Flammability Composition Diagram - (With more detail explanation using animation) -Watch until end..

FLAMMABILITY COMPOSITION DIAGRAM

This is the most important diagram to understand the concept of flammability.

Every point on the diagram represents a hydrocarbon gas/air/IG mixture in terms of its hydrocarbon and oxygen content.
When the tank is clean, the oxygen level inside the tank will be 21% by volume.
As hydrocarbon is introduced in the tank, the oxygen level will reduce until reaches the point where hydrocarbon is 100% by volume and oxygen is 0%. Line AB slope reflecting the reduction in oxygen content as the hydrocarbon content increases.
Any points to the left of the line AB represent mixtures with their oxygen content further reduced by the addition of Inert Gas (IG)
The point of “C” and “D” represent the lower and upper flammability limit mixture for hydrocarbon gas in the air.
As the IG content increases, the flammable limit mixtures change as indicated by the lines CE and DE, which coverage at the point E.
Only those mixtures in the shaded area within the loop CED can burn.

The possibility of the composition gas mixtures enters the shaded area of flammable mixture whenever an inerted mixture is diluted by air (During gas freeing operation). At this moment, the DANGER OF EXPLOSION may happen.
It is best practice to dilute the mixture in the tank with additional IG (by purging) to remove hydrocarbon gas and bring the level below line GA (critical dilution with air). Then after that, the inerted mixture is diluted by air in order to prevent the mixture become flammable on dilution

Hydrocarbon gas in petroleum tankers cannot burn in an atmosphere containing less than about 11% oxygen by volume (ISGOTT Chapter 11.1.1),
As a safety margin, SOLAS requires that the IG system keep positive pressure in cargo tanks and have an oxygen content of not more than 8% by volume (ISGOTT Ch. 11.1.3)

The minimum concentration of a particular combustible gas or vapor necessary to support its combustion in air is defined as the Lower Explosive Limit (LEL) for that gas. Below this level, the mixture is too "lean" to burn.

The maximum concentration of a gas or vapor that will burn in the air is defined as the Upper Explosive Limit (UEL). Above this level, the mixture is too "rich" to burn.

The range between the LEL and UEL is known as the flammable range for that gas or vapor where the concentration of a particular combustible gas or vapor is sufficient to support its combustion in air.


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