Fire and explosion risk in shot blasting

For a dust explosion to occur, five conditions must be present simultaneously. This is represented as the “dust explosion pentagon”: removing any single element prevents the explosion (though not necessarily a fire).

• Combustible dust, in particles fine enough to propagate the flame.
• Dispersion: the dust must be suspended in air as a cloud.
• Concentration within the material's explosive range.
• Ignition source with enough energy (spark, flame, hot surface, static discharge).
• Confinement in an enclosed space (equipment, ducts, building) and the presence of oxygen.

What sets it apart from a fire is speed: in a dust cloud, combustion spreads from particle to particle very fast, producing a pressure wave.

Not all dust is equally dangerous. The risk rises sharply with the finest fractions.

• Particles larger than 500 µm generally present a low combustion risk.
• Processes usually produce a mix of coarse and fine material (below 420 µm): the fines ignite easily and, in turn, ignite the coarser particles.
• Changes in the process —speed, alloy, lubricant or abrasive— can produce finer particles and raise the risk, so they call for a fresh explosivity assessment.
• Accumulated fine dust can cause secondary explosions: an initial deflagration lifts the dust deposited around the area and, when it ignites, produces follow-on explosions that are often more destructive than the first.

Prevention rests on reducing the available dust and removing ignition sources.

• Minimise the generation and dispersion of combustible dust and prevent it from accumulating on floors, ducts, hoppers and structures (focus on secondary explosions).
• Keep dust-collection systems running: they capture potentially explosive fine particles at source.
• Earth equipment, collectors and drums to dissipate static.
• Clean without raising dust clouds: by vacuum or direct collection, never with compressed air; use natural-fibre brushes and anti-spark tools.
• Discharge dust into earthed metal containers and empty them daily.
• Apply lockout/tagout (LOTO) before opening or servicing the equipment, and obtain a hot-work permit before welding or cutting.

The choice between dry and wet collector depends on the dust's explosivity, the location and costs; combustible metallic dusts usually point the decision toward a wet collector.

In typical shot blasting processes, with coarse mineral or metallic abrasives, the explosion risk is low. However, fine dust from wear, removed paint or contaminants can change the explosive behaviour, and a change of abrasive or the addition of lubricants can alter the effective Kst of the accumulated dust.

It is therefore advisable to avoid the build-up of fine dust, keep the extraction system always operational and test real samples whenever there is doubt about the dust composition.