The dangers of dust explosions has been highlighted again after an intial investigation showed that an accident may have been caused by an explosion of ultra-light dust in the polishing workshop at a Chinese factory that manufactures Apple's iPad 2.
Many materials produce flammable dust clouds that can explode if ignited. Sugar, carbon, grain, certain metals and approximately 85% of all organic powders behave in this way. Anything that can burn and which exists in a fine powdered form is a risk.
Obvious examples of safe powders are sand and cement. Flammability data is much less commonly available for dusts compared to gases and vapours because factors such as particle size can affect the figures so much.
The main risk of ignition of dust hazards is from hot surfaces. Dusts may settle on surfaces and the build up can give rise to a concentration that could be ignited. Layers of combustible material will burn relatively slowly owing to the limited surface area exposed to the oxygen in the air but if you have the same solid in the form of a fine powder and you suspend it in air as a dust cloud the result will be quite different. In this case the surface area exposed to the air is much larger, and if ignition occurs, the whole of the cloud may burn very rapidly. This results in a rapid release of heat and gaseous products and in the case of a contained dust cloud will cause the pressure to rise to levels which most industrial plant is not designed to withstand. Although a cloud of flammable dust in air may explode violently, not all mixtures will do so.
When a dust cloud ignites in an enclosed volume it results in a very rapid rise in pressure within the container e.g. a silo or closed room. The container may not be strong enough to withstand the pressure from the explosion and it will fail in a sudden and uncontrolled manner. The plant or building will only survive if the design or other protective measures deliberately allows for the high pressures.Where an item of plant fails, or an explosion vent opens as a result of a dust explosion, a fireball and shockwave will emerge. The fireball is usually much larger than the vessel from which it came, and is likely to spread burning particles a substantial distance. Dust clouds are formed by dust falling into an area or being raised by blasts of air. Dust is heavier than air and therefore it will eventually settle however an explosion within a piece of plant could stir up these dust deposits to create a secondary explosion that is generally more destructive than the primary explosion.
We received a very urgent request last week for a switchbox that could be used in a Zone where combustible dusts are present. Unfortunately a switchbox from another manufacturer had been fitted to an actuated valve that was about to be commissioned and on closer inspection it was noticed that it was certified "II 2 G". This meant it was suitable for use in gas Zones 1 or 2 only. To be used in dust Zones 21 or 22 it needed to be certified "II 2 GD". All K Controls products are certified for gas and dust and fortunately we were able to supply a replacement within a few days.
An excellent article appeared on the HazardEx website last month that explains the hidden dangers of combustible dusts and how to guard against explosions. Please click here.