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| | Table 2: Explosion Characteristics | |
| | Characteristic | Definition/Description | Remarks | |
| | Size of particle | Dust particles larger than 400 pm are not explosive. Dust particles are explosive when they measure less than 400 pm and up to 20 pm | Depending on friction, the transportation and processing of coarse dust result in the formation of fine dust. | |
| | Explosion limits | As with gases, dust is explosive within certain concentration limits: lower explosion limit: approx. 20...60 g/m3 air upper explosion limit: approx. 2...6kg/m3 air | These characteristics vary widely throughout the entire range. Extreme dusts can form an explosive mixture in concentrations of less than 15 g/m3. | |
| | Maximum explosion pressure | In enclosed containers of simple design, combustible dust can reach an explosive pressure between 6 and 10 bar. | In exceptional cases, such as with light metal dusts, explosive pressure of up to 20 bar may develop. | |
| | KSt-value | This is a classifying value expressing the shattering power of the combustion. Nummerically, it is equal to the value for the maximum rate of explosion pressure rise during explosion of a dust/air mixture in a 1m3 vessel. | This figure is the basis for calculating pressure relief areas. | |
| | Moisture | The moisture of a dust is a significant factor with regard to its ignitions and explosive behaviour. Although no exact limits exist, it is known that a higher moisture content requires a higher ignition energy and impedes the formation of dust swirls. | | |
| | Minimum ignition energy Emin | Lowest engery which is sufficient to effect ignition of the most easily ignitable explosive dust atmosphere under speciefied test conditons ( EN 13237). | Not every spark is capable of causing ignition. The decisive factor is whether sufficient energy is introduced into the dust/air mixture to initiate a self-substaining combustion of the entire mixture. A modified Hartmann tube (Figure 5) is used to determine the minimum ignition energy. | |
| | Ignition temperature of a dust cloud Tic | The lowest temperature of a hot inner wall of a furnace at which ignition occurs in a dust cloud in air contained therein (EN 50281-2-1). | The shape of the vessel in which the ignition temperature is determined has proved to be especially critical. It may be assumed that ignition on differently shaped surfaces is, in practice, only possible at much higher temperatures. In the case of dust from food products and animal feed, this figure is between 410 and 500 degrees C, depending on type. | |
| | Ignition temperature of a dust layer Til | The lowest temperature of a hot surface on which a ignition occurs in a dust layer of specified thickness on a hot surface (EN 50281-2-1). | This temperature describes the ignition behaviour of thin dust layers. If the layer is thicker, or if the ignition source is completely inundated by dust, the thermal insulation provided by the dust layer increases, resulting in quite different, sometimes significantly lower temperatures, which could trigger an exothermal reaction. Experiments have shown that the ignition temperature decreases nearly linearly with an increase in the layer thickness. The ignition temperature of a dust layer is sometimes considerably lower than the ignition temperature of a mixture of the same dust in air. The maximum permissible surface temperature for electrical equipment may be higher, depending on the dust's thermal conductivitiy. Unnoticed glowing spots can exist unnoticed for long periods in thick layers of dust and can, if the dust is swirled up, become effective ignition sources. | |
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