Standard air composition is the bulk average air composition encountered at or around sea level. Typically it contains the following composition.
| Component | Abbr. | %Wt | %Vol. | MW |
| Nitrogen | N2 | 75.47 | 78.08 | 28.01 |
| Oxygen | O2 | 23.20 | 20.95 | 32.00 |
| Carbon Dioxide | CO2 | 0.0590 | 0.038 | 44.01 |
| Hydrogen | H2 | 0 | 0.00005 | 2.02 |
| Argon | Ar | 1.28 | 0.93 | 39.95 |
| Neon | Ne | 0.0012 | 0.0018 | 20.18 |
| Helium | He | 0.00007 | 0.0005 | 4.00 |
| Krypton | Kr | 0.0003 | 0.0001 | 83.80 |
| Xenon | Xe | 0.00004 | 8.7x10^-6 | 131.30 |
From standard air composition at sea level, the oxygen content is approximately 20.95% by volume. Oxygen is the main component in fire triangle in generating of fire. From chemistry, without any one of the element, a fire will NOT form.
Although Oxygen is the major component in generating fire, there is still a minimum oxygen concentration (MOC) required present combustible mixture so that a fire can be initiated. Below this limit, a fire will not form. Base on this principle, hydrocarbon mixture is purge with inert gas such as Nitrogen, Carbon dioxide, etc so that the MOC is not reach and therefore combustible mixture is not form.
Common unit used for MOC is % oxygen in air plus fuel. Following table listed several
| Component | MOC (%O2) |
| Hydrogen | 4.0 |
| Acetylene | 6.2 |
| Methane (C1) | 12 |
| Ethane (C2) | 11.2 |
| Ethylene (C2=) | 9.9 |
| Propane (C3) | 11.6 |
| Propylene (C3=) | 11.5 |
| Butane (C4) | 12.3 |
| 1-Butene (C4=) | 11.0 |
| Pentane (C5) | 11.8 |
| Hexane (C6) | 11.8 |
| Benzene | 11.5 |
| Methyl Alcohol | 9.9 |
| Ethyl Alcohol | 10.5 |
| Dimethyl Ether | 10.5 |
| Diethyl Ether | 10.2 |
| Methyl Acetate | 11.0 |
| Methyl Ether Ketone | 11.0 |
| Carbon Disulfide | 5.0 |
Generally a mixtures of Hydrocarbon vapor MOC is around 10% Vol.
API STD 520 recommends MOC of 6 %Vol. for flare stack design to avoid combustible mixture present in flare system. Hydrogen rich flare gas shall consider lower MOC i.e. 3-4% vol.
Ref : "Application of Flammability Diagram For Evaluation of Fire & Explosion Hazards of Flammable Vapor", C.V. Mashuga, D.A. Crowl
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