The number of reliable sealing solutions for Low-Temperature Service in LNG plants is limited. The two main polymers currently used are PTFE spring energized seal or UHMW-PE seals.

The challenge of using liquefied natural gas

The leakage of LNG (liquefied natural gas) at low temperature is dangerous to the health and safety of workers and the operation of the plant. The problems such as toxicity, cold, suffocation, flammability and explosion caused by the rapid expansion of liquefied natural gas indicate that reliable leak proof sealing is required.

It may be challenging to find an effective sealing solution for low temperature working environment of LNG plant. Remember that nitrogen exists in liquid form at normal atmospheric pressure between - 346 ° F and - 320.44 ° F. Its liquid to gas expansion ratio is very high, 1:694, which means that when it boils (starting from the boiling point of - 320.44 ° f), it will expand to 694 times its original volume. If it occurs in a sealed environment, this may result in extremely high pressure changes, and most LNG seals must remain functional under vacuum pressure or extremely high pressure.

Low temperature sealing of LNG plant

The temperature involved in LNG is just at the position where many elastomeric and polymer materials lose elasticity and begin to behave as brittle materials. Some seals will also experience dimensional fluctuations associated with temperature changes, further increasing the probability of failure. If the temperature fluctuation is periodic, problems related to periodic stress will occur. Another problem associated with dynamic cryogenic seals is lubrication: at such low temperatures, standard lubrication solutions simply do not work.

Sealing options are limited to two UHMW or PTFE polymers, and full contact anti shrink springs are essential.

Both of these sealing jacket materials can be designated as PTFE and are usually known under the trade name Teflon or florolon 1000. Ultra high molecular weight PE or UHMW, fluorolon 5000 can handle low temperature in low temperature environment without embrittlement (some grades can handle temperatures as low as - 350 ° f), or quickly yield to the influence of cyclic stress. In addition, UHMW and PTFE are self-lubricating, low friction, support dry operation, and are non stick / sliding materials. In addition, both products are compatible with a variety of chemicals, including those that may be encountered in LNG plants.

A spring energized seal is a seal assembly comprising an energized spring that forces a seal lip against a mating surface to achieve a highly leakproof seal. When combined with PTFE lip, this sealing design is considered to be an ideal choice for low temperature applications involving LNG. Spring actuators add permanent elasticity to the seals and compensate for lip wear, eccentricity, hardware misalignment, and (perhaps most importantly, when LNG is used) extreme pressure and dimensional changes.

When low temperature and static, reciprocating or rotary motion are involved, the recommended geometry of the spring actuator is a simple coil spring. However, oscillating or static motion may require the use of solid springs. Recommended spring materials include 17-7 precipitation hardening stainless steel, 301 / 304 stainless steel, or, in some applications, Hastelloy, 316 stainless steel, Inconel, or Elgiloy.

Low temperature seal design for LNG plants may be challenging and must meet extremely high reliability and safety standards, but PTFE spring energized seal is an excellent starting point.