Characteristics of dust explosions
Description and key parameters of an explosion of a powder cloud
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| Section summary |
|---|
| 1. Pressure
profile of a dust explosion |
| 2. Maximum
explosion pressure |
| 3. Maximum rate of pressure rise |
| 4. Secondary explosion risks |
| 5. Pmax and Kst of common dusts |
| 6. Importance of the knowledge of dust explosion characteristics for risk analysis |
1. Pressure profile of a dust explosion
What happens during a dust cloud explosion ?
A dust explosion, when a dust cloud is ignited within a confined space, generates a pressure increase that develops quickly to reach a maximum pressure, then the pressure slowly releases.
The pressure profile will depend on the material generating the explosion.
It is necessary to know the characteristics of explosion of a particular material in order to protect the process, especially through pressure release systems or suppression systems.
2. Maximum explosion pressure Pmax
What is the Pmax value of a dust ?
The maximum pressure recorded during a powder explosion is called Pmax.
3. Maximum rate of pressure rise, constant Kst
What is the Kst value of a dust ?
Measuring how fast the pressure increases during a dust explosion is critical to correctly design protection measures like explosion panels or suppression system, which must act before the pressure reaches unsustainable levels.
The pressure increase rate is generally represented by a constant Kst, different for every material, and which can be measured experimentally by making powder explode in instrumented pressure resistant vessels. The maximum rate of pressure rise can be measured on the pressure vs time graph (see above) and then Kst calculated knowing the volume V of the test vessel :
(dP/dt)max = Kst.V-0.33 [Laurent]
Kst is expressed in bar.m/s
Depending on the value of Kst, powders can be classified in classes
St1 : 0 < Kst < 200
St2 : 200 < Kst < 300
St3 : 300 < Kst < 600
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4. Secondary explosion risks
It must be kept in mind that a 1st explosion may actually trigger others. Indeed, the pressure shock and burnt particles can travel through pipes to another process equipment like a silo and trigger an explosion there. It is through this mechanism that some grain silos were downed altogether.
As well, the pressure shock can put in suspension dust layers that was covering floor or equipment in a production area that was not well house kept and then trigger another explosion that could damage the building.
5. Pmax and Kst of common dusts
Please find below some Pmax and Kst 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.
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. Always consult the references given in
brackets for more precisions.
Table 1 : Pmax and Kst of common materials
| Material | Pmax bar | Kst bar.m/s |
|---|---|---|
| Adipic acid |
8 [Rhodes] | 97 [Rhodes] |
| Aluminium |
6.2 [Mills] 12.4 [Rhodes] |
1360 [Mills] 415 [Rhodes] |
| Coal |
5.9 [Mills] 9.2 [Rhodes] |
150 [Mills] 129 [Rhodes] |
| |
||
| Coffee |
3.4 [Mills] | 17 [Mills] |
| Cork |
9.6 [Rhodes] | 202 [Rhodes] |
| Cornstarch |
10.3 [Rhodes] | 202 [Rhodes] |
| Dextrin |
8.8 [Rhodes] | 106 [Rhodes] |
| Grain dust |
6.6 [Mills] | 190 [Mills] |
| Magnesium |
6.6 [Mills] 17.5 [Rhodes] |
1020 [Mills] 508 [Rhodes] |
| Nylon |
6.5 [Mills] | 270 [Mills] |
| Polyethylene |
5.4 [Mills] | 510 [Mills] |
| Polystyrene |
6.2 [Mills] | 480 [Mills] |
| Sugar |
6.1 [Mills] | 340 [Mills] |
| Sulfur |
6.8 [Rhodes] | 151 [Rhodes] |
| Wheat flour |
6.4 [Mills] | 250 [Mills] |
| Wood flour |
7.6 [Mills] 10.5 [Rhodes] |
380 [Mills] 205 [Rhodes] |
| Zinc |
3.4 [Mills] | 120 [Mills] |
6. Importance of the knowledge of dust explosion characteristics for risk analysis
In many countries and especially in US and Europe, the operator of the plant must carry out a risk analysis that will assess the risks of dust explosions and the potential consequences of such a dust explosion. In US, such Dust Hazard Analysis is governed by NFPA 652, while in Europe it is the ATEX directive which is defining how such risk analysis must be done.
The definition of the dust explosion characteristics of all materials present in the plant is a key data input for such risk assessment. They are also key to verify protections in place (venting of explosion, suppression...) and to design new ones.
Sources
# [Laurent] Securite des procedes chimiques, Andre Laurent, Tec et Doc, 2003, page 233
# [Mills] Pneumatic Conveying Design Guide, David Mills, Butterworth Heinemann, 2004, page 577
# [Rhodes] Principles of Powder Technology, Martin Rhodes et al., John Wiley and Sons, 1990, page 307