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Powder and Bulk Engineering's PBE News
Ask an Expert
photoways to identify your material's explosive
Q. We know our material has the potential to be explosive, so why do we need to test it?
imageRecognizing that your material is explosive is important but that's just the beginning. Your goals now should include determining your material's explosive characteristics and then performing a dust hazards analysis (DHA) as required in Chapters 5 and 7 in the NFPA 652: Standard on the Fundamentals of Combustible Dust.
Chapter 5 in NFPA 652 provides the methods for identifying the explosivity characteristics of your material. If that material is very common, such as cornstarch, most wood dusts, or other qualifying material, you might be able to use historical data that the appendix in NFPA 652 (and other sources) contains, but your dust may have characteristics that increase hazards not covered by multiple tests of similar materials. For example, characteristics of fine wood dust can make even low-energy electrostatic discharges sufficient for ignition. This isn't information that most historical data commonly provides.
You must test your material for at least four major characteristics: the maximum rate of pressure rise for dust clouds (KSt), maximum explosion pressure (Pmax), minimum explosive concentration (MEC), and minimum ignition energy (MIE). While the testing expense can be up to 5,000USD, the information you gain may save your facility from an even more costly fire, flash fire, and/or explosion event.
photoIdentifying a material's maximum rate of pressure rise for dust clouds (KSt)
KSt is one of the most well-known but often misunderstood material-explosivity characteristics. Too often, a low KSt value (below 100) is construed by those unfamiliar with the term to mean a material is less explosive than one with a KSt value of 300. However, the numerical value of KSt doesn't represent the likelihood of an explosion event. Instead, the term KSt more correctly represents the explosion event's severity should an explosion occur. The main significance of this number for a company investigating its potentially explosive material is in the KSt value's use in determining the level and type of explosion protection devices, systems, or both required for explosion mitigation and control.
photoIdentifying a material's maximum explosion pressure (Pmax)
Pmax is the maximum amount of pressure developed during an explosion event. The pressure is measured in bars (14.5 psig per bar). The pressure rise development in an explosion occurs virtually instantaneously an is measured in milliseconds. Combined with the information you've gathered on your material's KSt value, the Pmax value allows you to calculate the explosion vent area required and may indicate the need for additional or different types of explosion mitigation, such as chemical suppression.
photoIdentifying a material's minimum explosive concentration (MEC)
MEC is measured in grams per cubic meter (g/cm3). This value represents the minimum concentration of a combustible dust (material) suspended in air (for example a dust cloud created from aerated material) that will be sufficient fuel for a flash fire or explosion event. This value is critical for the DHA as it determines whether an area or enclosure can produce sufficient aerated material to represent an explosion hazard.
photoIdentifying a material's minimum ignition energy (MIE)
MIE is measured in millijoules (mJ) and represents the minimum energy required to ignite a dust cloud of a given material. In combination with MEC value, the MIE value indicates the risk level that the material represents. High MEC and MIE values normally indicate a significantly reduced risk of an explosive event, while low MEC and MIE values represent a higher risk. In addition, an MIE below 30 mJ represents a dust where electrostatic discharges can ignite the material.
If your material testing reveals a low MIE value (below 30 mJ), I recommend testing both with and without inductance. The addition of inductance for spark production tends to represent electrically based sparks as opposed to testing without inductance, which tends to represent electrostatic discharges only. The result of an MIE test with inductance tends to produce lower MIE values than without.
Additionally, if your material has an MIE value below 30 mJ and has an unknown volumetric resistivity, then I recommend an additional test for the material's level of volumetric resistivity. A test result that show's the material has high volumetric resistivity indicates that the material will retain electrical charges (has capacitance), which can represent a significant risk for electrostatic discharge.
Without knowing the measurements for these four characteristics, it's impossible to analyze the flash fire and explosion hazards or risks associated with your material.
Jack Osborn is the engineering manager at Airdusco.
July 10, 2019
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