected to a certain temperature for a certain period of time. This method determines the lowest temperature at which rubber products do not degrade due to cracking.
The ASTM D1329 test, also called the TR test, is carried out to measure the temperature at which frozen, vulcanized elastomeric products return to a flexible state. The method used is aimed at assessing the effects of crystallization and comparing the properties of rubber products during the temperature reversing process.
The performance of the sealing rings can be checked when exposed to low temperatures and what will happen to it when the temperature is raised. The ASTM D395 test checks how long-lasting compressed material will behave and how it can recover its original section thickness. The low compression set of the material indicates that it regains its original section thickness after prolonged compression and thus has a better sealing capacity. During prolonged compression, elastomers do not completely recover their original thickness.
Orings for cryogenic applications
An extremely important element is the chemical compatibility of materials that come into contact with cryogenic liquids. In order to ensure efficient sealing, we need materials that can withstand prolonged contact with extremely low temperatures. Thanks to the constant development of polymer engineering, many materials have expanded their operating temperature range and do not shrink in low temperature applications. The right choice of sealing material is crucial for cryogenic applications. An efficient seal in contact with low temperatures must also meet a number of other important properties (chemical resistance, mechanical properties, resistance to pressure spikes). Cryogenic orings must be designed in such a way as to prevent leakage of liquids.
O-rings made of polytettrapolyfluoroethylene (PTFE) filled with various polymers are used as standard. These polymers are polyimids and polyethylene materials with a very high molecular weight (UHMWPE). Polyethylene materials are evous and non-toxic. Chemical compatibility, low coefficient of friction, resistance to extrusion and excessive wear are the advantages of these materials. The combination of PTFE and various polymers ensures sealing efficiency over a wide temperature range. PTFE is a fluoropolymer that, thanks to its properties (chemical compatibility, low temperature resistance, low coefficient of friction) is used in the aerospace industry and in laboratory situations.
O-rings made of PTFE, despite their poor compression set, provide exceptional resistance to extremely low temperatures up to -200°C. The material also offers high-quality resistance to almost all industrial chemicals (chemical compatibility). PTFE orings are compatible in contact with acids, bases and solvents. In addition, PTFE has a low coefficient of friction, so it does not require lubrication during installation. At cryogenic temperatures PTFE retains its bending properties, does not crumble, and also offers durability in contact with UV radiation and does not swell due to moisture absorption.
Orings in FEP/PFA
Fep/PFA seamless seals demonstrate high-quality chemical resistance, ensuring reliable, long-lasting fluid sealing. The sealing rings in the wrap can have two types of elastomeric core: fixed and empty. The solid elastomeric core can be made of FKM (Viton®) or silicone (VMQ). The FKM elastomeric core offers excellent flexibility and a good compression set. O-rings in silicone wrap, in addition to the above-mentioned properties) are softer and show greater heat resistance. The silicone core in contact with very low temperatures remains flexible. The empty elastomeric core is mainly designed for applications where extreme flexibility is required. The enclosed elastomeric core can be made of FDA-approved plastic, so it can be used in the food and pharmaceutical industries and in other applications that require hygiene.
The pfa outer shell is chemically resistant to various corrosive substances (m.in. alcohol, acid, petroleum and aromatic solvents). It has low compression as well as a operating temperature range of -60°C to +260°C.
The FEP outer shell has similar properties to PFA, but it has greater mechanical strength and resistance to stress and cracking. This type of material is characterized by weaker mechanical properties and shorter service life than PFA. The operating temperature ranges from -60° C to +205°C.
FEP/PFA o-rings are prone to scratches, so avoid using them in applications that come into contact with abrasives. However, they are suitable for static applications. Spring-loaded seals are also available for cryogenic applications that are resistant to extremely low temperatures up to -250°C.
Cryogenic O-rings – Application
The use of liquid nitrogen in cryogenic seals is designed to work at temperatures from -196°C, while in liquid hydrogen applications up to -254°C. Cryogenic fluids also include liquid natural gas, liquid oxygen and helium. Cryogenic orings are most commonly used in the aerospace and oil and gas industries. In the aerospace industry, gaskets are used m.in. tanks and rocket valves. They protect the gas mixture from leakage, which is responsible for propelling the rocket. In the oil and gas industry, the use of gaskets corresponds m.in. for the transfer of liquefied natural gas (LNG) and liquefied petroleum gas (LPG). Here’s information about other apps, here they are:
- pharmaceutical research
- magnetic resonance imaging
- infrared telescopes
- cryogenic pumps
- production of special gas
- LNG fuel systems and compressors
- radio astronomy
- production of specialty gases
- pin seals for cryogenic valves
- infrared telescopes
- scientific instrumentation
What to look for when choosing cryogenic seals?
When purchasing orings for cryogenic applications, attention should be paid to several quality indicators to which the m.in. leakage rate (indicator should be maintained in the micro range), proper packaging, ease of installation. Properly packaged seals prevent various types of contaminants (e.g. dust) from entering them, which during installation can negatively affect the tight connection between the oring and the surface. Easy installation means less use of the force needed to insert it between two surfaces. In this case, polymer seals have an advantage over metal seals.
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