The locations of primary dust explosions are practically exclusively inside process equipment where explosive dust clouds can be created as an inherent consequence of the dynamic nature of the process itself. Examples of such processes equipment include mills, powder mixers and dryers, dust collectors and connecting ducts, pneumatic powder transportation pipes, bucket elevators, and silos during filling.
Fig. 5 shows a full scale dust explosion experiment in Norway for studying the propagation of dust explosions in inside a typical integrated process system where explosive dust clouds are generated as part of the process.
Because dust particles are so much bigger than gas molecules they are not likely to travel through narrow holes and slits of the order of 1 mm diameter and smaller in the same way as gas molecules will do. Furthermore, dust particles entering an enclosure in this way will not remain suspended in the air and eventually make up an explosive cloud inside the enclosure, as gas molecules would do, but settle out as layers on the internal surfaces of the enclosure.
It is interesting to note that NFPA 499  contains the following paragraph: "Walls are much more important in separating hazardous and non-hazardous zones in the case of combustible dusts than in the case of combustible gases. Only completely non-perforated solid walls make satisfactory barriers in the case of gases, whereas closed doors, light-weight partitions, and even partial partitions could make satisfactory barriers between hazardous and non-hazardous zones in the case of dusts."
For the purpose of the context of electrical apparatuses, the paragraph may be slightly re-phrased by replacing the first word 'walls' by 'enclosures'.
Well-proven Design Principles
Generally, irrespective of any fire and explosion hazards, the presence of dusts, whether combustible or non-combustible, is for a number of reasons incompatible with delicate mechanisms and components of electrical apparatus. Therefore IEC has produced a standard defining various 'degrees of protection' against ingress of solid objects, including ingress of dust particles.
Before the appearance of the gas/dust 'alignment'/ 'combination' standards, IEC as well as European standardization based safe design of electrical apparatus for use in areas with combustible dusts, on the two basic principles:
- Isolation of potential ignition sources by means of dust tight or dust protected enclosures as defined in the IEC standard mentioned above.
- Exclusion of the possibility that the dust of concern, whether as layer or cloud, could be ignited by the external enclosure surface (excessive surface temperatures, electrostatic discharges, or mechanically generated metal-particle sparks).
This article is protected by copyright. You want to use it for your own purpose? Contact us via: support.vogel.de/ (ID: 28876580 / Ex Protection & Safety)