Basics of Explosive Atmospheres

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An explosive atmosphere is an atmosphere that develops explosively because of an uncontrollable combustion. Explosive atmosphere consists of air and some sort of combustible material such as gas, vapours, mists or dust in which the explosion spreads after ignition. Typical examples of productions where combustible dust is of major concern, is the handling of cereals, animal feed, paper, wood, chemicals, plastics and coal.

Sources of Ignition
Sources of ignition that can cause the atmosphere to explode are listed below:

Common Sources of Ignition
Electrical sparks
Flames
Hot surfaces/ spots
Static electricity
Electromagnetic radiation
Chemical reaction
Mechanical forces
Mechanical friction
Compression ignition
Acoustic energy
Ionizing radiation

Fire Triangle
For an explosion to take place, three elements have to be present at the same time: fuel (such as explosive gas) , an oxidizer (such as the oxygen in the air) and a source of ignition (such as electrical sparks). The combination of these three elements is generally referred to as the Fire Triangle.

Explosion Limits for Typical Combustible Gases
For an explosive atmosphere to form, a certain concentration of combustible material must be present. When the concentration of combustible material is too low (lean mixture) or too high (rich mixture), no explosion will take place. In that case only a slow combustion or none at all will occur. It is only within the range of the upper and the lower explosion limit that the mixture of fuel and oxidizer reacts explosively when exposed to a source of ignition and becomes very devastating in terms of the scale of destruction.

Below are the lower and upper explosion limits of some common gases which may exist in the atmosphere where electrical or electronic equipment are installed:

Substance Designation
Lower Explosion Limit (Vol. %)
Upper Explosion Limit (Vol. %)
Acetylene
2.3
78.0 (self decomposing)
Ethylene
2.3
32.4
Gasoline
0.6
8.0
Benzol
1.2
8.0
Natural gas
4.0 - 7.0
13.0 - 17.0
Heating Oil/Diesel
0.6
6.5
Methane
4.4
16.5
Propane
1.7
10.9
Carbon Disulphide
0.6
80.0
Town gas
4.0 - 6.0
30.0 - 40.0
Hydrogen
4.0
77.0