Inert gas system with its alarms and trips are compulsory safety equipment on oil tankers under SOLAS Convention; Which also covers the rules and operation of inert gas system to prevent any explosion on tanker ships. Chapter two (1/4.5.5 and 2/16.3.3) of SOLAS make it compulsory for tanker ships of above 8000 Dwt (New) and 20,000 Dwt (Old rule up-to 2016) to have inert gas system installed.
They are installed to avoid high risk of explosion/Fire during the cargo operation. As air mixed with hydrocarbon vapors under flammable reason could be hazardous on ship.A ship can have dedicated inert gas generator or inert gas system with inert gas produced from boiler exhaust to avoid such scenario.
What is Inert Gas and Inert Gas System on Tanker’s?
Inert gas is a mixture of gases with not enough oxygen which can create or sustain explosion/Combustion. Typically inert gas used on tankers contains 2 to 5 percent of oxygen; 13 to 17 percent of carbon dioxide and balanced nitrogen. They are obtained by cleaning flue gas from the boiler or individual inert gas generator.
Flue gas from boiler is generally preferred as it is not just cheap but also easily available and easy to obtain 2-5% oxygen level by controlling combustion parameters. It also satisfy the other requirement for inert gas as it not react to cargo material, isn’t toxic and easy to produce.
While inert gas system is an integral part of safety (Operation and equipment) for the normal operation of ship. It pump inert gas into the tanks during the cargo operation. On flammability diagram it can be seen as the decreasing gap between the UFL ( Upper flammability limit ) and LFL ( Lower Flammability limit ). The UFL and LFL meets at point E; but inert gas is pumped in until it reach to the point F represented on the diagram.
Now during cargo discharge if the tank atmosphere is diluted with air (Oxygen); it will follow a dangerous path FA. From the diagram itself, we can see that the path FA pass through the flammable range and so can be hazardous. So to avoid such scenario; inert gas is purged into the tank before and during the cargo operation ( Discharging ).
By purging inert gas into the tanks, Oxygen-Hydrocarbon mixture is taken to point H from F. Now any further dilution with air while discharging won’t cause any risks to the ship; as the line HA represented on the flammability diagram don’t pass through flammable range. Now suppose for some reasons the tanks are filled with less inert gas than required. Then during discharging it will miss the flammable range by a small margin.
Q.Why Inert Gas is Required on Ship?
An oil tanker carry different grades of oil which produce high amount of highly commutable hydrocarbon vapors. Air is already in plenty all over the world with different source of heat (Boiler, hot work, smoking, charge discharge etc.) available on board. When hydrocarbon vapor mixed with air; it can lead to explosion damaging ship structure and crew.
To ensure safe operation of ship; Tank atmosphere is maintained under safe limits by using inert gas system. Basically, hydrocarbon vapors cannot burn in an atmosphere less than 11% of oxygen by volume. So oxygen level is always maintained much below that figure with the help of inert gas pumped into the tanks.
Flammability range decrease with increase in inert gas concentration; generally inert gas is pumped into the tanks until the upper flammability limit and lower flammability limit coincides. No hydrocarbon – oxygen gas mixture can burn under such condition. Inert gas system is essential for ship safety, it not just provide inert gas with oxygen 5% by volume but also help maintain positive pressure in tanks.
Component and Layout of Inert Gas System
A typical inert gas system consists of following systems:-
- Boiler Uptake Valve: It is the supply valve for the flue gas to the inert gas system. It is also termed as isolation valve as it can be used to isolate the system from the boiler.
- Scrubber: Flue gas from the boiler uptake came to the bottom of the scrubber; where it is washed by the spray of water and leaves from top after passing through the baffles. Water is sprayed from the top to cool and clean; it makes it clear of more than 90% of sulphur dioxide and 100% of shoot particles.
- Demister: The flue gas became moist after leaving the scrubber tower; so to make it free of excess moisture demister is placed just after the scrubber tower.
- I.G Blowers: A steam driven blower is fixed to supply the inert gas to the holds. Sometimes another electric driven blower is also fixed side by side for topping up.
- Pressure Regulating Valve: A pressure regulating or recirculating valve is fitted just after the I.G blower to recirculate the excess inert gas back to the scrubber to avoid excess load on the blowers when necessary.
