INERT GAS SYSTEM :

     The purpose of providing Inert Gas System for oil tankers is to endeavour to reduce the possibility of fire/explosion by reducing the oxygen content in the tanks below 8%  by volume.

SOLAS Regulation 62 states that the Inert Gas system shall be so designed and operated as to render and maintain the atmosphere of the cargo tanks non  flammable at all times, except when such tanks are required to be gas - free.

The system shall be capable of : (SOLAS REGULATION 62)

1)     Inerting and maintaining the atmosphere in any part of any cargo tank with an oxygen content not exceeding 8% volume and at a positive pressure at all times in port and at sea except when it is necessary for such a tank to be gas - free.

2)                    The system shall be capable of delivering inert gas with an oxygen content of not more than 5% by volume in the inert gas supply main to the cargo tank at any required rate of flow.

3)                    The system shall be capable of delivering inert gas at a rate of at least 125% of the maximum rate of discharge capacity of the ship expressed as a volume.

4)                    The inert gas supply may be treated flue gas from main or auxiliary boilers or from separate gas generators or any combination thereof.

5)                    Flue gas isolating valves shall be fitted in the inert gas supply mains between the boiler uptakes and the flue gas scrubber, with indication to show whether they are open or shut and to maintain them gastight. Arrangements shall be made to ensure that boiler soot blowers cannot be operated when the corresponding flue gas valve is open.

6)                    A flue gas scrubber shall be located aft of all cargo-tanks, cargo pump rooms and cofferdams separating these spaces from machinery spaces, which will effectively cool the gas and remove solids and sulphur combustion products. Provision shall also be made for an alternative supply of cooling water.

7)                    Filters or equivalent devices shall be fitted to minimise the amount of water carried over to the inert gas blowers.

8)                    At least two blowers shall be fitted and located aft of all cargo tanks, cargo pump rooms and cofferdams separating these spaces from machinery spaces, which together shall be capable of delivering to the cargo tanks at least the volume of gas required as stated above or only blower capable of delivering the total volume provided sufficient spares for the blower and its prime mover are carried on board to enable any failure of the blower and its prime mover to be rectified' by the ship's crew.

The inert gas system shall be so designed that the maxi mum pressure which it can exert on any cargo tank will not exceed the test pressure of any cargo tank. Suitable shut off arrangements shall be provided on the suction and discharge connections of each blower.         

10) A gas regulating value shall be fitted in the inert gas supply main which shall be automatically controlled to regulated the flow of inert gas or arrangement for automatically controlling the speed of inert gas blowers shall be required.

11)         At least two non - return valves, one of which shall be a water seal, shall be fitted in the inert gas supply main, in order to prevent the return of hydrocarbon vapour to the machinery space uptakes or to any gas - safe spaces under all normal conditions of trim,
list and motion of the ships. They shall be located between the automatic valve and the aftermost connection to any cargo tank or cargo pipeline.

12)         The water seal shall be capable of being supplied by two separate pumps, each of which shall be capable of maintaining an adequate supply at all times.  

 13)   Provision shall be made to ensure that the water seal is protected against freezing, in such a way that the integrity of seal is not impaired by overheating.

14)         A water loop or other approved arrangement shall also be fitted to each associated water supply and drain pipe and each venting or pressure - sensing pipe leading to gas - safe spaces. Means shall be provided to prevent such loops from being emptied by vacuum.

15)         The second device shall be a non - return valve or equivalent capable of preventing the return of vapours or liquids and fitted forward of the deck water seal and provided with positive means of closure.

16)         One or more pressure/vacuum - breaking devices shall be provided to prevent the cargo-tanks being subject to.

a)          positive pressure in excess of the test pressure of the cargo tank if the cargo were to be loaded at the maximum rated capacity and all other outlets are left shut; and

b)         a negative pressure in excess of 700 mm water gauge if cargo were to be discharged at the maximum rated capacity of the cargo pumps and the inert gas blowers were to fail.

Such devices shall be installed on the inert gas main unless they are installed in the venting system (also required) or on individual cargo tanks.

17)   Instrumentation shall be fitted for continuously indicating and permanently recording, when the inert gas is being supplied :

a)        temperature and pressure of the inert gas at the discharge side of gas blowers.  

b)         pressure of the inert gas supply mains forward of the non - return devices.

c)         oxygen content of the inert gas in the inert gas supply mains on the discharge side of gas blowers.

Above meters shall be fitted in cargo control room, navigation bridge and machinery control room.

18)   Portable instruments for measuring oxygen and flammable vapour concentration shall be provided.

Audible and visual alarms shall be provided to indicate :

A)   Low water pressure or low water flow rate to the flue gas scrubber

B)       High water level in the flue gas scrubber ;

C)      High gas temperature.

          D) Failure of the inert gas blowers ;

E)        Oxygen Content in excess of 8% by volume ;

F)  Failure of the power supply to the automatic control system for the gas regulating valve and to the indicating devices.

G)        Low water level inthe water seal ;

H)  Gas pressure less than 100 mm of water  gauge ; I) High gas pressure. For inert gas generator type, additional audible and visual alarms shall be provided to indicate.

a)        insufficient fuel oil supply ;

b)         failure of the power supply to the generator ;

c)    failure of the power supply to the automatic control system for the generator.

Application of Inert Gas :-

1.   To fill the void space in the cargo tanks which carry crude oil or other liquids of relatively low volatility thus preventing any inflammable mixture being formed.

2.                   To replace discharged cargo thereby maintaining an inert atmosphere.

3.                   To purge the Cargo tanks of flammable vapour prior to air entering the tanks so that inspection and maintenance can be conducted safely.

Inert Gas Specification :-

Oxygen - 5% or less by volume.

CO₂ - about 13% by volume.

SO₂ - 0.2 - 0.3%

Water - 5% by volume.

Particulate Content - negligible

Nitrogen - 77% by volume.

Methods of obtaining Inert Gas :-                          

1)   Utilise the exhaust gas from the boiler uptake.

2)         Supply a separate self - contained inert gas generator.

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