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Flammability of cargo vapours and associated hazards in liquefied gas carrier
All liquefied gases presently transported in bulk by sea, with the exception of chlorine and nitrogen, are
flammable. The vapours of liquefied gases are generally as easily ignited as those of oil cargoes. The
exception to this is ammonia vapour, which requires considerably higher ignition source energy to
ignite than the other flammable vapours. Statistically, therefore, fires following ammonia leakage are
less likely than those with other cargoes but it would be unwise to discount thereby the possibility of an
Because of the high vapour pressure and rapid vaporisation of spilled liquefied gases, the spread of
flammable vapour is likely to be more extensive than in the case of a similar liquid spillage of oil. The
chances of ignition following a spill of liquefied gas are thereby greater. Radiation from liquefied gas
fires, because of the rapidity of vapour production, may be intense and no fire-fighting should be
attempted without full fire-fighting protective clothing.
Leakage of a liquid or vapour from a pipeline under pressure will burn as a jet if ignited which will
continue as long as fuel is supplied.
A particularly destructive form of vapour burn associated with the transportation of liquefied gas in
pressurised containers is the BLEVE (Boiling Liquid Expanding Vapour Explosion). This arises from the
rise in pressure within the container together with the weakening of the uninsulated and uncooled part
of the container shell due to surrounding fire or due to radiation from the ignited vapour emission from
the safety relief valve. As a result, the container suddenly splits open, releasing the pressurised liquid
to atmospheric pressure. The consequent flash of liquid vapour provides fuel for a rising fireball and
parts of the ruptured container may be projected apart with considerable violence. The BLEVE is a well
known occurrence in road and rail transportation but has never occurred in marine transportation and
is unlikely so to occur for the following reasons:
Almost all cargo vapours are flammable. When ignition occurs, it is not the liquid which burns but the evolved vapour. Different cargoes evolve different quantities of vapour, depending on their composition and temperature.
- the likelihood of surrounding fire is small,
- safety relief valve emissions are piped away to mast head vents
- shipboard pressurised tanks are provided with water sprays and water for cooling purposes is
Flammable vapour can be ignited and will burn when mixed with air in certain proportions if the ratio of vapour to air is either below or above specific limits the mixture will not burn. The limits are known as the lower and upper flammable limits, and are different for each cargo.The risk of a flammable mixture being ignited may be substantially
preventing leaks from developing & ensuring that there are no sources of ignition when a leak occurs
and vapours may be within the flammable range.
Cargo temperatures may be very low but so too can the flash point.
Vapours and gases will only ignite if they are within the flammable
range that is there is enough oxygen present to support ignition and
there is neither too little vapour (too lean), nor too much (too rich) for
the mixture to burn.
Methane has a flash point of -175 degC
- Propane has a flash point of -105 degC
- Butane has a flash point of -60 degC
The flammable range of a gas is defined by the terms Lower
Flammable Limit (LFL) and Upper Flammable Limit (UFL), these are
sometimes known as the upper and lower explosive limits.
Combustion of vapour / air mixture results in a very considerable expansion of gases which, if constricted in an enclosed space, can raise pressure rapidly to the point of explosive rupture.
No fuel or petroleum product is completely safe: not
coal, oil, or liquefied natural gas, all of which are car-
ried on ships.
LNG is a fuel, and, when it becomes a
gas and mixes with air, it will burn. You can never
consider anything that burns completely safe, even
fairly innocuous materials like wood and cooking oil.
But some are worse than others, and liquefied natu-
ral gas is far from the worst.
When LNG vapor reaches an open flame, it easily catches fire and will burn
everything within the vapor-air mixture; the same as
when natural gas burns. Due to the extra care in
designing, maintaining, and operating LNG ships,
they all have excellent safety records. There have
been some fires at shore facilities, but those are rare
events. However, if a ship catches fire, it could be
very serious. That's why the LNG industry and the
Coast Guard are very careful about the movement of
liquefied natural gas.
- Design characteristics of liquefied gas carriers
- Liquefied gas carrier -applicable regulations
- Vapour Characteristics of liquefied gases
- Low temperature effects of Liquefied gases
- Reactivity of liquefied gas cargo and safety guideline
Liquefied gases - Health hazards
Liquefied gas cargo reactivity
Liquefied gas cargo corrosion
Liquefied gas cargo vapour characteristics
Liquefied gas cargo - low temperature effects
Liquefied gas carrier -monitoring cargo pressure
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