Fuels flammability diagram

The shape and size of the flammability zone on a flammability diagram change with a number of parameters, including fuel type, temperature, pressure, and inert species. Thus the flammability limits and the LOC also change with these parameters. 

The upper time-line diagram of figure 6.11 shows a burner startup situation where the air/fuel ratio control has erroneously been set too rich. The burner may have lighted as it entered the flammable zone (about 5% gas in a gas-air mixture, for natural gas), but its mixture soon became too rich to burn, exceeding the upper limit of flammability (about 15% gas in a natural gas-air mixture), exiting the flammable zone, with the flame going out. The pilot has its own controlled air and fuel supply, set at an air/fuel ratio between the flammability limits thus, it stays lighted even... 

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