The purpose of drying cargo tanks is to remove humid air, which would be present during a dry docking or tank inspection. As part of the process of preparing the cargo tanks ready to carry the next LNG cargo, they must be dried out to avoid the formation of ice when the cargo tanks are cooled down.
Dry-air, which is supplied by the inert gas generator in dry-air mode, is used to expel the moist cargo tank atmosphere via the displacement method of exchange, known as a ‘piston effect’. This can be achieved by introducing the dry air to the bottom of the cargo tank via the filling lines and venting at the top via the vapour lines and slowly displacing the cargo tank’s humid air with dry air. Alternatively, the dry air can be introduced at the top of the cargo tank via the vapour lines and expelled out of the bottom via the filling lines.
To decide upon which method to adopt depends upon the relative humidity of the cargo tank atmosphere, which will vary due to seasonal variations of humidity associated with the climate and location. However, it is assumed that in the summer months, the relative humidity is high, and in ‘Winter’ months, the relative humidity is low and thus dry.
Therefore, for summer months the dry air is introduced to the cargo tank bottom and the lighter warm humid air can be vented at the top of the cargo tank, whereas in winter months, the dry air is introduced at the top of the cargo tank and vented at the bottom.
The actual dew point should be measured to assess the relative humidity and thus the best method to achieve effective cargo tank atmosphere air exchange.