- Oxygen Analyzer: It is a device fitted after the pressure regulating valve; it sounds an alarm if oxygen level is more than 8% by volume.
- Deck Seal: A deck seal is installed in the system to avoid any back-flow of gases from tank holds to the engine room. Generally a wet type deck seal are proffered with heating coil, low level alarm and rubber lined internals.
- Deck Isolation Valve: It is a valve which isolates the inert gas system into two half; engine sides and deck side.
- P-V Breaker: It is a safety equipment for the system; as it protects the tanks from both being under charged or over charged conditions. They are provided with flame trap on their vents to avoid any fire during the cargo operation.
- Mast Riser: During cargo loading, a mast riser provide the exit point for the excess I.G-hydrocarbon vapors to escape avoiding over pressurizing the cargo tanks.
Working of I.G System
The idea of inert gas system is to use the flue gas from the boiler uptake and then use them as an inert gas to avoid combustion, explosion, reduce cargo loss due to vaporisation and assist cargo operation. The system is divided into two main parts based on production and distribution.
Inert gas is produced and then delivered to tanks through I.G blowers. Distribution system/channels are there to ensure they go to the right tank at right time.Hot flue gas from the boiler exhaust is taken from the boiler uptake valve to the bottom of scrubber tower. It is then washed and clean by spray of water and series of baffles before leaving at the top.
The flue gas from the scrubber tower is free of shoot and sulphur dioxide but contains high amount of moisture. So it is then passed through a dimmister to remove moisture before leaving to the blower suction. The blower discharge the inert gas through pressure regulating valve via deck seal and oxygen analyzer.
A deck seal acts as a non return valve avoiding possible back flow. A relief valve is also fitted in between the deck seal and pressure regulating valve to vent gas in event of their failure. It then goes to the designated tanks through the deck supply/distribution lines passing deck isolation and supply valves.
Inert Gas System (Starting Procedure)
Careful consideration is required on board for proper operation of inert gas system. The oxygen content must always be maintained at 5 percent by volume; further reduction in oxygen content can lead to mixing of impurities in gas that would be hard to separate. Certain precautions must be taken prior to starting the inert gas system.
- Open all the related valves to the fuel burner; check their is adequate fuel for the operation (boiler/IGG).
- Turn on the electric power to the control panel.
- Water drain lines on the scrubber must be opened.
- Ensure oxygen analyzer is working and calibrated.
- Set the pressure control setting for the inert gas in distribution lines.
- Set pressure control valve of the fuel burner.
- Ensure sea water supply to deck seal.
- Ensure the system lines are lined up.
- Start the inert gas generator before entering the port. (Actually it is done to avoid dark black smokes coming out visible when it start).
- Follow all the I.G system checklist as per the company guidelines.
- Ensure all cargo openings are closed.
- Line up the system prior to start.
- Take above precautionary measures.
- Start the Inert gas system.
- Check for the readings of oxygen analyzer.
- Supply inert gas to the deck opening I.G main supply valve.
- Monitor all pressure parameters.
- Monitor temperature and oxygen level of inert gas during the cargo operation.
- Increase the inert gas pressure before stopping the inert gas plant.
Note: The inert gas system is only operational during the cargo discharge; any abnormality during the process lead to stop cargo operation.
Alarms and Trips Installed on I.G System
Various safety equipments (Alarms and Trips) are installed to the inert gas system to help monitor and safeguard system, tank and machinery.
- Pressure gauge installed on the water supply line to the scrubber tower.
- Low pressure (0.7) in scrubber supply line raise the alarm and stop the IGG/Boiler.
- High water level alarms are fitted in scrubber tower which when sound initiates boiler shut down.
- High temperature alarm for flue gas at the outlet of scrubber tower.
- Low sea water pressure Alarm and shutdown for deck seal (1.5 bar).
- Low level alarm and shut down for water level in deck seal.
- High oxygen content alarm and shut down (more than 8% by volume).
- Other emergency stops and shut down.
Engine room is responsible for the production of inert gas while chief officer or deck officer is responsible for its distribution and operation. So any alarm that comes in the system will sound in both the places.
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