In brief
Combustible dusts such as flour, sugar, starch, wood and aluminium explode when they form a cloud in the air and meet an ignition source in an enclosed volume. The most severe damage often comes from the secondary explosion caused by settled dust lifted by the first blast. That is why dust collection filters and cyclones are protected with explosion venting and isolation devices, and settled dust is cleaned regularly with a suitable vacuum rather than compressed air.
How does a dust explosion happen?
A sack of flour, a pile of shavings or aluminium grinding dust looks harmless lying on the floor. When the same material mixes with the air as fine particles and forms a cloud, however, combustion starts simultaneously over a very large surface and a pressure wave is created. That is a dust explosion: the sudden combustion of a solid substance through its fine particles suspended in the air.
A dust explosion requires five conditions to come together. In practice these conditions are known as the "explosion pentagon":
- Combustible dust: A material that is fine enough and able to burn.
- Oxygen: The oxygen in the air is sufficient in most cases.
- Ignition source: A spark, hot surface, flame or electrostatic discharge.
- Dispersion: The dust becoming a cloud in the air.
- Enclosed volume: An environment where pressure can build up, such as a filter housing, cyclone, silo, duct or room.
The first three form the fire triangle; dispersion and confinement turn a fire into an explosion. Control measures are based on removing one or more of these five conditions.
Which businesses have combustible dust?
Combustible dust is not unique to large chemical plants. Common examples in the industrial structure of Bursa and the surrounding provinces:
| Sector | Dust | Where the risk is concentrated |
|---|---|---|
| Food (flour, feed, confectionery, milk powder) | Flour, starch, sugar, milk powder, spices | Mill, sieve, silo, bucket elevator, pneumatic conveying, bag filter |
| Furniture and woodworking | Wood dust, MDF and chipboard dust | Dust extraction cyclone, bag filter, sanding machines, shavings silo |
| Metalworking | Aluminium, magnesium, titanium dust | Grinding, polishing, sandblasting, dust collector |
| Plastics and chemicals | Polymer powder, powder coating, pigment | Electrostatic powder coating booth, mixer, dryer |
| Textiles | Cotton and synthetic fibre dust | Lint collection, filter rooms |
How fine the dust is, how moist it is and its concentration in the air affect the severity of the explosion. Whether a dust is explosive, and how violently it will explode, is determined from values established by laboratory tests. Where these tests are needed, we plan them with authorised laboratories and record the results in the risk assessment.
Dust cloud explosions and secondary explosions
What makes dust explosions particularly destructive is the secondary explosion. The process usually goes like this: a small primary explosion occurs inside a filter, silo or duct. The pressure wave and vibration lift dust that has accumulated for months on top of beams, cable trays, light fittings and machinery. This new and much larger dust cloud is ignited by the flames of the primary explosion and can affect the entire building.
For this reason, the Regulation on the Protection of Employees from the Hazards of Explosive Atmospheres makes an important note when giving the zone classification for dusts: places where layers, deposits or heaps of combustible dust are present must be considered as a source capable of forming an explosive atmosphere (Annex 1). Dust settled on the floor and at height is part of the classification.
Cyclone and filter explosions: means of protection
The dust collection system is where the dust in a business is gathered; that is why cyclone and filter explosions are one of the most common scenarios. Inside bag filters and cyclones a dust cloud is constantly formed by cleaning pulses; a spark or hot particle carried along the duct is enough for ignition. Protection is considered in three layers:
- Explosion venting: Vent panels fitted to the filter or silo housing open when the explosion pressure rises, discharging the pressure and flame in a safe direction. The vent should face an outdoor area where no people are present. Flameless venting devices can be used for equipment inside buildings. The vent area and panel selection are determined by engineering calculation according to the relevant standard (TS EN 14491).
- Isolation: To prevent the explosion travelling back through the duct to the production area or to other equipment, non-return flaps, rotary valves or similar isolation devices are used. Art. 5/2 of the Explosive Atmospheres Regulation requires measures to be combined, where necessary, with measures that prevent the propagation of an explosion.
- Suppression: Some systems use suppression devices that detect the onset of an explosion and discharge an extinguishing agent within milliseconds.
