Self Ignition Temperature of dust layers
Physical meaning and use in risk assessment of powder minimum ignition temperature of dust layers SIT
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Solids Handling Safety
# Dust explosion physical properties # MIT - Minimum Ignition Temperature of dust
1. Definition of SIT
When a powder deposit covers a source of heat, it can lead to self ignition of the dust layer which starts to smoulder. The minimum temperature required to self ignite a dust layer is called self ignition temperature SIT or smouldering temperature. As this temperature is varying with the layer height, it is necessary to precise it : 5 mm is a quite general value.
2. Experimental determination of SIT
The SIT is measured experimentally. The experiment consists in covering a hot plate in a oven with a dust layer, then heating up the plate observing if the dust starts to smoulder (observation of glowing, flame, or strong increase in powder temperature) [Polka]. It must be noted that the SIT is dependent on the dust layer height. Increasing the dust layer height can decrease the SIT, it is thus very important to always precise in which conditions the self ignition temperature has been measured. A 5 mm height is common and represents quite well what could happen in dusty environment, not well cleaned, when powder is depositing over time.
The SIT is the smallest temperature of the oven observed that ignites the tested dust layer.
To be representative, such experiences must be carried out following a clear protocol (example : ASTM E2021 in US, EN 50281-2-1 in Europe). The test machine mentioned in the norm is also important as values may be slightly different from one apparatus to another.
Those protocols should not be confused with the minimum ignition temperature of dust clouds which corresponds to other conditions and physical meanings.
3. SIT typical range
SIT values are generally in between 250 and 450 degrees celsius, of course dependent of the nature of the material, and typically lower than MIT.
Each processor must carry out a dust explosion risk analysis in order to assess the risk related to a specific material in a specific process and take necessary precaution and mitigation measures.
4. Use in risk assessment
|Material||SIT °c 5 mm layers|
||> 400 [Polka]|
|Lemon balm||290 [Polka]|
||> 400 [Polka]|
|Rice flakes||> 400 [Polka]|
|Semolina||> 400 [Polka]|
|Senna fruit||300 [Polka]|
> 400 for corn starch [Polka]
|Sunflower hulls||290 [Polka]|
|Wheat Flour||420 [Polka]|
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