Since inert gas does not refer to a specific composition of some particular gases but it broadly means any mixture wherein there is not enough oxygen for combustion to take place, there could be multiple sources of such a mixture on board ships. Some of the possible sources for inert gas include
The uptake or exhaust gases from the main engine or boilers used on ships
Gas turbine plants with afterburner or
There could be an independent inert gas generator dedicated solely for this purpose.
The benefit of using the boiler is that if properly controlled combustion is being done in the boiler, the flue gases contain around 5% oxygen which is ideal for the inerting process, and the designers only need to bother about installing a system to cool and clean these flue gases before use. This means saving of cost and space, which would be necessary for installing a dedicated IG generator. Currently we will focus our attention of systems which use flue gases from boiler uptake as that is one of the most commonly used systems on board tankers and combined carriers.
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The Overall Plant Layout
Before going into the details of various components and their workings, it would be very helpful for the reader to familiarize with the overall scheme of things. The best way to do that is to take a look at the diagram shown below which clearly depicts the various parts of the inert gas plant as well as the inert gas distribution system. I suggest to spend a few minutes analyzing this diagram and then read further.
As you can see the whole ship area is divided into two zones, with the inert gas plant zone designated as safe zone or non-hazardous area (on the left hand side of the diagram) and the part of the right hand side being shown as hazardous area or the dangerous zone which contains the distribution system of the inert gas.
We will take a detailed study of each of the components associated with the plant which would certainly require several subsequent articles on this topic but for the time being you can understand broadly about the system as a whole.
Flue or exhaust gases from the boiler are processed in the tower which contains the scrubber and demister and then passed on to the IG blowers finally on to the deck into the distribution system for use in individual tanks as required. These components perform different tasks related to cleaning, safety and distribution of inert gas on the ship.
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Schematic Diagram of IG System - International Maritime Organisation. ISBN 92-801-1262-7