Gas freeing of cargo containment systems is required for reasons of maintenance or technical inspection. By introducing nitrogen in a cargo containment system, the content on remaining vapours of a previous cargo is reduced so that contamination is avoided. For the change over from some products to other ones it’s recommended to first bring the cargo tanks under air for a visual cleanliness inspection and to reduce the remaining previous cargo content to zero. All such an operations requires the use of nitrogen vapours and implement a release of previous cargo vapours to the atmosphere or to a flare systems.

The GRS unit is perfectly designed to exclude the emission of organic and/or poisonous vapours in the atmosphere. While the vaporizing liquid nitrogen liquefying previous cargo vapours, the nitrogen vapours originating from the heat exchange in the unit are introduced in the cargo tank to be purged. A closed circuit is obtained, the emission of VOC’s or toxic vapours tot the atmosphere is excluded. Many tones of valuable cargo vapours are recovered and can be further used. The emission of carbon dioxide in the atmosphere is totally excluded.

After tanks, ships have been inerted prior to receive a certain cargo of liquefied gas, removal of all incondensable is necessary to allow a smooth operation of the cargo handling systems such as reliquefaction units etc… That means that the nitrogen vapours are to be substituted by vapours of the next cargo. Moreover, for products with a low boiling point, the tank material needs to be cooled down in accordance with certain procedures. This operation includes the use of a vapour return system in which commonly vapours are released to a shore incineration system.

The GRS system is even utmost efficient for this type of operations. The consumption of release of valuable product to the flare system is avoided. Product is liquefied and incondensable are simply released to the atmosphere. Sub-cooled condensate is returned to the cargo system and is reused in the cooling process of the tank. The fact that the condensate is sub-cooled makes the whole operations faster and much more efficient.