Other applications

Protecting storage tanks with chemicals and petroleum products

Containers containing flammable material, such as oil, petroleum products or chemicals, have to be protected against fire or explosion. Safety measures must be applied not only during storage but also when the tank is cleaned.

Any fire or explosion requires three necessary elements to initiate and sustain. These are:

1. flammable materials,
2. oxygen,
3. a source of ignition.

Heat or sparks can be the source of ignition, while oxygen is ubiquitous as part of the atmosphere. A fire or explosion can be prevented by removing one of the three elements. In case the presence of an ignition source cannot be prevented, the atmosphere in the tank can be inerted. Inert in scientific terminology means "not easily reactive with other elements" and forming no chemical compounds or anything that is chemically reactive. Thus, an inerted tank is non-combustible.

There are three tank inerting methods:

1. Reducing the oxygen content of the space above the fuel that contains air and fuel vapours (ullage) below the threshold required for combustion so that the tank atmosphere is considered inert, i.e. unable to ignite and thus inexplosive, or
2. Reducing the fuel air ratio of the ullage below the minimum threshold (Lower Flammability Limit) required for combustion, or
3. Increasing the fuel air ratio above the maximum threshold (Upper Flammability Limit) which can support combustion.

Oxygen reduction for tank inerting

A common tank inerting method refers to the process of rendering an inert atmosphere in the tank by reducing the oxygen content in the tank.

For oil tanks the oxygen content must be reduced to below 11% Vol. to prevent the risk of ignition. As an extra precaution, international standards, such as ISGOTT, are set to 8% Vol. For some petroleum products and chemicals the oxygen level of max. 5% Vol. is required for a safe atmosphere in the tank.

The atmosphere in the tank is not only inerted but also kept at a slightly positive pressure as compared to the ambient atmosphere to prevent any ingress of outside air which could increase the oxygen levels inside the tank.

Inerting a tank atmosphere requires replacing the oxygen content of the atmosphere with an inert gas. Two gases, nitrogen and carbon dioxide are widely accepted for this purpose., there are, however, several methods of generating and injecting an inert gas to tanks. For certain fluids carbon dioxide is not recommended as the inert gas, for instance for methanol. Hence, in many processes nitrogen is recommendable.

Typically, nitrogen with a residual oxygen content of 0.5-5% is used for this type of applications.

Nitrogen is often used to protect elements such as tanks and installations against destructive effects of corrosion. Reduction of oxygen content inside the installation can significantly limit corrosion, or – when applying relevant oxygen reduction levels –nearly completely stop corrosion and deoxidation processes on material from which the elements are made. This method is helpful for instance for protecting water tanks in heating systems.