Understand LNG Fire Hazards

Potential hazards resulting from intentional or accidental spilling of large quantities of LNG include thermal radiation from vapor cloud fires (also referred to as flash fires) and pool fires. There is general agreement among LNG experts regarding the following aspects of potential LNG fire and explosion hazards:

  1. Vapors from large, un-ignited spills of LNG cannot travel far into developed areas without finding an ignition source, igniting, and burning back to the source.
  2. Once delayed ignition of the vapor cloud occurs, and provided that the cloud is unconfined and rich in methane, the LNG vapors will burn in the form of a vapor cloud fire.
  3. A vapor cloud traversing over commercial and/or residential terrain will almost certainly encounter an ignition source early in its downwind drift and the resulting vapor cloud fire will burn back to the source.
  4. The vapor cloud fire will burn back to the source and cause a pool fire at the source if the release is a continuous release and the release duration is longer than the time it takes the cloud to find an ignition source.
  5. If the vapor cloud is confined and/or the vapors contain large amounts of heavier hydrocarbons (C2+), then the flame can accelerate and result in an explosion. The magnitude of the explosion and explosion damage will depend on several factors including the amount of vapors above the lower flammable limit, the presence of obstacles and degree of confinement, the composition of the vapor cloud, and the strength of the ignition source.
  6. If immediate ignition occurs, a pool fire will result. The extent of the pool spreading (diameter) and flame height will depend on several factors including the flow rate of LNG, the spill surface type (water or land), the spill surface geometry, spill surface roughness, release composition, release temperature, ambient wind speed, ambient temperature, and ambient relative humidity.
  7. If the liquid pool is unconfined and the inventory of LNG is large, the fire will continue to burn until all the fuel is exhausted by the pool fire. It is not practical or even possible to extinguish large LNG pool fires resulting from large spills of LNG unless the flow of LNG feeding the pool can be stopped.

The maximum vapor cloud fire hazard area is typically estimated by calculating a downwind dispersion distance to the lower flammable limit (LFL) and a cross-wind dispersion distance to ½ LFL at low wind speed and stable atmospheric conditions. This maximum fire hazard zone is very unlikely to be experienced in any situation where the cloud drifts over populated areas. As indicated in point 3 above, the cloud will soon encounter an ignition source and burn back to the source well before the maximum hazard area is reached.

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