Liquefied Gas Carrier

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Fire hazards and precautions - Atmosphere Control For Gas Carrier

When carrying a flammable cargo, the cargo system contains liquid and vapour. The atmosphere around the cargo tanks is normally inerted to prevent the formation of flammable mixtures. The IMO Codes use the term “environmental control” to describe this process. Some careful considerations are needed to ensure safety .

Hold and Inter-barrier Spaces

These spaces may have to be filled with inert gas if the cargo is flammable. Different cargo containment systems require different procedures, as follows:
LNG carrier underway
Fig:LNG carrier underway

Full secondary Barrier system: Dry inert gas or nitrogen; Maintained with make-up gas provided by the shipboard inert gas generation system, or by shipboard storage which should be sufficient for at least 30 days at normal rates of consumption.

Partial secondary Barrier system :Dry inert gas or nitrogen; Maintained with make-up gas provided by the shipboard inert gas generation system, or by shipboard storage which should be sufficient for at least 30 days at normal rates of consumption. Alternatively, subject to certain conditions, the space may be filled with dry air (see Regulation of the IGC Code).

No secondary Barrier system :Dry air or dry inert gas depending on the cargo; Maintained either with dry air provided by suitable air drying equipment, or with make-up inert gas provided by the shipboard inert gas generation system or shipboard storage.

Cargo Tanks and Piping Systems

The formation of a flammable vapour mixture in the cargo system should be prevented by replacing the air in the system with inert gas before loading, and by removing cargo vapour by inert gas after discharge, prior to changing cargoes or gas-freeing. Suitable pipe connections should be provided for this purpose. Inerting should be continued until the concentration of oxygen or cargo vapour in the space is reduced to the required level. The tank atmosphere should be monitored at different levels to ensure these are no pockets of excessive concentrations of oxygen or cargo vapour, particularly in tanks with complex internal structures or bulkheads.

Some cargoes require the oxygen content in the vapour space to be kept extremely low (in some case less than 0.2%) to prevent a chemical reaction occurring. For instance, ethylene oxide / propylene oxide mixtures can decompose spontaneously unless special precautions are taken to control the atmosphere; and butadiene can react with oxygen to form unstable peroxide compounds. The oxygen content in the tanks must be reduced as necessary before loading begins. While such cargoes remain on board, oxygen is excluded either by keeping the ullage space full of inert gas at a positive pressure or, in the case of butadiene, by keeping the cargo vapour above atmospheric pressure. In every case, shippers’ requirements should be strictly observed.

Spherical tank containment

Fig: Spherical tank containment system

Inert Gas Quality

Inert gas used for atmosphere control should be suitable for the intended purpose, regardless of source. In particular it should:

(1) Be chemically compatible with the cargo and the materials of construction throughout the full range of operating temperatures and pressures;

(2) Have a sufficiently low dew point to prevent condensation, freezing, corrosion, damage to insulation etc. at the minimum operating temperature;

(3) Have an oxygen concentration not exceeding 5%, but as los as 0.2% if the cargo can react to form peroxides;

(4) Have a low concentration of CO2 to prevent it freezing out at the anticipated service temperature.

(5) Have minimal capacity for accumulating a static electrical charge.

Inert Gas Hazards and Precautions

The main hazard associated with inert gas is asphyxiation of personnel due to lack of oxygen. Asphyxiation can occur in those parts of the cargo system which have been inerted, or in other enclosed spaces into which inert gas leaked. Nobody should enter spaces which are not in common use until it has been established that the atmosphere will support life.

At the inert gas plant Is often situated in the engine room, great care should be taken to ensure that cargo vapour does not flow back along inert gas supply lines; non-return valves should be tested for effectiveness, at regular intervals. Any temporary connection between the inert gas plant and the cargo systems should be disconnected and tightly blanked after use.

If a liquid nitrogen system is used, care should be taken to avoid contact with skin and eyes, or severe cold burns will be caused. Any metal structure or component likely to come into contact with liquid nitrogen could suffer brittle fracture unless it has been designed for a service temperature of -196 degree C. Great care should be taken to ensure that vaporizers are used correctly.

Related Information:

  1. Inerting of cargo tanks prior loading cargo

  2. Precautions against sources of ignition

  3. Matters That Require Attention to Onboard Work

  4. Statistic electricity precautions for liquefied gas carriers

  5. Personal protective equipments for people working onboard gas carriers

  6. Risk of Overfilling of Cargo Tank during Loading

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Inerting of Cargo Tanks prior loading LNG cargo

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Procedure for Water washing after Ammonia Cargoes

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