Flammability limits
When is a mixture flammable of explosive?
A mixture of a fuel and air is only explosive if the fuel and air are present in the correct ratio. With too little fuel in the air, the mixture is "too poor" and with too much fuel in the air, the mixture is "too rich"; the mixture cannot be ignited and is therefore not explosive.
The limits within which the mixture can be ignited are called the explosion limits of a gas or a substance. The area between the borders is called the explosion area.
Explosion limits LEL and UEL
Minimum and maximum Flammable Explosion Limit
The Lower Explosion Limit (LEL) indicates at what minimum percentage of gas or dust in air the combustible mixture is still explosive. Below this limit there is too little flammable substance in the air, the mixture cannot be ignited and is therefore not explosive. This limit is also called Lower Flammable Limit (LFL).
The Upper Explosion Limit (UEL) indicates at what maximum percentage of gas or dust in air the combustible mixture is still explosive, just short of being saturated. Above this limit, too little oxygen is present relative to the amount of gas and the mixture cannot be ignited. This limit is also called the Upper Flammable Limit (UFL).
The LEL and UEL are also important when you want to take the ATEX explosion hazards and guidelines into account
In the table below (Table 1.1) you can see the Lower and Upper Explosion Limits for different kind of gasses and vapors. So you can more precisely determine the ammonia explosion hazard for example.
| Gas or vapor | LEL value [%] | UEL value [%] |
| Acetone | 2,5 | 13 |
| Acetylene | 2,3 | 100 |
| Ammonia | 15 | 33,6 |
| Butane | 1,4 | 9,3 |
| Ethanol | 3,1 | 19 |
| Ethylene | 2,3 | 36 |
| Kerosene | 0,7 | 5 |
| Carbon monoxide | 10,9 | 74 |
| Methane | 5 | 14 |
| Propane | 1,7 | 10,9 |
| Hydrogen | 4 | 77 |
LEL and UEL values
An explosive atmosphere can also be avoided by diluting the combustible mixtures with chemically inert (non-reactive) substances. This precautionary measure is called inertization. For this, the minimum oxygen concentration (oxygen limit concentration) at which an explosion could take place must be known.
LEL and UEL values for dust
For flammable dust it is very difficult to determine an LEL value because the size and shape of dust particles vary greatly. In case of dust and fibers, the sensitivity and explosive power depend much more on particle size than on concentration; small particles are much more flammable than larger ones.
For a specific production process, the LEL value can be determined by examining a sample. The LEL values for a dust cloud are often between 50 and 200 gram/m3. If the LEL value is not known a conservative value of, for example, 20 gram/m3 must be assumed.
Determining the UEL value of flammable dust is even more difficult. Even with very high concentrations of dust (up to kilos per m3) it seems possible to ignite a dust cloud. Moreover, the heavier dust particles will settle more quickly and form a layer of dust. The lighter particles float longer, forming a more dangerous dust cloud because of greater flammability. The precipitated dust whirls up again during an explosion, thereby maintaining the dust cloud. It is therefore less relevant to determine the UEL value of a dust cloud.
The concentration of dust within a space is ultimately determined by the total amount of dust and the volume of that space, assuming that the dust is distributed homogeneously. In practice, this is never the case; locally dust concentration can be very much higher!
