Why IG is required?
To maintain a safe atmosphere within ship’s cargo tanks a fixed inert gas system is used. Hydrocarbon gas normally encountered in petroleum tankers cannot burn in an atmosphere containing less than approximately 11% oxygen by volume. Accordingly, one way to provide protection against fire or explosion in the vapour space of cargo tanks is to keep the oxygen level below that figure. This is usually achieved by using a fixed piping arrangement to blow inert gas into each cargo tank in order to reduce the air content, and hence the oxygen content, and render the tank atmosphere non-flammable. As inert gas is added to the hydrocarbon gas/air mixture, the flammable range decreases until a point, is reached where the LFL and UFL coincide. This point corresponds to an oxygen content of approximately 11%. No hydrocarbon gas/air mixture can burn at this oxygen level. For practical purposes and to allow a safety margin, 8% is taken as the level of oxygen at which no hydrocarbon gas/air mixture can burn under any circumstances. To prevent fire or explosion in a tank containing a hydrocarbon gas/air mixture, it is therefore necessary to produce and supply inert gas having an oxygen content not normally exceeding 5% and to displace the existing air in the tank until the resultant oxygen level throughout the tank does not exceed 8% by volume.
Inert gas systems should be capable of delivering inert gas with an oxygen content in the inert gas main of not more than 5% by volume at any required rate of flow; and of maintaining a positive pressure in the cargo tanks at all times with an atmosphere having an oxygen content of not more than 8% by volume except when it is necessary for the tank to be gas free.
Upper flammable limit (UFL): It is the concentration of hydrocarbon gas in air above which there is insufficient amount of air to support and propagate combustion. It is also referred to as Upper explosive limit (UEL). Mixture above this limit is called as rich mixture
Sources of inert gas; Possible sources of inert gas on tankers are:1. Uptake gas from the ships main and auxiliary boilers.
2. An independent inert gas generator.
3. A gas turbine plant when equipped with an afterburner.
When an independent inert gas generator or a gas turbine plant with afterburner is fitted, the oxygen content can be automatically controlled within finer limits, usually within the range 1.5% to 2.5% by volume, and not normally exceeding 5%.
In certain ports, the maximum oxygen content of inert gas in the cargo tanks may be set at 5% to meet particular safety requirements, such as the operation of a vapour emission control system. Where such a limitation is in place, the vessel is usually advised of the requirements in the pre-arrival information exchange.
The Inert gas, besides not supporting combustion, should also fulfill certain other requirements, such as –
1. It should be non-reactive with cargo (e.g. in case of chemical gas)
2. It should not taint cargo (e.g. product)
3. It should not react with tank material (e.g. not causing corrosion)
4. It should have negligible or no toxic constituents.
5. It should be easily available at reasonable cost.
Other advantages of using IG are:1. Positive pressure is maintained in the tank. This keeps out other gases that could cause combustion.
2. The positive pressure helps a faster discharge rate.
3. It prevents cargo loss due to evaporation.
Oil tankers may be fitted with an IG system, which utilizes the flue gases from engine room boilers as a means of reducing the oxygen content in cargo tanks. Some vessels may be fitted with IG Generators which are self contained units burning diesel oil to produce inert gas.
Usually, the inert gas composition is:Oxygen (02) % by volume: 2 to 4%
Carbon dioxide (CO2) % by volume : 12 to 14%
Nitrogen (N2) : Balance
Sulphur dioxide (SO2) : Traces
Carbon monoxide (CO) : Trace
Nitrogen Oxide (NOX) : Trace
Water vapour H2O : Trace (high if not dried)
Ash and soot (C) : Traces
GENERAL ARRANGEMENT OF THE INERT GAS SYSTEM.
Flue gas generated from the boiler flows through the Boiler Up-take Valve and into the Scrubber. There, the gas is cooled down and washed by sea water supplied by the Scrubber Water Pump. Before leaving the Scrubber the gas passes through the Demister where water droplets are removed before entering the Blower suction. On the discharge side of the Blower, oxygen content and temperature of the flue gas are monitored. High oxygen content and high temperature activate alarms.
Inert Gas from the blower flows in to the Deck Seal through Inert Gas Pressure Regulating Valve. Main line pressure is automatically controlled to keep desired pressure constant. Excessive pressure is avoided by the Inert Gas Pressure Regulating Valve working in conjunction with the Recirculation Valve.
The Deck Seal isolates the boiler up-take from the deck line by using sea water and to prevent the backflow of the hydrocarbon gas. Inert Gas from the deck seal flows into the deck supply line through the Non-return Valve and Deck Isolating Valve and then enters each tank through the Inert Gas Supply Valve (some vessels do not have individual valves). The Inert Gas System can also be used for Gas Freeing by opening the Fresh Air Inlet Valve.
The P/V breaker is installed to protect the cargo tanks from excessive pressure or vacuum.
Functions of the Inert Gas System UnitBOILER UP-TAKE VALVE
Flue Gas generated by the boiler flows into the Scrubber Unit through the Up-Take valve. This valve is opened by remote control on Blower start up and when the Blower stops this valve has to be closed in order to avoid Flue Gas entering the Scrubber Unit.SCRUBBER UNIT
This is installed to clean and cool the flue gas and to reduce the sulfur dioxide (SO2 ) from the flue gas.
The demister is provided to remove water droplets contained in the inert gas which have passed through the scrubber. Since the inert gas is cooled and cleaned at the scrubber, the gas at the scrubber outlet inevitably contains water droplets as a result of its direct contact with the sea water used for cleaning it and could thus overload the blower and damage and increase corrosion on the blower impellers.BLOWER
Total blower capacity is more than 125% of total cargo pumps capacity. And combination of two blowers is;-two blowers together giving 125% (i.e. 62.5% each) of total cargo pump capacity. Thus two blowers must be used during normal cargo discharging one blower having 125% capacity plus one standby/ auxiliary with either 30/ 60/ 125% of total cargo pump capacityRECIRCULATION LINE
This allows the blower to operate when the pressure regulating valve is being closed. Gas flows back through this line to the Scrubber and thus avoids pressure built up on the discharge side of the blower.
DECK SEAL UNIT
The IGS connects the boiler up-take indirectly with the cargo oil tanks, and while the system is not in operation, the backflow of the oil vapour under pressure from the cargo tanks must be protected against. The Deck Seal is provided for this purpose.DECK ISOLATING VALVE.
The inert gas coming out of the deck seal unit is distributed to each cargo tank through this valve.
P/V (Pressure Vacuum)
Under normal conditions, the Breather Valve controls the cargo tank pressure/vacuum automatically when the I.G.S. is off. As a back-up safety device, a Pressure/Vacuum Breaker is fitted to the deck main piping and is designed to release pressure from this piping and cargo tanks to atmosphere in the event that the Breather Valve capacity is exceeded while operating the I.G.S.The P/V Breaker does not have any moving parts and is filled to be required level by oil or fresh water containing an antifreeze solution.
Inert Gas system is required under SOLAS Regulation 60 of chapter II-2, for all Petroleum tankers of 20000 dwt and above, keel laid after 1984.
Exhaust from Main Engine is not used for inert gas for 2 reasons:
1. Inert gas is most required during discharge in port, when the main engine is not running.
2. To ensure complete combustion of fuel, extra air is fed into the engine (turbocharged) resulting in greater concentration of oxygen in the exhaust.
3. Air fuel ratio cannot be controlled for getting a certain O2% in exhaust.
# Various books, study material and other online sources has been refereed prior to writing this article but no part is copied or produced from any of the source but explained same thing in better detailed way.
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