COMBUSTIBLE DUSTS
COMBUSTIBLE DUSTS
Combustible dusts are not as well understood as a hazard as are, say, flammable gases and vapours.
Ignitions of combustible dusts can cause incredible destruction, usually due to the primary explosion raising the layers of dust, forming huge dust clouds which are ignited by the fires of the first explosion.
Dusts which have caused fires and explosions in the past are:
- MDF sandings (from final product sanding/parting saw area particularly)
- Foodstuff powders (sugar, flour, coffee, milk powder for example)
- Metal filings (aluminium, magnesium)
- Fibres (wool, cotton, paper)
- Trimmings (plastic from packaging plants)
- Industrial (coal, sulphur, carbon dust, shale dust)
- Medications (paracetamol for example)
DUSTS AIN'T GASES
For a gas or vapour to ignite, three things are required:
- Fuel (in the correct ratio with the oxidizer to react)
- Oxidizer
- Ignition Source
Combustible dusts also need following:
- Dispersion
- Confinement
Dispersion of dusts simply ensures maximum contact of all dust particles with oxygen in the air.
Clearly, dust dispersion (or indeed the presence of dust) is to be avoided. Sacrificial panels should also be employed in rooves and walls to ensure pressures do no buildup to a point where the integrity of the structure is compromised.
Confinement of dusts ensures that, if there is an ignition of the dust cloud, large pressures will build up until the structure confining the explosion fails (like the walls and roof of the plant). Clearly, this too is undesirable.
COMBUSTIBLE DUST WEB RESOURCES
The following sites provide some general information regarding Combustible Dust Hazards and their management. The appropriate standards from SAI Global should be referred to when determining local compliance requirements.
SECONDARY DUST EXPLOSION MECHANISM
Combustible Dust explosions can have small beginnings.
However, even a small explosion can cause dust layers to become airborne, creating large clouds of combustible dust which may only need an ignition source (probably from the fire cause by the first explosion) to create secondary explosions.
These secondary explosions can result in a cascade effect, causing the destruction of entire factories with multiple injuries and deaths.
HOUSEKEEPING - when cleanliness is truly next to Godliness. Poor housekeeping (and poor maintenance) will lead to the generation and buildup of large quantities of combustible dust, just waiting for a disturbance and an ignition source.
The right way - methods using a vacuum extraction system with antistatic properties are advised (including portable systems), with water under pressure as a last resort.
The wrong way - blowing the dust down with Compressed Air which simply creates dust clouds, probably worse than any initial explosion ever could. This is a near suicidal method and cannot be discouraged enough.
Strategy - during any maintenance shutdown, de-energise the plant in that area, have if declared SAFE by a hazardous area expert (with insurance) and clean all level surfaces, starting from the structural members in the roof and working down. Cyclic housekeeping will minimise problems.
