Inert gas system for tankers
To prevent the danger of fire / explosion of cargo tanks, where not only ships and cargo are lost, damaging the environment due to oil pollution that spills from the ship, but even many cause human casualties. accidents often occur on tankers when:
- Tank cleaning for ballast or repair purposes.
- Loading or discharge cargo.
For this reason, a preventative solution must be found to avoid explosion accidents or flares during such operations.
A fire can occur if it meets the fire triangle requirements or the fire triangle, namely:
- Source of ignition comes from sparks.
- Fuel in this case is Hydrocarbon which meets the requirements.
- Enough oxygen to cause a fire.
For safety from the explosion, the Inert Gas System was used on the ship. This can be a separate Inert Gas Plant or exhaust gas produced by the ship’s boiler.
IGS is the most important integrated system for oil tankers for safe operation of ships.
Inert Gas is an inert gas that contains insufficient oxygen (usually less than 8%) to suppress the combustion of flammable hydrocarbon gases.
IGS spreads the inert gas over the oil-charge hydrocarbon mixture which increases the lower explosion limit of LEL (lower concentrations at which the vapor can be ignited), simultaneously reducing the HEL higher explosion limit (Higher concentrations at which the vapor explodes). When the concentration reaches about 10%, the atmosphere is created in a tank where the hydrocarbon vapor (vapor) cannot burn. Inert gas concentrations are normally maintained at around 5% as a safety limit.
Components and description of the Inert Gas system :
The components used in IGS on oil tankers are as follows :
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Exhaust gases source : the source of inert gas is taken from the exhaust gas of the boiler or main engine because it contains exhaust gas in it.
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Exhaust gases valve : This functions as a supply valve from the exhaust to the entire system which isolates the two systems when not in use.
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Scrubbing tower : Exhaust gas enters the Scubbing Tower from below and passes through a series of water sprays and sealing plates to cool, clean, and wet the gas. SO2 levels are reduced by 90% and the gas becomes clean from soot.
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Demister : Usually made of polypropylene , used to absorb moisture and water from treated exhaust gases.
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Gas Blowers : Usually two types of blower fans are used, steam driven turbine blowers for IG operations and electrically driven blowers for the purpose of refilling.
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IG pressure regulating valve : Pressure in the tank varies according to the nature of the oil and atmospheric conditions. To control this variation and to avoid overheating from the blower fan, a pressure regulating valve is installed after the blower discharge recirculates the excess gas into the Scrubbing Tower.
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Deck seal : The purpose of Deck Seal is to stop the gas returning from the blower to the loading tank. Usually the type of wet deck seal that is used. A demister is installed to absorb the moisture carried by the gas.
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Mechanical non return valve : This is an additional non-return mechanical device parallel to Deck Seal
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Deck isolating valve : The engine room system can be completely isolated with the help of this valve system.
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Pressure Vacuum (PV) breaker : PV Breaker helps in controlling overloaded or under-loaded pressure. The PV breaker vent is equipped with a flame trap to avoid sparks when loading or unloading operations take place while at the port.
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Cargo tank isolating valves : A ship has a total load space and each space is equipped with an isolation valve. The valve controls the flow of inert gas to hold and ask is operated by the officer in charge on the ship.
- Mast riser : The mast riser is used to maintain the positive pressure of the inert gas while loading the cargo and during the loading time it remains open to avoid the pressure of the cargo tank.
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Safety and alarm system : Inert Gas Plant is equipped with various safety features to protect the tank and the engine itself.
The following are various alarms (with Shutdown) incorporated in the Inert Gas Plant on board :
- High Level in scrubbers leads to alarm and shutdown of blowers and scrubber towers.
- Low pressure sea water supply (approx. 0.7 bar) to scrubber tower leads to alarm and shutdown of blowers.
- Low pressure sea water supply (approx. 1.5 bar) to deck seal leads to alarm and shutdown of blowers.
- High inert gas temperature (approx. 70 deg C) leads to alarm and shutdown of blowers.
- Low pressure in line after blowers (approx. 250mm wg) leads to alarm and shutdown of blowers.
- Oxygen content high (8%) leads to alarm and shutdown of gas delivery to deck.
- Low level in deck seals lead to alarm and shutdown of gas delivery to deck.
- Power failure leads to tower alarm and shutdown of blowers and scrubbers.
- Emergency stop leads to tower alarm and shutdown of blowers and scrubbers.
The following are various alarms that are incorporated in the Inert Gas Plant :
- Low level scrubber.
- High level deck seal.
- Low O2 Content (1%).
- High O2 Content (5%).
- Low lube oil pressure alarm.
Working of Inert Gas Plant
The production base of Inert Gas in IG Plant is exhaust gas produced from ship boilers. The high temperature gas mixture from the exhaust boiler is processed at the IG Plant which cleans, cools and supplies the inert gas to each tank through a valve and PV breaker to ensure the safety of the structure and atmosphere of the tank.
This system can be divided into two basic groups :
a). Production Plant to produce inert gas and send it with pressure, through a blower, to cargo tanks.
b). Distribution system to control the passage of inert gas to the appropriate cargo tank when needed.
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