The following components are used in a typical inert gas system in oil tankers:

• Exhaust gases source: The inert gas source is taken from exhaust uptakes of the boiler as it contains flue gases in it.
• Inert gas isolating valve: It serves as the supply valve from uptake to the rest of the system, isolating both systems when not in use.
• Scrubbing tower: Flue gas enters the scrub tower from the bottom and passes through a series of water spray and baffle plates to cool, clean, and moist the gases. The SO2 level decreases up to 90%, and gas becomes clear of soot.
• Demister: Normally made of polypropylene, it is used to absorb moisture and water from the treated flue gas.
• Gas Blower: Normally, two types of fan blowers are used: a steam-driven turbine blower for I.G. operation and an electrically driven blower for topping-up purposes.
• I.G pressure regulating valve: The pressure within the tanks varies with the properties of oil and atmospheric conditions. To control this variation and to avoid overheating of the blower fan, a pressure regulator valve is attached after blower discharge, which re-circulates the excess gas back to the scrubbing tower.
• Deck seal: The purpose of the deck seal is to stop the gases to return back which are coming from the blower to the cargo tanks. Normally wet type deck seals are used. A demister is fitted to absorb the moisture carried away by the gases.
• Mechanical non-return valve: It is an additional non-return mechanical device in line with the deck seal.
• Deck isolating valve: The engine room system can be isolated fully with the deck system with the help of this valve.
• Pressure Vacuum (PV) breaker: The PV breaker helps in controlling the over or under-pressurization of cargo tanks. The PV breaker vent is fitted with a flame trap to prevent fire from igniting when loading or discharging operation is going on when in port.
• Cargo tank isolating valves: A vessel has several cargo holds, and each hold is provided with an isolating valve. The valve controls the flow of inert gas to hold and is operated only by a responsible officer in the vessel.
• Mast riser: The mast riser is used to maintain a positive pressure of inert gas at the time of loading of cargo, and during the loading time, it is kept open to avoid pressurisation of the cargo tank.

Working procedure:

• Boiler uptake gases are drawn to the scrubber unit via flue gas isolating valve(s).
• In the scrubber unit, the gas is cooled, cleaned and dried before being supplied into the tanks.
• Motor-driven inert gas blowers supply the treated gas from the scrubber tower to the tanks. They are mounted on rubber vibration absorbers and isolated from the piping by rubber expansion bellows.
• Regulation of gas quantity delivered to the deck is taken care of by the gas control valves, and the deck pressure is managed by the pressure controller. If the deck pressure is lower than the set point, the output signal will be raised to open the valve more, and vice versa. If the deck pressure is lower than the set point, these valves will then work in cooperation to keep both the deck pressure/blower pressure at their respective set point without starving or overfeeding the circuit.
• Entering the deck line, the gas passes through the deck water seal, which also acts as a non-return valve, automatically preventing the back-flow of explosive gases from the cargo tanks.
• After the deck seal, the inert gas relief is mounted to balance the built-up deck water seal pressure when the system is shut down. In case of a failure of both the deck seal and the non-return valve, the relief valve will vent the gases flowing from the cargo tank into the atmosphere
• The oxygen analyser, which is fitted after the blower separates the “production” and “distribution” components of the plant and analyses the oxygen content of the gas, if it is more than 8%, it alarms and shutdowns the plant

Alternate sketch of IG system.