Learn about the various methods used to replace air-hydrocarbon mixture with inert gas and vice versa on board ships as well as general cargo tank atmosphere control policy.
In the previous articles we learnt about the fire triangle, flammability graph and its various regions in context of Inert Gas Systems used on board ships. In this article we will see various operations such as dilution, displacement etc and the policies adopted for gas replacement.
Terms Associated with Gas Exchange
We already read a bit about the operations called gas freeing and purging in the previous article, similarly there is an operation known as inerting refers to introducing inert gas within the tank. Both these operations are carried out using either dilution or displacement processes wherein the former refers to the mixing process whilst the latter is more of a layering process. Just in case you are not clear about the terms associated with the Inert Gas systems here is a quick recap of some important terms
- Inert gas refers to a gas which is a mixture of flue gases and contains insufficient oxygen to sustain combustion of hydrocarbons.
- IG Plant refers to the overall system for the production, supply, control and monitoring of such an inert gas.
- IG Distribution system refers to the paraphernalia associated with distributing the gas generated in the IG plant to the required spaces.
- Gas freeing is the process of creating normal atmospheric conditions inside the tank wherein oxygen level is 21%.
- Purging refers to inserting inert gas in the tank when it is already having less than 8% oxygen to reduce oxygen and/or hydrocarbon volume even further than that.
- Topping up refers to the process of inserting inert gas into an already inert tank just for raising its pressure to a positive level with respect to atmosphere.
Processes of Gas Exchange
If the gas exchange process is carried out using the dilution theory it assumes that the incoming gas mixes uniformly with the original gas throughout the tank so that the concentration of the original gas reduces exponentially. The conditions required for such a process to happen ideally are high incoming velocity of the gas so that the jet could reach the bottom most portions of the tank which in turn depends on tank dimensions as well. In this case the points for entry and exit of the gas are both located towards the top of the tank as shown in the figure below
If the gas exchange is carried out through the displacement technique it requires the presence of a stable horizontal interface between the top of the tank so that the light gas entering from the top can push the heavier gas at the bottom out of the tank. Hence it requires a low velocity of gas entry as well as the outlet pipe suction located somewhere towards the bottom of the tank, thus requiring a different piping arrangement as compared to the previous technique. This arrangement can be seen in the schematic diagram below
As a matter of policy it is required that tanker ships that have inert gas systems should utilize to keep the tanks under inert conditions all the time which in turn means that whether the tanks are carrying cargo, ballast or whether it is about to arrive in port for loading, the tanks should have inert condition which means that oxygen content throughout the tanks is less than eight percent by volume.
In the next article we will learn about the various components of an Inert Gas Plant
Dilution & Displacement