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FIRE & ELECTRICAL SAFETY COMBUSTIBLE DUSTS


Mike Fletcher of Safety Management Limited (SML), comments on the prevention of fires and explosions due to organic dusts in food manufacturing plants.


When we think of dust, it is often more in terms of cleaning or inhalation in the work environment, rather than the explosive risk associated with it. In the UK explosions due to organic dust are fortunately rare in the food manufacturing industry, but nonetheless they are a threat with the capacity to cause substantial damage and fatalities.


Products like flour, custard powder, instant coffee, sugar, cocoa, dried milk and chilli powder if released into the air as a dust, have the potential to cause an explosion when exposed to an ignition source.


In 1981, a dust explosion occurred at the General Foods factory in Banbury, when a custard powder dust cloud was ignited by electrical equipment. Nine people were injured and there was substantial structural damage.


All plant-based material has the potential to produce combustible dusts. For a fire to occur three conditions, known as the Fire Triangle are required: air or oxygen, a source of ignition and a suitable fuel.


For an explosion to occur two further conditions are needed, these are the dispersion of the dust into a dense cloud and the confinement of this dust cloud in a vessel or building. This is known as the Explosion Pentagon.


At SML, we have recently produced a significant piece of guidance for the Agricultural Industries Confederation (AIC) on the control of risks in the manufacture of animal feed. These general principals are also relevant to food manufacturing.


When the fuel is in the form of large particles, for example, whole grain, or a heap of dust, the rate of combustion will be slow and will be limited by the rate at which air is supplied to the fire. The result is smouldering or low-intensity fire. The hot gases, smoke, that are released, disperse gently.


If fine dust is raised into the air to form a dense dust cloud, the flame can spread through this cloud extremely quickly giving rise to the rapid production of hot gases and a rise in pressure (an explosion).


The explosivity of the dust will depend on the concentration of the dust in the air, the size of the dust particles and the moisture content.


The initial dust explosion, often referred to as the primary dust explosion, is often quite small, but this initial explosion can generate a series of larger and very violent secondary explosions. Dust lying on the floor, on top of pieces of plant or building structures can all provide sources of dust for a secondary explosion. Just 1kg of dust sufficiently well dispersed can give rise to an explosive cloud of up to 20m3


and a dust layer of 30 “ 4.


“THE FACT THAT A POTENTIAL EXPLOSIVE ATMOSPHERE IS DETERMINED DOES NOT AFFECT ATEX CERTIFICATION BUT DOES REQUIRE THE MANUFACTURER TO PUT IN PLACE EXPLOSION PROTECTION MEASURES.”


only 2mm thickness over an extensive area is sufficient to give rise to a risk of secondary explosions in most circumstances.


The only solution to the risk of secondary dust explosions is to ensure that the plant is kept sufficiently clean on all levels.


Under UK law, where there is potential for dust-related explosions, companies must adhere to the Dangerous Substances and Explosive Atmospheres Regulations 2002 (DSEAR). This places duties on employers to protect people from risk from fires, explosions and similar events in the workplace.


For food manufacturing companies, this should be reasonably straightforward and if broken down into stages and followed in a logical manner, will ensure that the plant is compliant and safe to operate.


1. Identify the dangerous substances. 2. Analyse the plant and processes.


3. Determine the combustible atmosphere of each stage of the process under multiple conditions.


If it is considered that a combustible atmosphere would or could exist, the interior part of that piece of plant is classified as containing a potentially combustible atmosphere.


5. Once a location is classified, all equipment used within it needs to be suitable for that zone. This is known as ATEX rated equipment.


6. Identify potential ignition sources and determine likelihood of explosion.


7. Eliminate any ignition sources where at all possible.


8. Determine the actions required to prevent injury to personnel.


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