Many people are unaware of the combustible nature of dust and the issues that it can create.
Many organic dusts are combustible such as:
- wheat
- flour
- sugar
- wood
- coal
- some metals
- & more
The best way to determine the combustibility of a dust is to have it tested at a testing laboratory. Combustibility will depend on size, shape, material, moisture content amongst other properties.
Where dust is present when shifted and stored in silos, it will move around in a cloud where it mixes with air and is in its most volatile state. The dust usually ends up settling in layers on flat surfaces such as rafters, lighting, beams etc. These layers are not so bad if left undisturbed unless they cause overheating issues on electrical equipment.
Where an area has been identified as containing combustible dusts, it must have a Hazardous Area Classification carried out for combustible dusts to determine where the areas of risk are as far as electrical equipment which may introduce an ignition source. Electrical equipment can provide ignition in one of two ways – either by spark or temperature alone.
Once an area has been classified the only electrical equipment allowed in that area needs to be Certified to an acceptable scheme in relation to Electrical Equipment in Hazardous Areas, the electrical circuits need to be specially designed, equipment selected, installation, maintenance, testing, and inspection practices take place to ensure that the electrical equipment does not become a source of ignition.
If electrical equipment in the area has not been correctly selected and installed it can cause dust to heat and start to burn, this heat will cause the other dust in the layer to rise and form a cloud, when the dust air mixture reaches the explosive range (between the LEL and UEL) the dust cloud will explode sending an initial shock wave throughout the rest of the silo which will dislodge layered dust causing a bigger dust cloud and in turn, a bigger explosion – this cascading continues through the silos causing major damage and possibly injury or death.
By Brad Guy,
Senior Hazardous Area Specialist
