One of the major challenges in working with LNG (liquefied natural gas) is transporting it. LNG is stored and transferred at cryogenic temperatures which can be a major challenge when it comes to designing seals.
Polymer seals that can handle cryogenic temperatures are needed anytime LNG is transported or transferred, and can involve complex mechanisms such as valves, swivels, connectors, etc. In this blog post we are going to focus on the challenges specific to MLAs, or marine loading arms.
Basics of LNG
LNG (liquefied natural gas) is natural gas that has been cooled until it becomes a liquid, which occurs at -260°F, and is then stored at atmospheric pressure. The process reduces its volume to about 1/600th of its original volume. This allows it to be easily transported, and once it arrives at its destination it can be used as fuel after it is warmed back into its gaseous state.
Marine loading arms are used on ships and barges to facilitate loading and transfer of liquids. A key part of loading arm design is the swivel joint which makes the loading arm flexible. The swivel joints themselves are made of stainless steel and the mechanical seals used are made of either a polymer or an elastomer.
There are some major challenges when it comes to seals that must operate in cryogenic conditions. The first is the dangerous aspects of working with cryogenic fluids however, LNG is one of the safer cryogenic materials out there (e.g., it is stored at atmospheric pressure and it not explosive in its liquid state).
The cryogenic fluids themselves usually make for poor lubricators. Any added lubricants or even moisture can freeze onto the face of the seal, causing the seal to shatter or, worse yet, the system to lock up and experience catastrophic damage. However, not using lubrication can result in issues like slip-stick vibration.
In addition, there will be dimensional changes between the time the seal is installed and when it experiences the cryogenic operating conditions. Care must also be taken to make sure that the chosen polymer or elastomer doesnt become brittle at the cryogenic temperatures involved.
Its also important to make sure that the seal is compatible with the chemicals involved. For example, LNG contains 80% to 99% methane along with various other hydrocarbons and constituents.
There are some commonly chosen polymers for MLA seals, including PTFE, PCTFE, TFM and UHMW PE. These materials have excellent chemical compatibility, low friction and/or dry-running, and good dimensional stability. Dimensional changes can be accounted for using a spring-energized seal or sizing the seal by accounting for plastics coefficient of thermal expansion. Cryogenic seals made using materials like PTFE and its variants or UHMWPE offer a very high strength-to-weight ratio, excellent durability, and self-lubricating properties.
While cryogenic seals for LNG pose an interesting set of issues, there are several polymeric sealing solutions available. By using materials such as low-temperature PTFE or TFM, the major problems faced by cryogenic seals can be overcome.
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