The basics of dust-explosion protection - STAHL - #23

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Figure7: Correlation between the Maximum Permissible Surface Temperature and the Thickness of Dust Layers

The self-ignition of dust deposits is a critical problem. These processes are often caused by exothermal reactions involving oxygen from the surrounding air. It could be a chemical reaction (oxidation), a physical reaction (adsorbtion) or a decomposition process (mainly in the case of organic dusts). Self-ignition is determined of the temperature of the surrounding area, of the geometric factors and of the volume of the dust. The reaction heat produced during decomposition of materials can produce carbonisation gas, which in turn can lead to an explosive gas/air mixture.

Not every case of dust ignition necessarily leads to an explosion. Dusts with low rate of pressure rise may first of all be a dust fire. Under certain conditions, an explosion may occur, often in a completely different location than the ignition. The fire can spread from the place of origin to

other areas by way of transportation facilities. During this process, the flames swirl up unburned dust, which in turn takes deposited dust with it. A sudden change in the volume of the dust cloud as it enters a larger room (e.g. a silo) can produce a hazardous explosive atmosphere. The flames cause ignition.

The ignition temperature of a dust layer is determined using the method from EN 50281-1-2. The determination of the maximum surface temperature that an electrical apparatus can achieve must be done by the manufacturer, with any possible faults taken into consideration. Temperature limitation measures should be used to counter these faults (electrical/thermal fuse). The maximum surface temperature is measured as specified in Clause10 of EN 50281-1-1.

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