Characteristics of dust explosions

Description and key parameters of an explosion of a powder cloud

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Section summary
Pressure profile of a dust explosion
Maximum explosion pressure
Maximum rate of pressure rise
Secondary explosions risks

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

1. Pressure profile of a dust explosion

A dust explosion, when a dust cloud is ignited within a confined space, generate a pressure increase that develops quickly to reach a maximum pressure, then the pressure slowly releases.

Pressure increase and Pmax dust cloud explosion

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

The maximum pressure recorded during a powder explosion is called Pmax.

3. Maximum rate of pressure rise, constant Kst

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 explosed 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

4. Secondary explosion risks

It must be kept in mind that a 1st explosion may actually triggers other. Indeed, the pressure shock and burnt particles can travel through pipes to another process equipement like a silo and trigger an explosion there. It is through this mechanisms 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]



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




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