safety

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Definitions:

Inert gas- This is a gas which contains insufficient oxygen to support combustion of hydrocarbons

Inert conditions- This is where a space has had its oxygen content reduced to 8% or less

Inert gas plant- This is a system specially designed to supply cool, clean, pressurised, monitored and controlled inert gas.

Gas freeing- Opposite to inerting and is the replacement of an inert atmosphere with one of fresh air.

The choice of whether an inert gas system is fitted to a vessel is based on the initial cost of installation and maintenance, the planned cargos to be carried and the possibility of the being tainted by the inert gas and the possibility that the inert gas system in itself will introduce a risk. For an example of the latter procedures would have to ensure that the space is well ventillated of the inert gas before a person could enter.
Sources of inert gas
The use of the term Inert Gas is a misnomer in so far as the true inert gasses such as Helium and argon are prohibitively expensive to use.

Similarly the use of semi-inert gasses such as Nitrogen and Carbon-dioxide are too expensive to use on bulk, nitrogen is often seen in use on gas carriers in barrier spaces or for the clearing and inerting of pipelines and pumps.

The gas most commonly used is the exhaust product of combustion.

    This may have three sources
        Exhaust from an internal combustion engine such as a diesel engine or gas turbine. The relatively high Oxygen content in the output must be reduced to make it suitable and this is generally achieved by the use of an afterburner. This is an uncommon system and will not be dealt with currently
        Boilers
        Gas oil or heavy oil powered inert gas generator. This generally takens the form of an insulated combustion space similar in layout to a tank boiler with the insulation taking the place of the water,with rotary cup burner

    Purging- This is the introduction of inert gas into an inerted space to;
        further reduce O2 content
        reduce hydrocarbon level in the inert gas so that air may be introduced without the mix entering the flammable range.

Limits of flammability
Graph showing lower and upper explosive limits and area of combustion for hydrocarbons

Oxygen control
Theoretically any mix with less than 11.5% oxygen will not support combustion, However, for safety the level is reduced to 8% vol. This allows for calibration errors in monitoring equipment as well as any lack of homogeneity in the tanks.

The tank is kept at positive pressure to ensure no ingress of air.( say 100mmwg at the deck ).
Hydrocarbon control
The principle means of ensuring safe operation is the reduction in oxygen, high levels of HC should not effect the safe operation and may in fact aid by producing an over rich atmosphere.

If it is required to gas free then the level of HC must be reduced to prevent the mix entering the flammable range, then the HC level is reduced by purging.
Gas replacement
There are two principle means of gas replacement, these are;

Dilution-The important factors for these is that the vent is situated at the top of the tank and the inlet gas stream must have sufficient velocity to reach the bottom of the tank

Displacement- This requires a stable interface between the heavier and lighter gas, if the replacement gas is heavier it enters at the top with low velocity , the lighter gas is vented up a purge pipe reaching the base of the tank.
General policy of cargo tank atmosphere control.
It is the masters responsibility for keeping a non-explosive atmosphere within the tank, and to ensure all personnel concerned with the operation are well versed.

    To ensure the I.G. system is fulfilling its requirements it is the Chief Officers responsibility to
        Maintain Oxygen content at less than 8% especially when tank cleaning
        Maintain gas pressure at 100 mmWg
        Ensure correct level in Pv breaker
        Blanks and v/v's to be checked before operation
        Ensure no tank is overfilled when loading, ballasting.

The efficacy of the I.G. plant to produce inert gas at less than 8% Oxygen is the chief engineers responsibility.

    The correct operation of the I.G system should allow the following benefits;
        Closed loading procedures
        Reduced cargo discharge times
        Simplification of the tk cleaning procedures
        Reduction of oxygen dependent corrosion
        Limited repair work on hull without the need to gas free

Pyrophoric ignition
In an oxygen deficient atmosphere where there is Hydrogen sulphide present the iron oxide can be reduced to iron sulphide, with the reintroduction of air the iron reconverts to iron oxide with considerable heat and possible incandescence.

Hence, when gas freeing it is important to maintain the mix outside the flammable range.