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Heat leak into tank (

Boil-off Gas Sources

British Standard European Norm (BS EN) 1160 – Installations and Equipment for Liquefied Natural Gas – General Characteristics of Liquefied Natural gas defined physical properties of boil-off gas. 

Heat leak into tank (
Heat leak into tank (

Any heat leak into the tank will cause some of liquid to evaporate as a gas. This is known as boil-off gas (BOG). The composition of boil-off gas will depend on the composition of LNG. As a general boil-off gas can contain:

  • 20% nitrogen
  • 80% methane
  • Traces of ethane

The nitrogen content of the boil-off gas can be about 20 times that in the LNG.

At normal condition the density of boil-off gas will be approximately 0.6 of air.

Source of Boil-Off Gas

Source of BOG can be from:

  • Flash due to difference in ship and onshore operating pressures
  • Energy of pumping
  • Unloading line heat leak
  • Ship cargo tank heat leak
  • Onshore LNG tank heat leak
  • Displacement from LNG tanks due to unloaded LNG

Quantity of BOG during unloading mode (where there is LNG unloading activity from ship to shore) is much bigger than BOG during holding mode (there is no LNG unloading activity). All of BOG sources mentioned above contribute to total amount of BOG generated during unloading mode. Meanwhile, only BOG from onshore LNG tank heak leak is generated during holding mode.

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