Limiting oxygen concentration

Flammability diagram, green dotted line shows safe purging of an air-filled vessel, first with nitrogen, then with methane, to avoid the flammable region. The limiting oxygen concentration is shown in the lower right of the diagram.

Limiting oxygen concentration, (LOC),^[1] also known as the Minimum oxygen concentration, (MOC),^[2] is defined as the limiting concentration of oxygen below which combustion is not possible, independent of the concentration 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.

Limiting oxygen concentration for selected gasses and two inerts (volume percent oxygen)^[3]

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
Isobutane	12	15

The effect of increasing the concentration of inert gas can be understood by viewing the inert as thermal ballast that quenches the flame temperature to a level below which the flame cannot exist.^[4] Carbon dioxide is therefore more effective than nitrogen due to its higher molar heat capacity^[5]

The concept has important practical use in fire 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 vessel would first be flushed (purged) with nitrogen or another non-flammable inert gas, thereby reducing the oxygen concentration inside the container. When the oxygen concentration is below the LOC, flammable gas can be safely admitted to the vessel because the possibility of internal explosion has been eliminated.

See also

• Flammability limit
• Flammability diagram

Sources

Monographs

• Green, Don W.; Robert H. Perry (October 23). Perry's Chemical Engineers' Handbook. McGraw-Hill Professional; 8 edition. ISBN 0071422943.  Chapter 23

• Drysdale, Dougal (1985). An Introduction to Fire Dynamics. Wiley. ISBN 0 471 90613 1. 

• "Limiting Oxygen Concentration: Recent Results and their Presentation in Chemsafe". Physikalisch-Technische Bundesanstalt (Germany). 1998. http://www.ptb.de/en/org/3/34/343/moeller.htm. Retrieved 2009-03-11.  (Presented at 9th International Symposium on Loss Prevention and Safety Promotion in the Process Industries, May 1998, Barcelona (Spain))

References'

1. ^ Perry
2. ^ Drysdale
3. ^ Perry 8.ed:23-9
4. ^ Drysdale:84-85
5. ^ Drysdale:92