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Vapour characteristics of liquefied gases - Vaporisation of spilled liquid

When a gas is stored as a liquid, whether under pressure or refrigeration, it will vaporise when released to the atmosphere, taking heat from its surroundings in so doing. Depending upon the liquid spilled, the spill size and whether the spill is on land or water, the rate of vaporisation and the temperature and density of the ensuing vapour cloud will vary. Almost certainly the cloud will be low lying (only methane when warmer than -100 deg C, ethylene and ammonia are lighter than air), will be initially cold, and will drift downwind; its occurrence will, in general, be visible as a white 'cloud' which is condensed atmospheric water vapour.

One characteristic of liquefied gases is the large quantity of vapour readily produced by a small volume of liquid (1m3 of LNG will produce 600 m3 of vapour at ambient temperature). The venting of cargo vapour should therefore be avoided. However, if the venting of cargo vapour is unavoidable, it should be done with care and in full knowledge of the potential hazards. In most port areas the venting of flammable or toxic vapours is forbidden, and applicable local regulations should be observed.

Liquefied gases contain a great deal of energy, but so does a pile of coal. LNG is a liquid that won't burn until it becomes a vapor, and the vapor won't burn until it mixes with air and becomes diluted to between 5 per- cent and 15 percent LNG vapor in air. Above 15 per- cent, there's not enough air for it to burn, and below 5 percent, there's not enough LNG vapor to burn. LNG vapor clouds burn when they are in the 5-15-percent dilution range, but they don't explode.

U.S. Coast Guard tests have demonstrated that unconfined LNG vapor clouds do not detonate, they only burn.

Risk of LNG vapour cloud

If there is no immediate ignition of an LNG spill, a vapour cloud may form. The vapour cloud is long, thin, cigar shaped and, under certain meteorological conditions, may travel a considerable distance before its concentration falls below the lower flammable limit. This concentrate is important, for the cloud ignite and burn, with the flame travelling back towards the originating pool. The cold vapour is denser that air and thus, at least initially, hugs the surface. Weather conditions largely determine the cloud dilution rate, with a thermal inversion greatly lengthening the distance travelled before the cloud becomes non-flammable.

The major danger from an LNG vapour cloud occurs when it is ignited. The heat from such a fire is a major problem. A deflagration (simple burning) is probably fatal to those within the cloud and outside buildings but is not a major threat to those beyond the cloud, though there will be burns from thermal radiations.

When loaded in the cargo tanks, the pressure of the vapour phase is maintained as substantially constant, slightly above atmospheric pressure.

The external heat passing through the tank insulation generates convection currents within the bulk cargo, heated LNG rises to the surface and boils.

The heat necessary for the vaporization comes from the LNG and long as the vapour is continuously removed by maintaining the pressure as substantially constant, the LNG remains at the boiling temperature

If the vapour pressure is reduced by removing more vapour than generated, the LNG temperature will decrease. In order to make up the equilibrium pressure corresponding to its temperature, the vaporization of LNG is accelerated, resulting in an increase heat transfer from LNG to vapour.

Methane is an asphyxiant in high concentrations because it dilutes the amount of Oxygen in the air below that necessary to maintain life. Due to its inactivity, Methane is not a significant air pollutant although it is not a significant greenhouse gas and, due to it insolubility, inactivity, and volatility, it is not considered a water pollutant.

Related Information:

  1. vaporized gas (BVG) management system for carrying LNG cargo

  2. Cargo Machinery Room Precautions

  3. Fire fighting plan for LNG cargo

  4. LNG spill risk during marine transportation

propylene oxide mixtures procedure

Procedures for LNG cargo loading

Details of various cargo handling equipment onboard

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