With certain products, the empty space in the tank container during loading and discharge and the ullage space during transit is filled with an inert gas, usually nitrogen. For pressure loading or discharge, inert gas is used as the propellant medium by which the pressure is applied. Before loading it is normally necessary to purge the tank of atmospheric air.

Safety Note: Inert gases are normally non-toxic, but inhalation in confined spaces can cause asphyxiation. Tanks which have been purged of air should be fitted with a temporary seal and marked to indicate that the tank contains inert gas.

Preparing the tank

Products carried under a gas blanket require the normal pre-trip checks to ensure that the tank container is suitable for the purpose. There should be an additional pressure check for tightness of all valves and fittings to ensure that no loss of the inert gas will occur during transportation.

The air inlet connection pipe should be fitted with a closing valve, and a 0-60 psi pressure gauge (0-4 bar) fitted between the shut-off valve and the tank, so that the pressure in the tank can be checked during transportation.

Loading under a nitrogen blanket

Nitrogen Quality
Where nitrogen is used to protect the tank and its contents from contamination by water, commercially pure nitrogen with a guaranteed maximum water content should be used.

Purging the tank
Two methods of purging the tank of atmospheric air are available to the tank user depending upon the equipment available:

- By pressure
- Mixing and venting