The most dangerous and flammable cargo is carried across the world by sea, or to be more precise, by tankers. Considering this information, when was the last time a news heading stating a tanker explosion was mentioned. Minor fires on tankers have been reported a couple of times, but none of them have truly EXPLODED. Well, how do then modern tankers carry thousands and thousands of tons of dangerous cargo, safely?

To answer that we have to dive into the basics of burning something, or how an object catches fire. We all know that burning is just the oxygen in the atmosphere reacting with the hydrocarbons, or the fuel itself. In our case right now, the fuel is a hydrocarbon oil in the liquid state. The liquid itself doesn’t catch flames but the vapor released, when mixed with sufficient oxygen and an ignition source, will burn like a madman. The catch here is that the proportion of oxygen to fuel vapor does vary and the perfect amount of both will make it possible for the explosion to occur. We can represent the ratio on a flammability diagram.

Flammable Limit Diagram
Source: YouTube, Courtesy: Casual Navigation

As the diagram above illustrates, too little hydrocarbon won’t ignite with whatever amount of oxygen it is provided with, the fuel mixture is said to be “too lean”.

When the hydrocarbon level exceeds the amount of oxygen it is provided, the fuel mixture again will not burn, it’s “too rich”.

Now, the oxygen level in the atmosphere cannot go above 20.95%, and no mind will think of pumping more oxygen, so there is no point going above that. The slope in the oxygen line states that the oxygen amount will decrease when you add more fuel particles.

On the left hand side of the graph we have an inert section. When the oxygen level is too low, clearly no fuel will ignite. The mixture is said to be “inert”.

We do reach a point on the graph when the fuel percentage and oxygen percentage is just right to catch a fire with a given heat source.

Then as the oxygen level increases, the range at which the fuel could burn expands. Plotting it will give us the “flammable zone”. If a heat source is provided within this zone, the mixture will ignite.

Now with the already established information above, how do we prevent the explosion further? One way of doing so is by reducing the oxygen content inside the cargo space, as the percentage control of fuel is out of our hands.

One way of doing is by adding inert gases inside and pushing the oxygen out. The gases could carbon dioxide or nitrogen, and noble gases but the cost has to be kept in mind. Obtaining either N2 or CO2 isn’t that much hard. Almost 78% of our air has nitrogen in it, just find a suitable method to filter it out and you get an inert gas. Or the main engine of a ship does produce carbon dioxide as a by-product, pass the exhaust gas through a scrubbing tower and pretty pure carbon dioxide can be obtained, ready to get flushed inside the holds.

The atmosphere inside the cargo hold reaches the inert zone and voila, you’re fuel is ready to get shipped anywhere in the world.

Fun Fact: The last true tanker explosion due to flammable cargo was at Los Angeles port in 1976. The tanker was a Liberian oil tanker SS Sansinena.

Authored By:- Cdt. Shubham Shivne, TMI

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