Dust Explosions Overview
The Legislation' (ATEX/DSEAR)
The Methodology for legislation compliance and a safer environment -
Area Classification
Ignition protection
Safety Systems Risks
Explosions
In the last 20 years industrial dust explosions have accounted for several hundred deaths and hundreds of millions of pounds of insurance money and yet they are one of the least recognized of industrial fire hazards.
Where do they occur ?
They can occur within any process where a combustible dust is produced, and can be triggered by any energy source, including
static sparks, friction and incandescent material.
Around 50 dust explosions are reported per year, ranging from small deflagrations to building destroying detonations which lead to a large numbers of fatalities.
They are often associated with the grain and mining industries.
However they can occur whenever a process uses particulate materials, either as feed stocks, intermediates or products.
There are a few basic rules to observe to see whether a dust is capable of causing a dust explosion:
The dust must be combustible.
The dust must be capable of becoming airborne.
The dust concentration must be within the explosive range.
An ignition source must be present.
The atmosphere must contain sufficient oxygen to support and sustain combustion.
What is an Explosion
Explosions are sudden releases of energy, in this context resulting from a chemical reaction, that lead to sudden and significant pressure rise.
These can be classified as
Detonations, if the flame front speed is greater than the speed of sound in the explosion medium
Detonations are much more destructive than deflagrations.
Deflagrations, when the flame front speed is less than the speed of sound
Dust explosions are unlikely to cause detonations due to the relatively slow process of combusting solid particles.
When are they likely to occur ?
A dust explosion is very similar to a gas or vapour cloud explosion, i.e. when a volume of a flammable mixture is ignited, resulting in a rapid pressure increase and fire moving through the cloud.
A dust explosion occurs when a combustible material is dispersed in air forming a flammable cloud and a flame propagates through it.
This of course also depends on the supply of oxygen to the fire, and the concentration of the fuel, if either of these are in too high or low a concentration then the explosion will not occur.
Any solid material that can burn in air will do so at a rate that increases with increased surface area.
If the area available for combustion is high enough, then a flame can propagate through the combustible mixture at high speeds, if this is rapid enough then the fire will become an explosion.
The maximum pressure in a dust explosion is typically around 5-12bar(g)
Specific Surface Area
We can easily understand that the degree of subdivision of a solid dictates the likelihood of an explosion.
A useful measurement is the specific surface area, which allows us to see exactly how much surface area there is per unit mass.
Essentially the higher the specific surface area the more danger there is of dust being involved in a dust cloud explosion.
What Materials are explosive
Generally dust explosions arise from the simple chemistry of combustion:
Fuel + Oxygen Oxides + Heat
Organic materials (grain, linen, sugar, etc)
Synthetic Materials (plastics, pigments, etc)
Coal and Peat Metals (aluminum, zinc, iron, etc)
What Materials are NOT explosive
Materials that are stable oxides cannot produce dust explosions:
Silicates
Sulphates
Nitrates
Carbonates
Phosphates
There is no risk of dust explosions in industries such as cement manufacture, sand quarrying, limestone excavation etc.
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