FLAMMABILTY DIAGRAM

 Lower Flammable Limit

It is the concentration of CH gas in air below which there is insufficient CH to support and propagate combustion. 

Upper Flammable Limit

It is the concentration of CH gas in air above which there is insufficient air to support and propagate combustion. 

The flammable range for 

- Hydrocarbon gases is 1% and 10%

- Oxygen 11.5% and 21%

If 10% of CH gas is introduced in air the proportion of oxygen reduces to 18.9%.

If 1% of CH gas is introduced in air the proportion of oxygen reduces to 20.8%.

When oxygen is minimum (11.5%), CH gas must be 1.5%.

If the concentration of CH gas and Oxygen is outside the flammable range, the mixture will not burn. 

When we dilute a CH gas - air mixture with air, air will slowly replace the hydrocarbon gas till the mixture become 100% air. The concentration of CH gas and oxygen will take 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.