Pollution
Humain
Environnement
Economique

A leak of liquefied natural gas fuel was detected at a filling station after it had been unloaded by a contractor. After LNG is unloaded, it is reconditioned so that it is stored under the right pressure and temperature conditions. It was during this phase that the gas detector detected a leak at the rear of the dispensing tap, causing the station’s automatic safety valves to close. The tanker’s driver informed his company, which also manages the station. The on-call manager saw all the parameters on her remote console, identified the source of the leak, and dispatched a technician. At the same time, a customer who had arrived to get fuel alerted the firefighters. Upon their arrival they saw that the station’s safety valves were closed. Given the limited duration of the leak, the amount of LNG that spilled out – and subsequently evaporated to gas – was very low.

Breakaway coupling

Construction: stainless steel and polytetrafluoroethylene (PTFE).

Operating principle: during normal operation, the ends of the two check valves are in contact with one another to allow fluid to flow. If the breakaway coupling is yanked out, the two halves split apart. This moves the valves into position so that they completely seal off each side of the coupling.

The leak occurred when the breakaway coupling split apart but not enough to allow the two valves to completely seal off the flow of fluid. No mechanical stress had been applied to the dispensing tap. The split appears to have been caused by mechanical strain from repeated contraction and expansion of the coupling, which is used to convey a cryogenic fluid.

Two systems were considered and tested on site:

  • A compensator was fitted to the line between the wall of the bund and the dispensing tap;
  • The tube between the wall of the bund and the dispensing tap was formed into an expansion loop.

The second system was chosen because it is widely used in the LNG industry. It limits stresses placed on the breakaway coupling as much as possible whilst ensuring that it splits as designed if abnormal loads are placed on the dispensing tap. A similar accident occurred a few months earlier (ARIA 49845).