Calming the Seas: The Invisible Role of Inert Gases in Maritime Safety

One of the most important safety mechanisms in preventing accidents while transporting dangerous cargo by sea are inert gas systems. It's these silent workers that tame every spark and prevents cargo tanks from becoming IEDs. This article gives a brief overview of what these gases are and how they are employed for the safety of everyone around them.

Inerting is the process of replacing a reactive gas with an inert gas, most commonly oxygen with the chief goal being preventing ignition. An inert gas is any gas that does not create chemical reactions with other substances in a certain context, in this case, handling cargo in the maritime industry. Unlike noble gases, which are all inert, an inert gas is not strictly comprised of one element and is often a compound of several elements. Their non-reactivity is explained by their tendency and veering to having a full outermost, valence electron shell in its atoms, thereby achieving extremely high stability.

Carbon dioxide and nitrogen are the two most commonly used inert gases in the industry, of which nitrogen is cleaner and because of it, is more expensive and harder to produce in large quantities, which is why it's often the case that it must be imported from shore based distribution networks. The concentration of oxygen generally needs to be reduced to a level where it occupies a maximum of 5% of the volume, but recommendations are lower and depending on the cargo, it may even need to be reduced all the way down to 0.1%, as is the case with vinyl chloride and butadiene.

There are two main ways of inerting tanks and their subgroups, the choice of the best method comes down to the shape and type of tank, equipment and its configuration as well as relative vapor densities of the cargo, among other considerations.

Each cargo is different, but one important thing to note is that tanks into which ammonia is loaded must be inerted even though it is not overly flammable. Inert gas produced from combustion generators should not be used in this case even though it goes through purification because ammonia reacts with carbon dioxide and produces carbamates, one of the main components of neurotoxic insecticides. In addition, ammonia should never be loaded with the spray-loading method because there is an increased risk of creating static charge, and ammonia itself in combination with oxygen accelerates stress-induced corrosion in the tank walls, which further illustrates the importance of inerting.

Inerting by displacement

This method relies on stratification of the tank's atmosphere due to differences in gas densities between the gas entering the tank and the gas already in the tank. The denser gas is released below the less dense and lighter gas at a slow speed to reduce turbulence. In practice, mixing still occurs and often more than one tank volume in inert gas must be used to completely replace the tank's atmosphere. This way of inerting is often the most economical because it uses the least amount of inert gas and time compared to other arrangements.

One way to increase efficiency with this method is to simultaneously inert multiple tanks in parallel, which reduces mixing due to reduced entry speed while simultaneously accelerating the flow of inert gas due to less flow resistance in the pipe.

Inerting by dilution

Inerting by dilution causes mixing of gases in the tank, and utilizes it for replacing the tank's atmosphere in the following three ways:

Dilution by repeated pressurization of atmosphere

This method can be applied if the ship uses type "C" tanks. As it goes, inert gas is pressurized into a tank using a cargo compressor, then the compressed gases are released into the atmosphere. This process is repeated until the entire tank's atmosphere is replaced. The better quality of inert gas, i.e., the less oxygen content in it, the fewer repetitions are needed. Generally speaking, more repetitions at lower pressure are more effective in expelling oxygen from the tank spaces.

Dilution by repeated introduction of vacuum

As in the previous method, type "C" tanks are needed because they are made to withstand significant vacuum. A vacuum is created with a cargo compressor which is then interrupted with an inert gas, and this is repeated until all oxygen is replaced.

Continuous dilution

This is the only method available for type "A" tanks due to their low tolerance for excessive pressure and vacuum. It's continuously diluting the contents of the tank until the atmosphere is clear of oxygen. When used on type "C" tanks, to increase flow rate of inert gas and thus better mixing and shorter operation time, a tank can be kept under vacuum using a cargo compressor.

Knowing where and how to apply inerting for the purposes of preventing dire consequences is as in dire need as the consequences themselves, this way we ensure the safety of ourselves and everyone around us.

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