Onboard every tanker Inert Gas System is fitted to provide safety of the ship from explosion. It is a safety system which is a requirement by SOLAS. In this article we will learn about Inert Gas System.
What is Inert Gas?
Inert gas is a mixture of gases, such as flue gas, which contain insufficient oxygen to support the combustion of hydrocarbons. It consists of N, CO2, CO, O2, SO2, H2O, NOx, etc.
Inert Gas System (IGS)
Inert Gas System is a requirement by SOLAS. With the help of inert gas system, tank oxygen content can be reduced and maintain positive pressure inside the tank. Simply we can say that it is used to protect tank explosion by introducing inert gas into the tank.
Sources of Inert Gas
- The uptake from the ship’s main or auxiliary boilers.
- An independent inert gas generator.
- A gas turbine plant when equipped with an afterburner.
Components of IG System
1. Source of Flue Gas: Inert gas can be generated from either uptake of boilers, IG generator and gas turbine plant.
2. Flue Gas Isolating Valve: It is a supply valve through which flue gas entering to the scrubber can be controlled. It isolates both the system when not in use.
3. Scrubber: IG form boiler uptake enters the scrubber tower from bottom and moves up inside the tower, where the gas is cooled and cleaned by sea water spray. Here most of the sulphur dioxide and particulate soot is removed.
4. Demister: It is a polypropylene mattress or cyclone dryer, which absorbs the water from the gas.
5. Blower: It is used to deliver flue gas to the cargo tanks. There should be at least two blowers, which together shall be capable of delivering at least 125% of the maximum rate of discharge capacity of the ship.
6. IG Pressure Regulating Valve: It is used to prevent backflow of gas and to regulate the flow of inert gas to the deck main.
7. Deck Seal: It permits inert gas into the tank but prevent any backflow of cargo gases like HC even when the system is shut down. Types of Deck seal are:
- Wet Type
- Semi-dry Type
- Dry Type
8. Deck Mechanical Non-Return Valve: The purpose of this non-return valve is same as Deck Seal.
9. Deck Isolating Valve: This valve can isolate engine room IG system with Deck IG system.
10. Pressure/Vacuum Breaker: It prevents the excess pressure/vacuum inside the tanks. It is filled with fresh water/glycol mixture. which helps to achieve this safety feature. It normally operates at 1800 mmWG and -400 mmWG.
11. Pressure/Vacuum Valve: These are fitted on deck for every cargo tank to provide for the flow of tanks atmosphere due to thermal variation. It normally operates at 1400 mmWG and -250 mmWG.
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12. Cargo Tank Isolating Valve: On tanker every tank is provided with a isolating valve. By the help of this valve IG entering into the tank can be controlled. This valve should be locked and it’s key should be kept with Chief Officer or any other authorised person.
13. Mast Riser: When cargo is being loaded into a tank, pressure starts building up inside the tank to release this excess pressure mast riser is used. Every tank is connected to the mast riser. It is fitted on the deck and with a good height so when the gas releases it do not spread around the deck.
14. Alarms in the IGS: These are audible and visual alarms which are required to be fitted as per Reg 62 of SOLAS.
- Low flow rate.
- High / Low water level in scrubber.
- High inert gas temperature.
- Failure of IG blowers.
- High oxygen content above 8% by Vol.
- Low / High IG pressure in the main line.
- Failure of power to automatic control system.
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Working of IG System
1. Flue gas form boiler uptake enters the scrubber from bottom and rises up in the scrubber tower, here 90% of the sulphur dioxide & soot particle form the gas is removed and the gas become cleaned and cooled. Moisture content from the flue gas is also absorbed by the demister.
2. Treated flue gas enters to the blower through blower inlet valves which helps the gas to enter inside the tank with pressure.
3. In the outlet of blower there is a sensor which measures temperature and O2 content of the gas, if temperature and O2 content does not reaches the permissible limit, IG will be stopped till it reaches the permissible limit.
4. Inert gas which pass through these sensors will go through regulating valve where the pressure can be controlled.
5. Now gas will pass through deck water seal and non-return valve to the Deck IG main line and latter to the IG branch valve.
6. By operating IG branch valve IG will enter inside the tank.