6 replies to this topic

[Diederik Zwart]

This topic was discussed previously:

I think you'll find what you're looking for if you go to: Limiting Oxygen Concentration

If the link doesn't work, do a search on "limiting" AND "oxygen", the topic was posted first on March 10 2003.

Good luck,
Diederik Zwart

[gvdlans]

What you are looking for is the "flammability envelope" of your compound. If you draw a triangle with 100% flammable gas at the top corner, 100% oxygen at the lower lefthand corner and 100% nitrogen at the lower righthand corner. Inside this triangle (or tertiary diagram) you can draw a region inside this triangle where you have a flammable mixture. See figure 5 in the article found via following link (for hydrogen): Article with flammability envelope for hydrogen

You can construct the flammability envelope when you have literature data of flammable limits both in air and in pure oxygen. When plotting these upper and lower flammable limits in the tertiary diagram, and connecting the points by straight lines, you get a pretty good idea what the flammability envelope looks like. The figure 5 in above mentioned article is a good illustration of this method.

With this diagram you can find for any oxygen/nitrogen ratio what the flammability limits for your flammable gas are... Just draw a line from the top corner (with 100% flammable gas) to the point on the base line with the composition of your atmosphere (for example 30% oxygen, 70% nitrogen). Where this line crosses the flammability envelope you find the flammability limits. For most compounds you will find that for oxygen concentrations below about 10% (the limiting oxygen concentration) that the line does not cross the envelope. Here you really have an inert atmosphere...

See also NFPA 69 for more details.

[gvdlans]

Ramesh,

I strongly recommend you to get a copy of NFPA 69. It contains a good explanation on how to use the ternary diagram in Annex B, as well as a listing of Limiting Oxygen Concentrations (LOCs) in Annex C, both for N2/Air and for CO2/Air atmospheres. These LOCs were obtained at atmospheric temperature and pressure (so they are not valid at different process conditions..). For N2/Air atmospheres, the LOC for most gases is above 8%. but there are exceptions, for example:
Carbon Disulfide: 5.0%
Carbon Monoxide: 5.5%
Hydrogen: 5.0%
Dimethyl hydrazine: 7.0%

Note that this list in NFPA 69 is not complete, for example it does not contain ethylene oxide. So you cannot simply assume that LOC is above 8% just because it is not one of these four examples! It should also be noted that you should have a safety margin below this LOC. This is explained in chapter 5 of NFPA 69.