Explosive Situation

Sugar and flour, when mixed with air, form a potentially explosive mixture. Such conditions frequently occur in storage vessels in plant bakeries. To prevent this happening, silo manufacturers are governed by the directive ATEX 100. But bakers are also required, under the ATEX 137 directive, to be able to provide documentation showing their silos are ATEX 100 compliant.

All hazardous areas in a workplace, which were used for the first time after June 30, 2003, have had to comply with ATEX 137. But the transitional period for companies using silos before that date will run out on June 30, 2006.

How to comply Bakeries must ensure that any on-site storage vessel, where there is a risk of explosion, is built to ATEX standards and has the necessary documentation to prove it.

Of particular note is that ‘own design’ vent membranes and doors are no longer acceptable unless certified by explosion testing in conjunction with a notified body.

The following points must be considered under ATEX 137: • Explosion characteristics of the stored product • Silo design and explosion vent specifications • Vent area • Silo supply documentation

Individual powders and dusts have their own explosion characteristics. Sugar and flour, for example, behave differently both in susceptibility to explosion and pressure rise/maximum pressure if they do explode. These characteristics are fundamental to silo design calculations and explosion vent specification.

Put simply, the silo must be strong enough to contain any explosion without sidewall rupture. This is dependent upon having calibrated explosion vents with sufficient vent area to limit the internal pressure rise to a calculated maximum during any explosion.

Not only should the silo supplier design and build the silo to meet the ATEX requirements, but documentation must accompany the product which outlines the silo design calculations, drawings, explosion vent calculations and silo certification.

In the zone ATEX 137 focuses on minimising the risk to workers by classifying the working environment into zones and then specifying the category of equipment that can be used in each zone. The dust hazard classification is as follows: Zone 20: Permanent presence of combustible dust Zone 21: Incidental presence of combustible dust during normal duty Zone 22: Accidental presence of combustible dust, but not during normal duty Mark Barton is MD of Barton Fabrications, which supplies ATEX-compliant silos to the food industry.

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