On the ignition prevention side, the basic measures are spark detection and extinguishing at the duct inlet, magnetic separators that stop metal parts entering the mill, earthing and equipotential bonding of all conductive parts, selecting equipment of a category suitable for the explosive atmosphere, and making hot work around the filter subject to a permit to work. For the permit system for hot work, see our article on the hot work permit.
Cleaning dust accumulation: why not compressed air?
Blowing settled dust with compressed air means creating a dust cloud with your own hands. Compressed air lifts the dust and provides exactly the "dispersion" condition required for an explosion; if there is an ignition source nearby it leads to an explosion, and if not, the dust settles on higher and inaccessible surfaces. For the same reason, dry sweeping with a stiff brush also puts dust into the air.
Dust accumulation should be cleaned as follows:
- Clean with a vacuum. Use an industrial vacuum designed for combustible dust and, where necessary, of a category suitable for explosive atmospheres. A domestic vacuum cleaner can be an ignition source.
- Do not forget high surfaces. The tops of beams, cable trays, ventilation ducts, light fittings and machinery should appear as separate lines in the cleaning plan.
- Set the frequency according to accumulation. The cleaning interval is determined not by the calendar but by how long it takes for a visible layer of dust to form. If a layer is forming, the interval is shortened.
- Capture it at source. Seals, duct joints and sieve covers that leak dust are repaired; extraction hoods are positioned to capture dust at source.
- Record the cleaning. Who cleaned which area, and when? The record shows where accumulation is accelerating. You can enter the review dates of the cleaning plan and the Explosion Protection Document in the OHS calendar tool.
In narrow places where compressed air is unavoidable and a vacuum cannot reach, the surrounding ignition sources are shut off first, low pressure is used and the work is carried out under a permit to work.
The health effects of dust and exposure limits are a separate subject; for these, see our article on the Regulation on Dust Control.
Training and emergency preparedness
Annex 2 of the Explosive Atmospheres Regulation requires employees in places where explosive atmospheres may occur to be given adequate and appropriate training on explosion protection, and written instructions and permits to work to be applied where deemed necessary. What operators need to know: not to stand in the venting direction of the filter and cyclone, not to use spark-producing tools when cleaning filters, to report dust leaks, and not to tackle a small dust fire in a way that lifts dust into the air.
We covered the fire side of wood dust in the furniture sector in our article on fire in İnegöl furniture workshops and on our İnegöl region page, and dust extraction on panel cutting lines in our article on panel sizing and CNC routers. Bursa Sistemer incorporates dust explosion risk into the risk assessment work for food, furniture and metal businesses in Bursa and the surrounding provinces, and teaches the correct response to dust fires in a hands-on way as part of fire training. For other risks specific to the food sector, see our food page.
Frequently Asked Questions
Does flour really explode?
Yes. Food dusts such as flour, sugar and starch can explode when they form a cloud in the air and meet an ignition source in an enclosed volume. That is why mills, silos and filters are assessed for explosion risk.
Why are cyclones and bag filters protected with explosion venting?
A dust cloud forms inside this equipment during normal operation, and a spark coming from the duct can be enough to ignite it. The vent panel discharges the explosion pressure in a safe direction, limiting the rupture of the housing and the spread of damage.
Can settled dust be cleaned with compressed air?
As a rule, no. Compressed air lifts the dust into the air and creates an explosive cloud. A suitable industrial vacuum is used for combustible dusts; compressed air is used only where unavoidable, with ignition sources shut off and under a permit to work.
Which document is prepared for dust explosion risk?
In workplaces where explosive atmospheres may occur, an Explosion Protection Document is prepared before work begins. The document includes the classification of dust zones (20, 21, 22), the measures taken and the suitability of equipment, and is updated when significant changes are made.
A risk assessment that identifies and scores hazards on site, links each measure to an owner and a date, and has a clear renewal schedule.
Bursa SİSTEMER OSGB Technical Team — written from the field experience of our OHS specialists and workplace physicians. Legal information is valid as of the publication/update date; contact us for an assessment specific to your business.