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Behaviour of LNG in the cargo tanks - Dispersal of vented cargo vapours

When loaded in the cargo tanks, the pressure of the vapour phase is maintained substantially constant, slightly above atmospheric pressure. The external heat passing through the tank insulation generates convection currents within the bulk cargo, causing heated LNG to rise to the surface where it vaporizes.
The heat necessary for vaporization comes from the LNG, and as long as the vapour is continuously removed by maintaining the pressure as substantially constant, the LNG remains at its boiling temperature.



If the vapour pressure is reduced by removing more vapour that is 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.

Cargo vapour, whether toxic or flammable, should be vented to atmosphere with extreme caution, taking account of regulations and weather conditions.

If the temperature of the vented vapour is below atmospheric dew point, clouds of condensed water vapour will form. Condensed water vapour (fog) is heavier than air whereas the cargo vapour may or may not be heavier than air, depending on temperature. The cargo vapour cloud is likely to be oxygen deficient, and should only be entered by personnel wearing breathing apparatus. Furthermore, it should never be assumed that the cargo vapour is contained entirely within the boundaries of the visible water vapour cloud.

If the cargo vapour is heavier than air it may accumulate on deck and enter accommodation spaces. Standard precautions should therefore be observed. In some cases it may be possible to heat vapour before venting to reduce its density and assist dispersion. If such facilities are provided they should be used.

Any cargo vapour, whether toxic or flammable, is to be vented to atmosphere with extreme caution, taking account of regulations and weather conditions. In some cases venting may be prohibited.

If the vapour vented is at a temperature below the atmospheric dewpoint clouds of water vapour will form and these are heavier than air. The cargo vapour may or may not be heavier than air, depending on temperature. It is never to be assumed that the cargo vapour is contained entirely within the boundaries of the water vapour cloud. The cargo vapour cloud is likely to be Oxygen deficient and is only to be entered by personnel wearing breathing apparatus.


Void spaces / duct keels and pipe tunnels

Because of restricted natural ventilation these spaces may be oxygen deficient. In addition, they are adjacent to cargo holds and ballast tanks, so both hydrocarbon vapour and inert gas may leak into them. It must be recognised that the rescue of an unconscious or injured person may be very difficult. Hydrocarbon vapours may also be released from adjacent cargo tanks due to leaks in pipelines or hairline cracks in the tank structure. It is therefore essential that checks are made of these spaces for the presence of hydrocarbons.

Some ships will be equipped with automatic detectors and recording devices for this purpose. Ships that do not have such equipment must carry our manual checks at least weekly and the results recorded in the table within the deck log book.

Where the Classification Society rules do not require the permanent lighting systems in these spaces to be isolated during gas trading, the following procedures are to be complied with in order to eliminate the hazards that could result from damaged flame proof fittings.

The space must be tested and proven gas free, before the lights are switched on.

The mechanical ventilation system is to be in operation before the lights are switched on, and must remain in operation until after the lights are switched off.






Related Information:

  1. Engine and Boiler Room Precautions


  2. Cargo Machinery Room Precautions


  3. Use of cargo as fuel -Cargo conditioning, reliquefaction and boil-off control for LNG carriers


  4. Type of gas carriers - variation in the design, construction and operation



Preparatory operations for drydocking

Hazards of electrical equipment on board

Discussion prior to cargo transfer in liquefied gas carrier

Handling LPG And ammonia cargo - preparation for loading

The risk of discharging cargo - a brief outline to liquefied gas carriers

The risk of laden voyage - a brief guide to liquefied gas carriers

Connection and disconnection of cargo hoses and hard arms







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