A large amount of asset damage and several unplanned process shutdowns can be avoided through control of dust. Most of the organic fine dusts ignite at a temperature below 500OC. Common high-risk zones include filters, silos and cyclones.
Recently an initial post-accident investigation in China has again highlighted the danger of dust explosion. The accident might have been caused by an explosion of ultra-light dust. Every year many such accidents go unreported worldwide, and there is hardly any authentic statistical report available, especially in developing countries.
Many materials produce flammable dust clouds that can explode if ignited. Sugar, carbon, grain, certain metals and approximately 85 per cent of all organic powders behave in this way. Broadly speaking 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, as factors – such as particle size – can affect the figures so much.
Sources of Risk for Dust Explosions
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 materials 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 – this in turn will cause the pressure to rise to levels that most industrial plants are not designed to withstand. Although a cloud of flammable dust in air may explode violently, not all mixtures will do so.
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