This well-proven enclosure philosophy was the basis of two central CENELEC dust standards issued in 1998, and IEC seriously considered adopting these standards. However, probably encouraged by the lack of adequate distinction between gases/vapours and dusts in the European Directive 94/9/EC, they decided to depart from the sound and well proven common European/American approach.
Instead they initiated the production of the new series of dust standards 'aligned' or 'combined' with existing gas/vapour standards. Fortunately IEC has more recently issued a new standard for 'dust ignition protection by enclosures', which revitalizes the well proven basic design basic principles mentioned above.
No Clarification of Gas/Dust Differences
Directive 94/9/EC, which addresses the design of equipment/apparatus, has no doubt been produced largely from the point of view of gas/vapour explosions. Only modest attention has been paid to the special aspects of explosive dust clouds. The same applies to the dualistic appearance of dusts, as either clouds or layers.
This is evident right from the definition of 'explosive atmosphere' in Article 1, item 3: An explosive atmosphere is a "mixture with air, under atmospheric conditions, of flammable substances in the form of gases, vapours, mists or dusts, in which, after ignition has occurred, combustion spreads to the entire unburned mixture". It is clear from the context in the directive that this definition addresses explosive clouds only.
However, dust layers/deposits are also "mixtures of air and dust in which combustion can spread to the entire unburned mixture". Therefore, the definition in Directive 94/9/EC of 'explosive atmosphere' also embraces dust layers/deposits. But this fact is totally neglected in the existing standards based on the directive.
Directive 1999/92/EC addresses the general safety inside of a factory or plant. Also in this context it is essential to account for the dualistic flame propagation regime of dusts, viz. in clouds and layers. However, neither the directive nor the IEC and European area classification standards based on the directive address this fact.
The logical approach would be to include combustible dust layers/deposits that can propagate open or smouldering fires in the definition of 'hazardous areas' alongside with explosive dust clouds, irrespective of whether the layers/deposits may become dispersed into explosive dust clouds.
Another shortcoming of this directive is that the term 'source of release', which is used throughout the directive for 'explosive atmospheres' at large, does not apply to dusts.
With dusts this term is generally inappropriate and confusing and should be replaced by a more relevant term e.g. 'area of dust cloud generation'. This is because, as opposed to gas explosions, dust explosions in the process industries are practically always initiated in dust clouds generated inside process equipment, and very seldom in dust clouds that have been unintentionally 'released' to the outside.
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