Dust explosion concentration

Physical meaning and use in risk assessment of powder minimum explosive concentration (MEC)

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Section summary
Definition of MEC
Determination of MEC
Typical range
Use in risk assessment
MEC of common dusts

Bulk Solids Handling Safety
# Dust explosion physical properties # MIT - Minimum Ignition Temperature of dust

1. Definition of dust explosive concentration

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.

2. Experimental determination of 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.

3. Dust explosive concentration typical range

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

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.

4. Use in risk assessment

Indications of minimum explosible concentration is interesting in risk assessment to determine the zoning of the production process and its environement. 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 litterature 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.

5. MEC of common dusts

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
Acetylsalicylic acid
0.015-0.050 [Laurent]
Adipic acid
0.035 [Rhodes]
Aluminium
0.030-0.140 [Laurent]
0.040 [Rhodes]
0.045 [Mills]
Benzoic acid
> 0.010-0.030 [Laurent]
Caprolactam
0.070 [Laurent]
Coal
0.055 [Laurent]
0.055 [Rhodes]
0.055 [Mills]
Coffee
0.085 [Mills]
Cork
0.035 [Rhodes]
Corn starch
0.040 [Rhodes]
Dextrin
0.050 [Rhodes]
Epoxy resins
0.010 [Laurent]
Flour
0.050 [Laurent]
0.050 for wheat flour [Mills]
Grain dust
0.055 [Mills]
Steel
0.100-0.120 [Laurent]
Magnesium 0.030 [Rhodes]
0.020 [Mills]
Nylon 0.005-0.030 [Laurent]
0.030 [Mills]
Paper
0.030 [Laurent]
Phenol formaldehyd resins 0.015-0.025 [Laurent]
Sugar 0.015 [Laurent]
0.035 [Mills]
Sulfur 0.020 [Rhodes]
Wood flour 0.050 [Rhodes]
0.040 [Mills]
Zinc 0.400-0.460 [Laurent]
0.480 [Mills]
Zirconium
0.010-0.045 [Laurent]
Polyethylene
0.010-0.020 [Laurent]
0.020 [Mills]
Polystyrene
0.015 [Mills]



Sources

# [Laurent] Securite des procedes chimiques, Andre Laurent, Tec et Doc, 2003, page 237
# [Mills] Pneumatic Conveying Design Guide, David Mills, Butterworth Heinemann, 2004, page 577
# [Stahl] The basics of dust explosion protection, manual edited by Stahl, page 7
# [Rhodes] Principles of Powder Technology, Martin Rhodes et al., John Wiley and Sons, 1990, page 307




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