BRE IP14/08 PDF

INTRODUCTION TO DUST EXPLOSIONS

A dust explosion will take place if a number of conditions are met:

• The dust is combustible.

• The dust has a particle size distribution with sufficient fines to support flame propagation (typically 200 micron or less).

• The dust is dispersed in air with a concentration above the minimum explosible concentration (MEC). This value varies widely with different materials, but is not normally less than 20 g/m³.

• An ignition source is present with sufficient energy to ignite the dust cloud. The most common ignition sources are electrical sparks or high surface temperatures.

• The dust cloud is enclosed within plant equipment or building.

When these conditions are met simultaneously, flame will propagate away from the ignition source, forming a flame front. Figure 2 shows a flame front expanding with a flame speed of around 10 m/s. The sudden heating of the air by the fireball creates a pressure wave that travels outwards at the speed of sound (330 m/s), much faster than the flame front itself. The difference in speed between the flame front and the pressure it generates allows sufficient time for the explosion to be detected and extinguished before damage occurs – the concept of explosion detection and suppression.