When we think of dust it is often more in terms of cleaning or inhalation in the work environment. What we don’t think so much about is the explosive risk associated with dust. 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 – 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.
As experts in the field of health and safety, 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, e.g. 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 smoldering 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, i.e.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, building structures etc. 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 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). These place 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.
- Identify the dangerous substances
- Analysis of the plant and processes
- 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
- Once a location is classified, all equipment used within it needs to be suitable for that zone. This is known as ATEX rated equipment
- Identify potential ignition sources and determine likelihood of explosion
- Eliminate any ignition sources where at all possible
- Determine the actions required to prevent injury to personnel
The original three fire factors – air, fuel, heat source (plus the two extra factors for explosions – dispersion of dust and confinement of dust) all need to be present to support fire and explosions. So, the exclusion of any of these will prevent fire or explosion occurring or will extinguish it. In practical terms then, there is the need to exclude the fuel, i.e. the dust or the source of heat, since air will certainly be present.
The main ignition sources that need to be eliminated where feasible are:
- Sparks created by metal striking metal or metal striking other hard materials e.g. flint
- Mechanical friction (bearing failure, belts miss tracking and rubbing on plant)
- Hot work, such as angle grinding or welding
- Static electricity
- Unsuitable/overloaded/faulty electrical equipment
If during the DSEAR assessment process it is determined that there is likely to be a combustible cloud of dust and an ignition source present – it is usually difficult to control all ignition sources, then a decision has to be made on how this risk should be controlled.
Generally, controls fall into three categories:
Explosion mitigation involves designing the plant in such a way as to allow an explosion to occur but without creating a hazard to personnel by allowing the excess pressure and flame to vent to a safe area.
Explosion containment involves designing plant and equipment to be strong enough to withstand the results of an explosion within it without failing.
Explosion suppression involves a system which detects the early signs of an explosion and extinguishes it by the injection of an inert material.
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. We advise using the services of a reputable Health & Safety Consultancy such as SML that can provide a bespoke assessment with recommendations for compliance, and identification of all the potential danger points.
We have carried out fire and explosion risk assessments for many food manufacturers and it’s worth pointing out that it is not just an adherence to DSEAR that is crucial. It is also an understanding of the potential threats and outcomes and the implementation of the correct regular procedures to minimize risk on a daily basis.
The DSEAR Regulations are aimed solely at preventing injury to personnel, they are not concerned with reducing damage to plant and equipment. Accordingly, there are minimum standards of operation and design which must be implemented, so far as is reasonably practicable, to prevent injury to personnel. There are however a series of extra measures which go above and beyond the minimum legal requirement and as a result, reduce the risk of damage to plant and equipment and interruptions to production. The implementation of these extra measures becomes an economic decision for the operator in question and will be of some interest to the operator’s insurance company.
Safety Management Limited (SML)
Mike Fletcher is MD of Safety Management Limited (SML). SML is one of the UK’s leading health and safety management consultants, working with clients across a wide range of sectors to ensure their safety standards go beyond compliance with even the strictest legislation, thereby safeguarding their people, their customers and vitally, the reputation of their business.
For further information, please visit: www.safety-management.co.uk