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Preparation for loading LNG cargo - drying of cargo tanks

During dry docking or inspection, cargo tanks which have been opened and contained humid air, must be dried to avoid the formation of ice when they are cooled down and the formation of corrosive agents if the humidity combines with sulfur and nitrogen oxides which might be present in excess in the inert gas.



The drying operation need not be performed independently by using dry air, instead during inerting operation by supplying dry inert gas, drying operation can be achieved. During such operation special attention is required to the delivery temperature of inert gas to prevent condensation of humid air inside the tank.

Dry air, with a dew of -70ºC to -40ºC, can be produced by the onboard IGG system.

Operation procedures and precautions

i) It is essential that cargo tanks are thoroughly inspected for cleanliness, free of liquid, any loose objects and all fittings are properly secured. Once this inspection has been completed, the cargo tank should be securely closed and drying operation can be started

ii) During drying operation, measure the atmosphere at different levels at regular intervals. When the dew point of the cargo tank drops below than the planned temperature, finish the drying operation.

Drying of Hold Spaces

The drying operation of a hold space is carried out in order to prevent tank insulation damage due to condensation of moisture inside it prior to initial cool down operation and periodically during a voyage. Fresh air is dehumidified by the IGG and sent to a hold space as dry air with a dew point of -70ºC to -40ºC through its bottom section, humid air inside the hold space is released through the vent pipe provided in the upper portion of the tank. The hold space should be maintained at a higher pressure than the atmospheric pressure.

Operation procedures and precautions:

i) Before delivering dry air into a hold space, completely dry up the bottom section of the hold space, particularly the bilge well.

ii) When drying a hold space after completing the inerting operation of a cargo tank, purge relevant equipments and inerting/aerating lines with dry air to prevent the ingress of inert gas into the hold space. This is because the hold space holding dry air sent into it is kept almost sealed till the next dry docking and, in addition, about 15% CO2 gas is present in the inert gas, which may corrode aluminum cargo tanks and destroy insulation materials.

iii) During drying operation, measure the atmosphere at different levels at regular intervals. When the dew point of the hold space drops below than the planned temperature, finish the drying operation.







Related Information:

Inerting of Cargo Tanks prior loading LNG cargo

Gassing-up requirement for cargo tanks

Initial Cool Down of cargo tanks

The risk of discharging liquefied gas cargo

Procedure for Changing Liquefied Gas Cargoes

Procedure for Liquefied Gas Cargo stripping

Displacing with Vapour of the Next Cargo (Purging)

Procedure for Water washing after Ammonia Cargoes



Inerting of Cargo Tanks prior loading LNG cargo

Gassing-up requirement for cargo tanks

Initial Cool Down of cargo tanks

The risk of discharging liquefied gas cargo

Procedure for Changing Liquefied Gas Cargoes

Procedure for Liquefied Gas Cargo stripping

Displacing with Vapour of the Next Cargo (Purging)

Procedure for Water washing after Ammonia Cargoes



Procedure for loading Liquefied Gas Cargoes

Procedure for Cargo Conditioning in Liquefied Gas Carriers

Cargo Transfer between Vessels (STS Operation)

Procedure for segregation of Liquefied Gas Cargoes

Preparation for Cargo Transfer

Procedure for discussion prior cargo transfer













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