Limiting oxygen concentration
Limiting oxygen concentration, often abreviated LOC, is defined as the limiting concentration of oxygen below which combustion is not possible, independent of the concentraion of fuel. It is expressed in units of volume percent of oxygen. The LOC varies with pressure and temperature. It is also dependent on the type of inert (non-flammable) gas.
| Gas or vapor | Nitrogen / Air | Carbon dioxide / Air |
| Hydrogen | 5 | 5.2 |
| Methane | 12 | 14.5 |
| Ethane | 11 | 13.5 |
| Propane | 11.5 | 14.5 |
| n-Butane | 12 | 14.5 |
| i-Butane | 12 | 15 |
Limiting oxygen concentration below which combustion cannot take place (volume percent oxygen)[1]
The effect of increasing the concentraion of inert gas can be understood viewing the inert as thermal ballast that limits the flame temperature below which the flame cannot exist.[2]. Carbon dioxide is therefore more effective than nitrogen due to its higher molar heat capacity[3]
The concept has important practical use in safety engineering. For instance, to safely fill a new container or a pressure vessel with flammable gasses, the atmosphere of normal air (containing about 21 volume percent of oxygen) in the vesseel would first be purged with nitrogen or another non-flammable gas, thereby reducing the oxygen concentration in the container. When the oxygen concentration is below the LOC, the flammable gas can be safely admitted to the vessel without risk of internal explosion.
See also
Sources
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