Every year in the UK there are between 15 and 20 dust explosions which occur in the food, beverage and animal feed industry. Powdered food products such as flour, cake mix powder, dried milk, sugar, coffee, tea, starch, powdered potato, soya beans, maize, barley, grain and many more can explode if they form a dust cloud at certain concentrations. Dust explosions arise from solid particulates that become suspended in air, together with an adequate source of ignition. These particulates can be quite harmless as they occur naturally, but when reduced to fine powders through grinding, sanding or milling, they can become highly explosive. The finer the particles, the greater the surface area per unit of mass and the more explosive the dust is likely to be. The moisture content of the dust also affects the explosive risk. Dry dusts of very small particle size are easily ignited and can cause the most violent explosions. There are many processes used in the food industry that can produce explosible dust, these include milling, grinding, spraying, drying, conveyance and storage of finely ground food stuffs. Ignition sources include electrostatic discharges, friction, mechanical and electrical sparks, spontaneous heading and welding. Even an electrical spark that may occur when pulling a plug out of a socket can cause an explosion.
Legislation concerned with dust explosions in the UK include the 1992 Management of Health and Safety at Work Act, the 1974 Health and Safety Act, the 1961 Factories Act and DSEAR (the Dangerous Substances and Explosive Atmospheres Regulations 2002).
Safety measures from food dust cloud explosions
- An explosible dust cloud should never be allowed to occur
- To prevent combustion, oxygen in the air should be depleted or kept to a minimum for working levels
- Ignition sources should be removed where possible, electrical installations and sockets should be protected to be explosion proof. Electrical equipment which is dust protected to IP5X or IP6X should be installed. Surface temperatures should be no higher than 200 degrees celcius
- Cleaning of equipment should be done regularily so there is no thick dust layers forming that could cause an explosion risk
- There should be a permit system to control hot work areas
- Any leakage points around powder handling systems should be sealed to prevent dust escape and accumulation into surrounding plant items
- There should be a centralised pipe vacuum cleaning system
- Silos or bins should be fitted with an explosion relief system and vented to an unoccupied place of safety. Explosion relief panels need to be ATEX certified
Explosions due to dust fires can have very serious consequences on human life and buildings. When maintaining and designing premises priority should be given to venting, explosion suppression, sprinkler systems and plant design. All powdered food processing plants need to carry out assessments on the risks and hazards in the production, handling and storage methods of powdered food stuffs. Laboratory test data results are generally essential in order to mitigate against risks. These tests can establish at what concentration there may be ignition and then precautions can established to prevent this.