Inertization of cryogenic ISO tank containers

Inertization, also known as inerting or nitrogen purging, is the replacement of the vapor content of a cryogenic ISO tank container and its pipework with an inert gas, so that no flammable or reactive mixture is possible. Bofort carries out inertization as a standard preparation procedure, part of preparing its tank containers for maintenance, repair, or service, typically using dry nitrogen (N₂) as the purging medium.

Why is inertization required?

Three elements are needed to initiate and sustain combustion: an ignition source (heat), fuel and oxygen. Combustion is prevented by removing any one of these three elements. Inside a closed tank container an ignition source or oxygen content can only be ruled out completely through inertization by:

reducing the oxygen (O₂) concentration in the tank to below the concentration capable of supporting combustion (the combustion threshold), or
keeping the relevant concentration below the lower explosive limit (LEL), i.e. the minimum concentration capable of supporting combustion, or
keeping the relevant concentration above the upper explosive limit (UEL), i.e. the maximum concentration capable of supporting combustion.

In practice, the tank vapor content is displaced by (dry) nitrogen. This brings the oxygen concentration below the combustion threshold before any flammable product is introduced.

At cryogenic temperatures inertization has a second purpose: purging with dry nitrogen removes moisture and CO2 before the tank is cooled down.

Bofort inertises the complete cargo system of its cryogenic ISO tank containers: the inner vessel together with its pipework and valves. The purge is continued until the atmosphere measured at the vent meets the oxygen content and dew point specified for the product to be loaded.

What Bofort interiss

When is a tank container inertised?

Before loading. A tank going into service with a flammable gas such as Liquid Biomethane / LNG, ethane or ethylene is first purged with dry nitrogen. The tank is then gassed up with product vapour and cooled down before loading; see our Cooldown page.
After maintenance or inspection. Whenever the cargo system has been opened and exposed to air, the tank is inertised again before it returns to service.
At every product change. When a tank container is switched from one product to another, the previous atmosphere is purged out with nitrogen to prevent contamination of the new cargo and to rule out any reactive mixture during the changeover.
Before a tank is opened. In the other direction, flammable product vapour is first displaced with nitrogen, and only then is the tank safe for access.

For non-flammable products such as nitrogen, oxygen, argon and CO₂ the same purging technique serves product purity and dryness rather than explosion prevention.

With which gases?

Dry nitrogen is the standard inerting gas for cryogenic tank containers: it is inert, moisture-free and readily available, and it remains gaseous at the temperatures involved. Argon and CO₂ are also inert, but CO₂ freezes to dry ice at cryogenic temperatures and argon is considerably more expensive, so neither is commonly used for this purpose.

A nitrogen-inerted tank contains no breathable oxygen and will not sustain life. The inner vessel itself is not accessible, so there is no entry into the tank. The precaution that matters is outside it: nitrogen released during purging displaces oxygen in the surrounding air, so a safety zone is marked out around the tank in which the oxygen concentration can drop, resulting in a risk of asphyxiation.

Technician in an orange hard hat connecting a transfer hose to a Bofort liquid CO2 ISO tank

Safety & Quality Assurance at Bofort

The procedures described on this page follow recognised industry standards. Bofort applies these standards to all its technical activities, from cooldown and inertization to leak testing and pressure testing, so that every tank we lease is prepared, tested and maintained to the same level.