Dust to Dust
Author : J GALE
28 February 2011
Dr Julian Hought of risk management specialists, HFL Risk Services, explains the hidden dangers of flour and other dry ingredients and how we can guard against potentially fatal explosions.
Dr Julian Hought
Every year an estimated 2000 dust explosions occur in factories and refineries in Europe. There are approximately 50 reported dust explosions in the UK alone – that’s roughly one every single week. But these explosions are not solely the preserve of chemical or wood processing companies.
A staggering 24 per cent of them occur within the food industry.
In fact dry ingredients such as flour, custard powder, instant coffee, sugar, dried milk, potato powder, soup powder and cocoa powder have been responsible for 120 deaths in the past 30 years.
All dust explosions are preventable. However in order to prevent them you first need to know how they can occur. Essentially for a dust explosion to occur, five conditions must co-exist:
An oxidiser (such as air)
These five factors are known as the dust explosion pentagon. Practically all organic-based dusts can explode, given the right conditions. It’s important to be aware that it is not just a question of a build-up of dust on a surface that can have devastating results. Explosions can also occur inside vessels; during storage, handling and transportation; and during any processes such as blending, milling or spray drying.
By way of an example, the overfilling of a hopper with cornstarch during custard making triggered a dust explosion in 1981 at General Foods, Banbury. This created a dust cloud which was ignited by nearby electrical equipment, leaving nine men badly burnt. Thankfully there were no fatalities, but unfortunately the same cannot be said for the explosion which occurred in a grain storage complex at Société d’Exploitation Maritime Blayaise in August 1997. This killed 11 people in nearby offices and the ferocity of the explosion is comprehensible when we consider that significantly-sized debris from the explosion was found up to 100 metres from the silo.
You might argue that these are examples from several years ago and that things have now changed – sadly not. Less than 3 years ago 13 workers died and 40 were hospitalised following a sugar dust explosion at the Imperial Sugar Refinery in Georgia, USA – and many more explosions occur on a smaller scale every week. In the case of Imperial Sugar, it was the secondary dust explosion caused by dust layers in the workplace (often referred to as ‘fugitive dust’) that caused the major loss.
Of course there is legislation in place to prevent the occurrence of such tragedies. Under UK law, where there is the potential for dust explosions, companies must comply with the Dangerous Substances and Explosive Atmospheres Regulations 2002 (DSEAR). This requires companies to:
Find out what dangerous substances are in their workplace and what the fire and explosion risks are
Put control measures in place to either remove those risks or, where this is not possible, control them
Put controls in place to reduce the effects of an incident involving dangerous substances
Prepare plans and procedures to deal with accidents, incidents and emergencies involving dangerous substances
Identify and classify areas of the workplace where explosive atmospheres may occur and avoid ignition sources (from unprotected equipment, for example) in those areas
Where a workplace containing potentially flammable atmospheres exists, the employer must ensure that the overall explosion safety measures are confirmed (verified) as being safe. This must be done by a person or organisation competent to consider the particular risks in the workplace and the adequacy of the explosion control and other measures put in place.
To safeguard against a potential dust explosion a ‘Basis of Safety’ must be defined. This should ideally be based on preventative measures, but where this is not possible, adequate protection measures must be employed.
Effective prevention measures will prevent the fire triangle (fuel, oxygen, heat) from being formed through either the avoidance of a flammable atmosphere or the elimination of ignition sources.
Protection measures on the other hand include explosion venting, explosion suppression or containment. Where explosion protection measures are employed it is critical that suitable ‘chokes’ are in place to prevent the explosion propagating up or downstream.
In a bakery or food manufacturing environment, where dry powders such as flour and sugar are handled, potential flammable atmospheres can arise in equipment such as silos, bins, hoppers, cyclones, mills and filters. There are also numerous possible ignition sources. The most common of these include mechanical friction, electrostatic discharges, hot surfaces, smouldering nests, and mechanical sparks (e.g. during maintenance).
Since it is difficult to guarantee that ignition sources will not arise during the life of the plant, explosion prevention is not usually employed alone. Safety for the types of equipment listed above is usually based on protective measures.
