Explosive limit

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Mixtures of flammable gases, vapors or dusts with air and thus the oxygen they contain are explosive in certain mixing ratios typical of the substance . These mixing ratios determine the explosion range, which is described by its two explosion limits , the upper and lower explosion limits (UEG or LEL):


Explosion limits are the limits of the so-called "explosion range". The lower explosion limit (LEL) and the upper explosion limit (UEL) are the lower or upper limit value of the concentration (proportion of substance) of a flammable substance in a mixture of gases, vapors, mists and / or dusts, in which a Flame independent of the ignition source can no longer propagate independently.

The range below the lower, including the explosion limit itself, in which the concentration of the combustible substance is too low, is also referred to as a mixture that is too lean . The range above and including the upper explosion limit is referred to as a mixture that is too rich . Here the concentration of the flammable substance is too high to react explosively. A rich mixture can be further diluted by supplying air and thus get under the OEG and thus become explosive . A lean mixture can become greasy through the addition of fuel, for example through evaporation or the supply of fuel gas, and thus reach an explosive substance concentration. The explosion limits also change with changes in pressure and / or temperature. The humidity also has a not inconsiderable influence.

The explosion limits are also referred to as ignition limits ; they are temperature and pressure dependent, among other things. In the case of dusts, the particle size and the particle size distribution of the solid also have an influence on the explosion limits.

The explosion limits are not physical constants, but so-called “ safety-related parameters ” which, in contrast to the physical constants, are very much dependent on their method of determination.

In the context of a risk or hazard assessment, it must therefore be ensured that, in the case of practical application, it may be necessary to verify in individual cases whether the information on the explosion limits given in the literature can actually be used in terms of safety.

If the concentration of the flammable substance in the air is above the lower explosion limit and below the upper explosion limit, the mixture is called an explosive atmosphere under atmospheric conditions . Under other than atmospheric conditions or other oxidation partners than atmospheric oxygen, we generally speak of an explosive mixture. In general, atmospheric conditions are understood to mean an ambient temperature range between −20 ° C and 60 ° C and a pressure range between 0.8 bar and 1.1 bar. [ATEX guidelines, directive 94/9 / EC] [guidelines for explosive atmospheres].

The concentration of the combustible gas or vapor ( fuel to air ) is given in vol .-% or mol-% . In contrast to this, the air ratio lambda is used for the ignition limit with the specification of the weight fraction ( air to fuel ). The following relationship applies to ideal gases: 1 vol .-% = 1 mol% = 10,000 ppm . The stoichiometric volume ratio and lambda = 1 means approximately the highest risk of explosion and the best possible ignition.

The following table shows the lower and upper explosion limits for some gases when mixed with air. Further numerical values can be found in tables and for some substances in the safety data sheets .

Flammable gases LEL (lean)
in% by volume
in vol .-%
OEG (fat)
in% by volume
Carbon monoxide 10.9 30th 76
Methane (natural gas) 4.4 9.5 16.5
hydrogen 4th 30th 77
Ethyne (acetylene) 2.5 7.7 80
propane 1.7 4.0 10.8
Monosilane 1.5 ? 98
Flammable vapors LEL (lean)
in% by volume
in vol .-%
OEG (fat)
in% by volume
Nitromethane 7.3 ? 22.2
Methanol 6.7 ? 36
Ethanol (ethanol) 3.4 6.5 19th
petrol 1.4 ? 7.6
Kerosene 0.7 ? 5.0
diesel 0.6 ? 7.5

Individual evidence

  1. ^ Roy Bergdoll, Sebastian Breitenbach: Die Roten Hefte, Heft 1 - Burning and Extinguishing . 18th edition. Kohlhammer, Stuttgart 2019, ISBN 978-3-17-026968-2 .
  2. Final report on the research project, the dependence of safety parameters on the pressure below atmospheric pressure. In: D. Pawel, E. Brandes. Physikalisch-Technische Bundesanstalt (PTB), Braunschweig, 1998, accessed on February 29, 2020 .
  3. a b E. Brandes, M. Thedens: Characteristics of explosion protection under non-atmospheric conditions. ( Memento of March 4, 2016 in the Internet Archive ) (PDF; 432 kB). Physikalisch-Technische Bundesanstalt, 2003.
  4. a b Gases - Explosive and Flammability Concentration Limits , (English).
  5. E. Brandes, W. Möller: Safety-related parameters. Volume 1: Flammable Liquids and Gases. Wirtschaftsverlag NW - Verlag für neue Wissenschaft, Bremerhaven 2003.
  6. Chemsafe. Retrieved March 12, 2020 .
  7. New lower explosion limit values ​​(LEL) for flammable gases and vapors in air. (PDF) State Fire Brigade School Baden-Württemberg, September 7, 2007, accessed on August 9, 2016 .