Dust Explosions – Non-electrical Sources of Ignition
20 November 2011
One of the first recorded dust explosions was in a flour warehouse in Turin in 1795 that injured two boys but did not kill anyone. As the scale of human industrial processes has grown so sadly has the size of the dust explosions.
Figure 1 - Explosion of the Washburn A Mill, Minneapolis Photographer: William H. Jacoby (1841-1906)
In 1878 the Washburn ‘A’ Mill , then the largest flour mill in the United States, exploded and claimed 18 lives and destroyed a large amount of the surrounding area. (See Figure 1 for a remarkable stereograph that was taken at the time.) This explosion led to reforms in the milling industry to reduce the dust in air during milling and to improve housekeeping.
Since these explosions occurred before the deliberate use of electricity they illustrate the problem of non-electrical sources of ignition.
The traditional approach to Ex rated equipment has been to concentrate on electrical equipment but a review of dust explosions in Germany by BIA (reference 1) shows this to be a mistake. As the chart below shows, the percentage of ignitions caused by electrical equipment is only 3.2% and even static electricity only accounts for a further 8.5%. This means that 88.2% of explosions are caused by non-electrical ignition sources.
The low incidence of explosions due to electrical equipment is possibly because of the use of the correct electrical equipment and the awareness of this source of ignition. On the other hand, the percentage of ignitions caused by mechanical sparks/friction is 32.7% illustrating the importance of including non-electrical equipment as a source of ignition. Static electricity is also a problem but perhaps not as much as would have been expected.
All of the ignition sources shown in Figure 2 are likely to occur in manufacturing operations. Some of them will occur as a result of normal processing and others during maintenance or abnormal operation.
Equipment Involved in Dust Explosions
The BIA report also included the type of equipment that was involved in the reviewed explosions. These data are shown in Figure 3 below and again it is clear that most manufacturing processes that handle powders and dusts will use some of the items shown in this Figure. The means that any hazards study looking at a powder/dust handling process will almost certainly have to take into account non-electrical equipment.
In November 2006 the US Chemical Safety and Hazard Investigation Board published an investigation report No. 2006-H-1 “Combustible Dust Hazard Study” (Reference 2). This indicates the number of dust explosion incidents in the US rose from 7 per annum in 1980 to more than 25 per annum in 2004, showing that much work remains to be done to prevent dust explosions.
The US report includes a number of important conclusions three of which relate directly to non-electrical equipment.
1. Facility management failed to conform to NFPA standards that would have prevented or reduced the effects of the explosions.
2. Dust collectors were inadequately designed or maintained to minimize explosions.
3. Process changes were made without adequately reviewing them for the introduction of new potential hazards.
HSE data from the UK presented by Lunn (Reference 3) for the years 1979 to 1988 shows that there were 136 explosions indicating an average of 15 explosions per year which is comparable to the US data. Zeeuwen (Reference 4) writing in 1997 suggested that the data available then did not indicate a trend but did suggest that the increased use of dust extraction as a result of the need to protect workers from dust was at that time probably causing a rise in explosions whilst increasing awareness of dust explosions was tending to push in the opposite direction.
In Europe the ATEX Directive 94/9/EC requires that non-electrical equipment must be certified and this has resulted in a number of European standards being written to cover non-electrical equipment.
EN 13463-1: 2009 Non-electrical equipment for use in potentially explosive atmospheres. Basic methods and requirements
EN 13463-2: 2004 Non-electrical equipment for use in potentially explosive atmospheres. Protection by flow restriction enclosure “fr”
EN 13463-3: 2005 Non-electrical equipment for use in potentially explosive atmospheres. Protection by flameproof enclosures “d”
EN 13463-5: 2003 Non-electrical equipment for use in potentially explosive atmospheres. Protection by constructional safety “c”
EN 13463-6: 2005 Non-electrical equipment for use in potentially explosive atmospheres.
Protection by control of ignition source “b”
Whilst there may be no direct legal requirement to consider non-electrical in many countries outside the European Economic Area, the data presented above show how important non-electrical equipment is as a source of ignition. This means that non-electrical equipment should be considered a part of risk assessment for fires and explosions whether it is a direct legal requirement or not. The European standards above, particular EN 13463-1:2009 form useful guidance on how to reduce the risk of non-electrical equipment acting as an ignition source.
1. BIA Report 11/97 Dokumentation Staubexplosionen – Analyse und Einzelfalldarstellung.
2. Report 2006-H-1, November 2006, US Chemical Safety and Hazard Investigation Board
3. Lung G., Dust Explosion Prevention and Protection, Part 1 – Venting, 2nd Edition, IChemE
4. Zeeuwen P., Dust Explosions: What is the risk? What are the statistics? Euroforum conference, Paris – March 1997