Cryogenic sealing: Getting it right and overcoming the common pitfalls

Cryogenic sealing is typically required for Liquefied Natural Gas (LNG) processes. To achieve the sealing requirements of LNG processes, thermoplastic materials are commonly used in place of elastomers. At such low temperatures, elastomers become brittle and ineffective as sealing materials. The point at which natural gas changes from gas to liquid is at -162°C. This reduces the volume to approximately 1/600th of the gas form for much more efficient transportation in LNG tankers between the original source and where the gas is needed.

This article provides an overview of cryogenic sealing, the common pitfalls and how to get it right in your equipment. 

Temperature is the first and most obvious challenge relating to LNG processes. Industry standards and specifications are actually starting to suggest the use of thermoplastic seals for equipment operating below -29°C. Whilst there are elastomers that can seal at these temperatures, it just shows how important the use of thermoplastics are when dealing with much lower temperatures.

There are a number of thermoplastic materials available to carry out this sealing (primarily PTFE, PEEK, PCTFE, TFM and UHMW PE) however overcoming the common pitfalls is not just as simple as choosing one of these materials and making parts to a drawing.

The key considerations are:

• Coefficient of thermal expansion - through large temperature swings the thermoplastic and the spring materials will both change dimensions according to the temperature. Bear in mind that fitting happens at ambient temperature whereas operation can be extreme low or high so there are a number of factors to consider with temperature. The resulting effects could cause parts to seize at high temperature or shrink at low temperature and no longer form an effective seal. 
• Media - this might not just be the liquefied gas that is being transferred, there may also be lubricant or a lack of lubricant to account for.
• Movement - whether the equipment you have is dynamic or static will have an impact on the material and seal design chosen
• Size and pressure - a lot of LNG processes are at relatively lower pressure but make sure this is considered along with the seal housing as it will impact the design
• Cold set / creep – the seal design may need to be modified to help reduce shrinkage and or permanent deformation at cryogenic temperatures, spring energisers may also be required to compensate for shrinkage.

At lower temperatures, thermoplastic materials become more brittle and less adaptable to rough sealing faces. It is critical that housing and adjacent equipment design, surface finishes and materials all have this in mind.

The key here is testing across the operation range of the equipment in the media. Being able to understand and report the data from dynamic testing across the temp range from cryogenic up to high temperatures is essential.

To make sure you cover all bases, speak with a supplier that has material processing expertise, design, manufacture and testing capabilities all under one roof. This grouping of knowledge will make sure that every stage in the process is done with the other in mind and will ultimately achieve success in your challenging cryogenic sealing applications.

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