Liquefied Gas Carrier

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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 ammonia fire.

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:
  1. the likelihood of surrounding fire is small,
  2. safety relief valve emissions are piped away to mast head vents
  3. shipboard pressurised tanks are provided with water sprays and water for cooling purposes is readily available.
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.

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 reduced by: 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. For example:
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.

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.

LPG carrier

Related Information:

  1. Design characteristics of liquefied gas carriers

  2. Liquefied gas carrier -applicable regulations

  3. Vapour Characteristics of liquefied gases

  4. Low temperature effects of Liquefied gases

  5. Reactivity of liquefied gas cargo and safety guideline

Liquefied gases - Health hazards

Safety equipment

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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