A general way to represent the flammability of a gas or vapor is by a triangle diagram shown below. Concentrations of fuel, oxygen and inert material are plotted on the three axes. Each apex of the triangle represents either 100% fuel, oxygen or nitrogen. The air line in the figure represents all possible combinations of fuel plus air. The air line intersects the nitrogen axis at 79% nitrogen and 21% oxygen which is the composition of pure air. The UFL and LFL are shown.
As the intersection of the flammability zone boundary with the air line.
The stoichiometric line represents all stoichiometric combinations of fuel plus oxygen. The combustion reaction can be written in the form,
Fuel + z O2 ? combustion products
Here, ‘z’ represents the stoichiometric coefficient for oxygen.
The intersection of the stoichiometric line with the oxygen axis is given by
The stoichiometric line is drawn from this point to the pure nitrogen apex. The LOC is the line that shows any gas mixture containing oxygen below the LOC is not flammable. 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.
Reference: 1. D.Crowl and J. Louvar, Chemical Process Safety: Fundamentals with Applications, Prentice Hall (2002) Chapter 2; 2. MSDS sheet from online; 3. http://www.unisa.edu.au/ohsw/procedures/hazard.asp