Ice-cold precision: seals for extreme temperatures in cryogenics

Cryogenic applications involve extreme temperature ranges, often below -150 °C. (Image source: Vulkan Verlag)

Cryogenics, which deals with the handling and application of extremely low temperatures, poses special challenges for the sealing systems used. Unlike conventional seals, which are designed for use at room or moderate operating temperatures, seals for cryogenics must withstand extreme temperature ranges without losing their sealing or mechanical properties. Typical applications include the storage and transport of cryogenic liquids such as liquid nitrogen, oxygen, hydrogen or helium. This requires special materials and designs in order to fulfil the requirements for flexibility, tightness and chemical resistance.

Special requirements for seals in cryogenics

Cryogenic applications involve extreme temperature ranges, often below -150 °C, which place severe stress on materials. At these temperatures, many materials shrink and lose their elasticity, which can lead to embrittlement and leaks. At the same time, the risk of so-called cold flow increases, in which materials change their shape under stress and do not return to their original state. Seals must therefore remain flexible in cryogenic environments and retain their shape despite changes in temperature and pressure.

Cryogenic seals are frequently used in medicine and research. (Image source: AdobeStock / svastix)

Another key problem is leak tightness. Cryogenic liquids such as liquid hydrogen or helium are extremely volatile, which means that even the smallest leaks can lead to losses. In addition, many of these gases are highly reactive or flammable, which increases the risk of explosions or other dangerous incidents. Seals used in cryogenics must therefore have extremely low leakage rates and be able to maintain them even in the event of temperature fluctuations.

In addition, the seals used must have a high chemical resistance in order to withstand the aggressive conditions. Liquid oxygen and other cryogenic gases can react with various materials and chemically attack them. For this reason, special materials are used that are not only resistant to low temperatures, but also to chemical influences.

Choice of materials and sealing technologies

Materials that retain their elasticity even at extreme temperatures are preferred for seals in cryogenics. The most common materials include polytetrafluoroethylene (PTFE), fluoropolymers such as FEP and PFA as well as special elastomers. These materials not only offer high flexibility at low temperatures, but are also resistant to many chemicals, which makes them particularly suitable for cryogenic applications. PTFE, for example, has an operating temperature range of -200 °C to +260 °C and shows excellent chemical resistance to most cryogenic liquids.

Spring-loaded seals are another important technology in cryogenics. These seals utilise a coil spring made from materials such as stainless steel or Elgiloy to maintain constant pressure even at low temperatures. Such seals are used particularly in dynamic applications where the seals are exposed to repeated pressure and temperature changes. The spring ensures that the seal can adapt to changes in the environment while remaining tight at all times. The combination of elastomers and spring-loaded systems makes it possible to minimise leakage and extend the service life of the seals.

Liquid nitrogen is an important substance in cryogenics. (Image source: AdobeStock / HealthyShot)

In addition to PTFE and fluoropolymers, graphite is also used in gaskets for cryogenic applications, especially in areas where high temperatures can occur in combination with low temperatures. Graphite gaskets offer high oxidation resistance and are particularly suitable for applications where a long service life is required. These gaskets are often used in the chemical industry and in applications where both high temperatures and aggressive chemicals are present.

Applications in the industry

The areas of application for seals in cryogenics are diverse. In the aerospace industry, for example, they are used in tanks and valves that contain liquid fuel or oxygen. Here, the seals ensure that no gases escape and that the pressure in the tanks remains constant. Similar requirements exist in the oil and gas industry, particularly in the transport and storage of liquefied natural gas (LNG). LNG is stored at temperatures of around -162 °C, which is why seals must fulfil particularly high requirements in terms of temperature resistance and tightness.

Cryogenic seals are also used in medicine and research, for example in magnetic resonance tomographs (MRTs) and cryogenic pumps. These devices use liquid nitrogen or helium to cool magnets and thus create superconducting conditions. Seals are required here that prevent leaks and have a long service life in order to minimise costly maintenance and downtime.

Another important field of application is the food industry, where cryogenic processes are used to store and process food. Liquid nitrogen is used here to freeze food quickly and thus preserve its freshness and quality. The seals in the systems not only have to withstand low temperatures, but also meet the strict hygiene standards of the food industry.

To summarise, it can be said that seals for cryogenics must meet high technical requirements. They have to withstand extremely low temperatures, chemical influences and mechanical stresses without losing their tightness. The choice of material and design of these seals are crucial for the safe and efficient operation of systems that work in extreme temperature ranges. Thanks to modern technologies such as PTFE, spring-loaded seals and special elastomers, it is now possible to offer sealing solutions that work reliably even under the most extreme conditions.