What is Flammability Diagram?
Flammability diagrams is a type of ternary diagram that show the regimes of flammability in mixtures of
fuel, oxygen and an inert gas, typically nitrogen. Mixtures of the three gasses are usually depicted in a
triangular diagram, also known as a Ternary plot. Such diagrams are available in the specialty literature.
The same information can be depicted in a normal orthogonal diagram, showing only two substances,
implicitly using that the sum of all three components are 100 percent. The diagrams below only concerns
one fuel, the diagrams can be generalized to mixtures of fuels.
Image from google
How to read the flammability diagram?
Reading the flammability diagram consist of three simple steps…
1.
Consider the first triangular diagram below, which shows all possible mixtures of methane, oxygen
and nitrogen. Air is a mixture of about 21 volume percent oxygen, and 79 volume percent inerts (nitrogen).
Any mixture of methane and air will therefore lie on the straight line between pure methane and pure air
- this is shown as the blue air-line. The upper and lower flammability limits of methane in air are located
on this line, as shown.
2.
The stoichiometric combustion of methane is: CH4 + 2O2 → CO2 + 2H2O. The stoichiometric
concentration of methane in oxygen is therefore 1/(1+2), which is 33 percent. Any stoichiometric mixture
of methane and oxygen will lie on the straight line between pure nitrogen (and zero percent methane)
and 33 percent methane (and 67 percent oxygen) - this is shown as the red stoichiometric line. The upper
and lower flammability limits of methane in oxygen are located on the methane axis, as shown.
3.
The actual envelope defining the flammability zone can only be determined based on
experiments. The envelope will pass through the upper and lower flammability limits of methane in
oxygen and in air, as shown. The nose of the envelope defines the limiting oxygen concentration (LOC)).
What actually the lines in the diagram represent?
1. Triangular diagram showing all possible mixtures of methane, oxygen and nitrogen. Any mixture
of methane and air will lie on the blue air-line.
2. Any stoichiometric mixture of methane and oxygen will lie on the red stoichiometric line.
3.
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6.
7.
The actual flammability envelope defining flammable mixtures of methane.
The vapor-air mixture will not burn when the composition is lower than the lower flammability limit (LFL);
the mixture is too lean for combustion.
The mixture is also not combustible when the composition is too rich; that is, when it is above the upper
flammability limit (UFL).
The stoichiometric line represents all stoichiometric combinations of fuels plus oxygen.
Limiting oxygen concentration (LOC): Minimum oxygen concentration required to propagate a flame.
Source from Wikipedia.