Components in line: boiler uptake/ flue gas source -> sea water washing/scrubbing tower (flue gas scrubber with centrifugal blower drawing flue gas) -> demister/moisture separator -> inert gas (flue gas) blower -> inert gas cooler (optional) -> non-return/PT (pressure/vacuum situations) valve -> deck water seal -> distribution main along deck -> cross-over valves / branch lines -> gas risers (drop lines) into each cargo tank -> tank pressure/vacuum relief or venting. Automated controls: oxygen analyser and SO2/temperature/density monitor on the discharge, relief valve to atmosphere, overboard discharge/dump line, and connection to the tank atmosphere control.

The inert gas system (IGS) supplies inert gas, normally the flue gas from the main or auxiliary boilers, to keep the cargo tank atmosphere above the flammable range and to prevent the formation of an explosive mixture during cargo handling. Flue gas, which contains only a small residual oxygen (about 2-5 per cent depending on combustion), is drawn from the boiler uptake by a centrifugal/inert gas blower and passed into a sea-water scrubber/cleaning tower. In the scrubber the gas is cooled and washed by sprays of sea water, which removes sulphur dioxide, soot and ash and reduces its temperature, while also lowering the oxygen slightly. The cleaned, cooled gas then passes through a demister (mist eliminator) to remove entrained water droplets, and then through the deck water seal - a tank of water through which the gas passes - which acts as a non-return barrier preventing cargo vapour or tank gas from flowing back into the machinery space. From the water seal the gas goes to a deck distribution main, and from there through branch lines and drop lines (risers) down into each cargo tank via tank valves. The gas flows into the tank pressing the atmosphere; the excess gas and vapours are displaced out through the vent lines and the relief (or through the tank's PV valve) as the inert gas purges the tank of air. Continuous oxygen analysis on the discharge diverts gas overboard if the oxygen content exceeds the safe limit. A high-pressure/high-vacuum relief valve protects the system, and during loading the IGS is adjusted to maintain a slight positive pressure in the tanks so that air cannot enter.

With good (complete, slightly excess air) combustion the flue gas will typically contain about 2-5 per cent oxygen (often stated as approximately 3 per cent), the balance being mainly nitrogen and carbon dioxide, with the carbon dioxide content typically 12-14 per cent and little carbon monoxide. The IGS is required to supply gas with oxygen content not exceeding about 5 per cent (SOLAS requires below 8 per cent entering the tanks, but good practice well below that).