Flammability diagrams show the control of flammability in mixtures of fuel,

oxygen Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as ...

and an inert gas, typically

nitrogen Nitrogen is the chemical element with the symbol N and atomic number 7. Nitrogen is a nonmetal and the lightest member of group 15 of the periodic table, often called the pnictogens. It is a common element in the universe, estimated at se ...

. Mixtures of the three gasses are usually depicted in a triangular diagram, known as a

ternary plot A ternary plot, ternary graph, triangle plot, simplex plot, Gibbs triangle or de Finetti diagram is a barycentric plot on three variables which sum to a constant. It graphically depicts the ratios of the three variables as positions in an equ ...

. Such diagrams are available in the speciality literature.Crowl 2003 The same information can be depicted in a normal orthogonal diagram, showing only two substances, implicitly using the feature that the sum of all three components is 100 percent. The diagrams below only concerns one fuel; the diagrams can be generalized to mixtures of fuels.

Understanding flammability diagrams

Triangular diagrams are not commonplace. The easiest way to understand them is to briefly go through three basic steps in their construction. #Consider the first triangular diagram below, which shows all possible mixtures of methane, oxygen and nitrogen.

Air The atmosphere of Earth is the layer of gases, known collectively as air, retained by Earth's gravity that surrounds the planet and forms its planetary atmosphere. The atmosphere of Earth protects life on Earth by creating pressure allowing f ...

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 Mixtures of dispersed combustible materials (such as gaseous or vaporised fuels, and some dusts) and oxygen in the air will burn only if the fuel concentration lies within well-defined lower and upper bounds determined experimentally, referred to a ...

of methane in air are located on this line, as shown. #The

stoichiometric Stoichiometry refers to the relationship between the quantities of reactants and products before, during, and following chemical reactions. Stoichiometry is founded on the law of conservation of mass where the total mass of the reactants equ ...

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

of methane in oxygen are located on the methane axis, as shown. #The actual envelope defining the flammability zone can only be determined based on experiments. The envelope will pass through the upper and lower

of methane in oxygen and in air, as shown. The nose of the envelope defines the

limiting oxygen concentration The limiting oxygen concentration (LOC), also known as the minimum oxygen concentration (MOC), is defined as the limiting concentration of oxygen below which combustion is not possible, independent of the concentration of fuel. It is expressed in ...

(LOC)). Image:Flammability diagram methane 1 w air line.svg, Triangular diagram showing all possible mixtures of methane, oxygen and nitrogen. Any mixture of methane and air will lie on the blue air-line Image:Flammability diagram methane 2 w stoich line.svg, Any

mixture of methane and oxygen will lie on the red stoichiometric line Image:Flammability diagram methane.svg, The actual flammability envelope defining flammable mixtures of methane

Sources

* (His main work, 6.3Mb download) * * *{{cite book , last = Crowl , first = Daniel A. , title = Understanding Explosions , publisher = Wiley-AIChE , year = 2003 , isbn = 0-8169-0779-X

References

Safety engineering Fire protection Combustion

Understanding flammability diagrams

See also

Sources

References