We have already established that the presence of dust from dry powders combined with an ignition source has the potential to cause an explosion. But just how likely is an explosion to occur? And with what magnitude? In order to demonstrate the safety of a plant it is sometimes necessary to measure the flammable properties of the powders involved. The violence of a dust explosion can be measured by measuring the rate of pressure rise (Kst) and determining the maximum pressure that could be achieved in an unvented explosion (Pmax).
Depending on the Kst value, the dust risks can be classified as follows:
Dust explosion class KST ( Bar m s-1) Characteristics
St 0 0 No explosion
St 1 >0 <200 Weak explosion
St 2 >200 <300 Strong explosion
St 3 >300 Very strong explosion
Other characteristics that may need to be tested are:
Vertical tube apparatus – this explosibility test is used to identify which powders require an explosion risk assessment and which don’t. Under this test, dust can be classified as either combustible or non-combustible (Group A or B respectively).Obviously if dust is classed as non-combustible it cannot explode.
Minimum ignition energy – a test to assess how sensitive a dust is to ignition by determining the minimum ignition energy required to ignite the dust/air mixture.
Layer ignition temperature – This is often used to assess the ignition risk of equipment and is a test to find the minimum ignition temperature of a hot surface which will ignite a layer of dust deposited on it.
Minimum ignition temperature – again used to assess the ignition risk of equipment, this test determines the minimum ignition temperature of a dust cloud on a hot surface.
The DSEAR regulations require areas where there is the potential for dust explosions to be ‘zoned’ and equipment within these zones to be suitably protected.
Zone 20: A place in which an explosive atmosphere in the form of a cloud of combustible dust in air is present continuously, for long periods, or frequently. In general these conditions arise only inside containers, pipes, vessels etc. i.e. usually only inside plant (mills, dryers, mixers, silos, etc.)
Zone 21: A place in which an explosive atmosphere in the form of a cloud of combustible dust in air is likely to occur in normal operation occasionally. This zone can, for example, include places in the immediate vicinity of for example powder filling and emptying points.
Zone 22: A place in which an explosive atmosphere in the form of a cloud of combustible dust in air is not likely to occur in normal operation but, if it does occur, will persist for a short period only. This zone can include places in the vicinity of the plant containing dust, if dust can escape at leaks and form deposits in hazardous quantities.
Once these areas have been identified then a suitable plan and (if beneficial) elevation drawing is required. The phrase ‘ATEX compliant is often used to refer to electrical and mechanical equipment to be used in hazardous (zoned) areas. Zone 20 requires Category 1 equipment (also covers protective devices), Zone 21 Category 2, Zone 22 Category 3. The intent is to have more ignition protected equipment in the more hazardous area. Thus an overall acceptable level of risk is achieved. Old equipment predating the DSEAR Regulations can be continued to be used provided that a suitable and sufficient ignition risk assessment is carried out.
It is critical that housekeeping within the workplace is kept at a very high standard. It is often the secondary dust explosion that causes the most destruction (as we know was the case at the Imperial Sugar Refinery). This can occur when the pressure wave from a small primary explosion rouses dust layers that are then ignited by the following flame front. A dust layer of just 1/32 inch thickness over 5 percent of the floor area is sufficient to cause a very destructive dust explosion. That’s about the thickness of a paper clip. Regular visual checks should be made of the floor, but also of overhead pipes and vents etc.
Despite the fact that there are regulations in place to guard against and prevent them, dust explosions in the food industry will continue to kill people. Why? Because employers have not carried out their risk assessment and ensured appropriate measures are in place.
The law requires that explosion risk assessments must be carried out by someone ‘competent in the field of explosion protection’ through experience or professional training. There are several risk management companies with the expertise to undertake a full assessment of all credible release scenarios, determine what is required and then provide the necessary support to ensure that your workforce and your business are fully protected. If you cannot demonstrate that you have met your statutory duties under DSEAR, don’t delay – act now before it’s too late. You will not only be saving lives, but also saving your business from loss of revenue due to forced plant closures and potential compensation claims.
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