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|1. Definition of
|3. MIT typical values
|4. Use in risk assessment
|5. MIT of common dusts
If a dust cloud enters in contact with a hot surface, it may auto-ignite and explode. The Minimum Ignition Temperature is the minimum temperature for which a hot surface will ignite a dust cloud [Laurent].
The MIT is measured experimentally. The experiment consists in putting in suspension dust of the material to be tested in a cloud that has a concentration in between the minimum explosion concentration and maximum explosion concentration (determining this concentration may need additional experiments) and submitting the dust cloud to a hot surface in an oven.
The MIT is the smallest temperature of the oven observed that triggers an explosion at the concentration tested.
To be representative, such experiences must be carried out following a clear protocol (example : ASTM E1491 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. Norms and literature are mentioning the Godbert Greenwald furnace and the BAM furnaces as apparatuses to perform the tests.
Those protocols should not be confused with the minimum ignition temperature of dust layers which corresponds to other conditions and physical meanings.
MIT values are generally in between 150 and 700 degrees celsius, of course dependent of the nature of the material.
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.
The MIT must be compared to the maximum temperatures that can be observed in a process. This can be within the process or even outside (motors for example). The process plant operators must make sure that dust is not entering in contact with hot spots. At design stage, the MIT of the dusts involved in the process can be used to specify equipment and make sure by design that temperature cannot reach the MIT of any component of the process.
Maximum admissible temperatures that can be put in contact with a powder cloud is given as 2/3 of TMI.
Tmax = 2/3*TMI [Stahl]
Please find below some MIT data given in the litterature. WARNING : these are general values given without guarantee, a risk assessment and design must ALWAYS refer to the MSDS of the ACTUAL product used for tests carried out specifically on the ACTUAL material by a reputable institute.
Table 1 : Minimum Ignition Energy MIT of common materials
|Grain dust||430 [Mills]|
|Lemon balm||480 [Polka]|
|Rice flakes||430 [Polka]|
|Senna fruit||520 [Polka]|
460 for corn starch [Polka]
|Sunflower hulls||460 [Polka]|
|Wheat Flour||380 [Mills]|