How is combustible dust tested?
Combustible dust analysis consists of a series of tests both to determine particle size and moisture content, and to calculate how potentially “explosive” the dust is. The sampling part is actually fairly simple — dust is carefully collected using a brush and put into a glass or plastic jar. It is suggested to collect samples from the appropriate areas in the production areas and also from elevated surfaces. A sample consisting of 1 kilogram of dust must be collected and submitted to the lab for analysis. Air sampling is not necessary to determine whether or not a dust is combustible.
For combustible dust sampling and analysis, EMSL recommends following the OSHA ID-201SG sampling method guideline. The OSHA Combustible Dust Emphasis Program (CPL 03-00-008) also provides information on sample collection.
Combustible dust is typically analyzed by a suite of testing:
Initial Dust Characterization
Determines particle size and moisture content. The most important information determined in this stage are Percent Combustible Dust. This is the percentage of the sample that has the potential to be combustible when it is dry and fine enough to pass through a 40 mesh sieve (less than 420 μm in size). The testing includes:
• Percent through 40 Mesh Screen
• Percent Moisture Content
• Percent Combustible Material (calculated)
• Percent Combustible Dust (calculated)
Go – No Go Testing (Explosive Screening) – ASTM E1226
This is an economical and practical way to determine if the dust in the sample has the potential to be explosive. Testing consists of exposing the fine dust in the sample to low energy igniters inside the 20-Liter Siwek explosion chamber and determine the explosion overpressure. If the dust is not found to be an explosive threat, the analysis can be aborted to avoid unnecessary fees. If the sample turns out to be explosive on the screen testing, the more comprehend analyses listed below should be conducted.
Explosion Severity (Kst, Pmax, [dP/dt]max) – ASTM E1226
This testing provides an indication of the severity of the dust explosion by determining the deflagration parameters. The larger the value of Kst, the more severe the explosion is. For this test, the dust is suspended and ignited in the Siwek chamber and the maximum pressure and the rate of pressure rise are measured.
Minimum Explosion Concentration (MEC) – ASTM E1515
MEC is the minimum concentration for explosivity of a combustible dust cloud. It is determined by suspending the dust in the Siwek Chamber.
Minimum Ignition Energy (MIE) – ASTM E2019
MIE is the electrical energy discharged from a capacitor, just sufficient to produce the ignition of the most ignitable mixture of air and dust. It is determined by suspending the dust in a Hartmann Lucite explosion chamber.
Minimum Ignition Temperature Test (MIT) – ASTM E1491 (dust-cloud)ASTM E2021 (dust layer)
This test method covers the minimum temperature at which a dust cloud will autoignite when exposed to air and heated in a furnace at atmospheric pressure. It is determined by introducing the dust into a BAM oven. As an alternative, the minimum temperature of self-ignition of dust layer can be measured using a hot plate set-up.
Class II Testing
This level of testing involves a number of parameters that determine if the sampled dust is considered a Class II hazardous material. Class II locations are defined as locations with combustible dust having Ignition Sensitivity(I.S.) greater than or equal to 0.2 or Explosion Severity (E.S.) greater than or equal to 0.5. I.S. is calculated from MIT, MIE, and MEC for the sample normalized to Pittsburg coal dust, whereas E.S. is calculated from Pmax and [dP/dt] max for the sample, also normalized to Pittsburg coal dust.
Resistivity Testing (for metal dust in particular)
The resistivity testing is particularly important for metal dust. The electrical nature of the dust is one criteria to determine if it is necessary to take special precaution with regard to electrical insulation of the equipment operating in a location with Class II dust.
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