There are two reasons for nitrogen padding or blanketing in tank containers. “Safety” & increasing product “Shelf Life”. In both the cases this is achieved by reducing oxygen level in tank. Basic composition required to start a fire is Oxygen, Fuel & Ignition source. Nitrogen padding removes or reduces the oxygen to such level to avoid possibility of ignition. Food industry use nitrogen padding to avoid oxidation and increase shelf life.
Why nitrogen? Nitrogen is widely used in padding as its inexpensively available. Carbon dioxide or argon can also be used for padding purpose however carbon dioxide may be reactive to certain cargo. Oil tanker ships employ inerting of cargo tanks. Inert gas supply to tanks can be done by scrubbing & cooling of flue gas from ship’s boilers. Reducing the ullage oxygen level achieves high level of safety in storage and during transport.
Lower explosive limit (LEL) is the lowest concentration of vapour or gas in air which can be ignited and Upper Explosive Limit (UEL) is the highest concentration of vapour or gas in air which can be ignited. Both LEL & UEL is measured in per cent. LEL & UEL may vary depending on the pressure and temperature. Hydrocarbon oil vapours may burn with less than 11% oxygen in air. Achieving oxygen level to below certain percentage for different cargo is necessary for correct inert condition.
Flammability range of crude ethanol is LEL 3.3% to UEL 19%. To achieve inert condition for ethanol oxygen need to be limited to below 8% LOC (Limiting Oxygen Concentration).
In ISO Tank containers purging the tank can be done by pressure or by mixing and venting. Purging by pressure require more nitrogen. Up to 6 x volume is required to achieve level of oxygen to 0.5%. In mixing and venting with 4 x volume change of nitrogen can achieve oxygen level to 0.4 %.
Loading under nitrogen blanket requires the tank to be pressurized with nitrogen then commence loading. While loading the addition of cargo will increase the pressure which can be maintained by operating airline valve. Correct safe operation procedure must be strictly adhered to while carrying out this operation.
Apart from safety nitrogen blanketing can increase the product shelf life by arresting oxidation. Nitrogen is used in blanketing edible oil to avoid rancidity, in wine shipments and in many other food grade products.