|1. Definition of
|3. Typical range
|4. Use in risk assessment|
|4. MEC of common dusts
A dust cloud can explode only if there is the right proportion of powder in suspension in air. If the concentration of dust is too low or too high, explosion will not happen. The smallest concentration (kg of powder / m3) below which the explosion cannot happen is called the minimum explosive concentration or MEC.
Determining the minimum explosive concentration and the maximum explosive concentration requires a lot of experiments. Such concentration measures are also very sensitive to particle size distribution, the ignition energy and the volume and shape of the vessel in which the tests are carried out. [Laurent]
Values of concentrations should therefore be considered primarily as indications rather than taken as absolute and definitive values.
As it is quite difficult to determine the explosive concentrations of powder clouds, there are different references that can be found in literature.
[Laurent] is giving the range : 0.020 kg/m3 as minimum concentration to 1-10 kg/m3 as maximum concentration
A commercial dust explosion manual by [Stahl] is giving the range : 0.020-0.060 kg/m3 to 2-6 kg/m3
A concentration of around 0.5 kg/m3 is sometimes given as "optimal". MIE testing are done around this concentration.
All these values are in air only, please note that the presence of flammable gas will totally change results.
Each processor must carry out a dust explosion risk analysis in order to assess the risk related to a specific material in a specific process and take necessary precaution and mitigation measures.
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Indications of minimum explosible concentration is interesting in risk assessment to determine the zoning of the production process and its environment. Note that the values reported are low but actually even at these small concentration a dust cloud is visually very dense. References are found in literature reporting that a 0.040 kg/m3 coal dust cloud is so dense that you cannot see a light bulb at 2 m distance through it [Laurent].
The maximum concentration limit can also be interesting as many equipment like mixers may not be at risk of explosion during processing at the dust concentration is very high.
All decision in the risk analysis should however be supported by documented assessment of the concentrations observed in the process.
Please find below some MEC data given in the literature. WARNING : these are general values given without guarantee, a risk assessment and design must ALWAYS refer to the MSDS of the ACTUAL product used for tests carried out specifically on the ACTUAL material by a reputable institute.
As mentioned above, the determination of this value is difficult
thus sometimes a range is given when different research results have
been reported. As results reported may have been obtained following
different protocols, not always precised, it adds to the care needed
when using those values.
Table 1 : Minimum Explosive Concentration of common materials
|Material||MEC in kg/m3 in air|
||> 0.010-0.030 [Laurent]|
0.050 for wheat flour [Mills]
|Phenol formaldehyd resins||0.015-0.025 [Laurent]|
|Wood flour||0.050 [Rhodes]