Inerting of Cargo Tanks

The Fire Triangle is made up of three factors:

• Fuel.
• Oxygen.
• Ignition.

Tankers carry flammable cargo, these cargoes represent the first factor of the fire triangle, fuel. Inside the tank, there is also the presence of oxygen which is the second factor of the fire triangle. With that said, the only thing stopping the completion of the fire triangle is the ignition. Considering the risk to life, the ship, and the environment, this one single barrier (ignition) is not enough.

Having said that, it is for this reason that the oxygen levels inside the cargo tanks are lowered in order to ensure that should ignition be present, the lack of oxygen will still prevent the fire triangle from completing. This process of reducing the oxygen level inside the cargo tank is known as Inerting.

An oxygen content of less than 8% is required to be able to remove the flammable hazards inside a tank. In order to be able to bring the cargo tank to that oxygen level, an inert gas needs to be introduced in the cargo tank. Having said that, there are two methods of Inerting a tank, these are;

• Inerting by Dilution.
• Inerting by Displacement.

Inerting by Dilution Method

In this method, we reduce the oxygen content in the tank by diluting the air with inert gas containing less than 5% oxygen by volume. The process keeps going until the oxygen in the tank is less than 8% by volume.

The Inert gas is supplied through the IG line (Inert Gas Line), and the purge flap of the purge line is opened to let the air inside the tank out. It is important to note that the velocity of the inert gas entering the tank in this method needs to be high, this is because we need to dilute the air that is inside. If the velocity of the inert gas is low, half of it may escape through the purge pipe and the dilution of the air will not be carried out efficiently and will result in the operation taking longer than needed.

The disadvantage of this method is that if we need to inert more than a couple of tanks, the dilution method will not allow us to open all the tanks that need Inerting, for the simple reason, that in such as case, the velocity of the gas would be too slow to do the job and be efficient. With that said, the maximum amount of tanks being inerted together at the same time with the dilution method, should not exceed two. 

Inerting by Displacement Method

This method is a little different from the dilution method. Instead of diluting the air, this time we displace the air inside the tank with Inert Gas.

With this method, an oxygen content of 8% from the initial 21% is achieved three times as quickly as the dilution method. This time, the Inert gas is introduced from the bottom of the tank, achieved by supplying the inert gas through the drop line. In this case, the IG line is connected to the cargo lines at the manifold.

If multiple tanks are to be inerted, they can be connected to the common line, and then the IG line will be connected to the common line too. In this method, there is no limitation on the number of tanks being inerted at the same time. The cargo lines are set up in the same way they would be set up for loading through the drop line, and the Inert Gas is started.  

As the inert gas fills the bottom of the tank the air consisting of 21% oxygen, will start exiting through the open purge pipe. This way, the air is gradually displaced by the inert gas entering the cargo tank.

An important note to keep in mind during the operation, when using the displacement method, is to ensure that the Inert Gas is not mixed with the air flowing and exiting through the purge pipe, otherwise, the procedure of reducing the oxygen will take longer. It is advisable to introduce the Inert Gas inside the tank at a slow rate and speed in order to avoid just that. This is clearly opposite to when using the dilution method, where we mentioned that a very high velocity is required for the Inert Gas to be efficient.