INERT GAS SYSTEM Inert Gas is a gas which contains insufficient Oxygen to support combustion and

which is used to prevent explosion by eliminating or removing at least one of following component;

Hydrocarbon gas Oxygen Source of ignition Eliminate Explosion

Inert Gas mainly consists of Nitrogen and combination of small percentage of other gases as follows;

Nitrogen Approx. 80%Carbon Dioxide Approx. 14% Oxygen 2 to 5 %Water vapour Approx. 5% Carbon monoxide Approx. 0.01%Nitrous gases Approx. 0.02%Sulphur dioxide Approx. 0.3%Soot 300mg/m3

Inert Gas can obtain either from vessel’s Inert Gas generator or vessel’s boiler exhaust gas due to low oxygen content of exhaust gas from boiler.

Typical Inert Gas system consists of following main components; Boiler uptake valve Scrubber I.G blower Oxygen analyzer Pressure Regulating valve Recirculation valve Deck water seal Mechanical non return valve Isolating valve Individual branch valves for each tank Pressure / Vacuum Breaker Mast riser

Gases from Boiler uptake are hot and include impurities before reaching the scrubberThe scrubber’s main purpose is to clean and cool the flue gas from the boiler uptakes and to remove impurities such as soot and sulphur oxides by continuous water spray and demister . Cleaned and cooled gases from the scrubber outlet are forced by the I.G fan/blower through the oxygen analyzer to check oxygen content in the gas. If the content of the oxygen in the flue gas is less than 5% by volume then, these gases are supplied to cargo tanks via deck water seal otherwise re-circulating valve will automatically open to re-circulate the gases to the scrubber. The oxygen analyzers are fitted at the outlet of the Scrubber and at the I.G main deck line to maintain the oxygen level less than 8% (5% for most of the new ships). A Cargo tank is considered to be “Inerted” when the oxygen content in the tank is less than 8 % by volume.

Low oxygen content Inert Gases are now passing through the Deck water seal which mainly prevent the back flow of Hydro Carbon gases from the Cargo Tanks to Machinery Space and helps for further dry, cool and purify the gases. There are three types of Deck seal as follow;

Wet Type Semi-dry Type Dry Type

Deck Water Seal - Semi-dry type

Instead of bubbling through the water trap the inert gas flow draws the sealing water into a separate holding chamber by venturi action thus avoiding or at least reducing the amount of water droplets being carried over. Otherwise it is functionally the same as wet type.

Deck Water Seal - Dry Type

Water gets drained when inert gas plant is in operation (as flowing to the tanks).

It is filled with water when the inert gas plant shuts down or tank pressure exceeds inert gas blower discharge pressure.

Filling and drainage are performed by automatically operated valves controlled by the levels in the water seal and drop tanks and by the operating state of the blowers.

The correct level of water in the deck seal is necessary to maintain at all time and to be check prior every operation.

The Inert Gas is then delivered to the Cargo Tanks through individual branch lines. All cargo tanks are fitted with common Mast Riser for releasing the gases during loading and pressure/vacuum water breakers to prevent excessive stresses in case of malfunctioning Mast riser and Individual branch valve.

The main usages of Inert Gas System are;

Inerting during Unloading Cargo, Crude oil washing and Tank Cleaning operation Purging of Cargo Tank with Inert Gas Gas Freeing with fresh air by means of I.G blower

Advantages of Inert Gas system

Preventing of explosion by blanketing the surface of the cargo from the atmosphere

Reduce rate of corrosion due to low content of Oxygen in the tank Increase in discharging rate by maintaining the tank with positive pressure Less evaporation rate during transit Reduce risk of explosion due to static electricity during tank cleaning.