Draw Flammability Diagram & explain it briefly ?

Upper Flammable Limit (UFL) or Upper Explosive Limit (UEL): It is the concentration of CH gas in air above which there is insufficient air to support and propagate combustion.

Here, line AB represents the mixture of oxygen ( Air ) and particular hydrocarbon gas. Anything that is left to the line AB represents the condition where inert gas is also mixed with the hydrocarbon-air gas mixture.

The gas mixture represented by AC on line AB is too lean to catch fire while DB is too rich mixture to get fire.

When we dilute a CH gas – air mixture with air, (let’s say having the concentrations as shown at point F), air will slowly replace the CH gas till the mixture becomes 100% air. The concentrations of CH gas and oxygen will take the path FA. The path passes through the flammable range and at some time during the dilution, the mixture could explode if a spark was applied. We must try to avoid passing through the flammable range during dilution with air. To do this, the mixture is diluted with inert gas (along the line FH) till a point H is reached below the critical dilution line. This is known by sampling and testing the tank atmosphere with gas measuring instruments.

Now if oxygen (air) is let in freely, the dilution will take place along line HA, without passing through the flammable range at any time causing no danger of fire or explosion.