“We have first raised a dust and then complain we cannot see.” — George Berkeley
You don’t need to be convinced that combustible dusts are hazardous. You’ve seen the photographs of demolished grain elevators, of the Imperial Sugar plant in Port Wentworth, Georgia looking like the Dresden firebombing in World War II. You’ve heard about the explosion pentagon, that adds confinement and dispersion to the three sides of the fire triangle (fuel, oxidizer, and ignition source). You know that NFPA standards call for Dust Hazard Analyses (DHAs) and you agree that anyone handling, processing, or producing combustible dusts should do one.
If you’ve seen, heard, and know all this, then you are aware that NFPA 652, Standard on the Fundamentals of Combustible Dust, states that “the owner operator of a facility with potentially combustible dusts shall be responsible for…characterizing their properties as required to support the DHA”. (§ 5.1)
What properties are those? And what do you do with them in your DHA?
The Properties of Combustible Dusts
There are several properties associated with combustible dust, the most important being Kst, Pmax, MEC, and MIE.
Kst is a dust deflagration index that allows a comparison of dusts and gives an indication of how powerful a dust cloud explosion would be if an optimal mixture of dust and oxidizer were ignited. It is typically tested using Standard Test Method for Explosivity of Dust Clouds, ASTM E1226 and reported in bar-m/s. The larger the value, the more powerful the explosion can be. The “st” in Kst is for “staub”, which is German for dust.
Pmax is the maximum pressure that a dust deflagration will produce, assuming that the initial pressure is atmospheric pressure. It is typically reported in bar and is measured in the same apparatus used to measure Kst. Pmax and Kst are related, and to a certain extent, correlated. One such correlation,
Kst = 2.13Pmax2 – 5.96Pmax – 4.75
was reported recently at the International Seminar on Fire and Explosion Hazards in St. Petersburg.
MEC, minimum explosive concentration, is analogous to the lower explosive limit (LEL) of flammable vapors. It is the concentration below which a dust cloud is too lean to explode, typically tested using Standard Test Method for Minimum Explosible Concentration of Combustible Dusts, ASTM E1515 and reported in oz/ft3 or g/m3, which are not equivalent. The smaller the value, the more prone to explosion the dust is.
MIE, minimum ignition energy, is the amount of energy required to ignite a combustible dust cloud, typically tested using Standard Test Method for Minimum Ignition Energy of a Dust Cloud in Air ASTM E2019 and reported in mJ. The smaller the value, the more prone to explosion the dust is.
What Use Are These Properties?
Kst and Pmax tell you how strong an explosion will be when it happens. Because of that, anyone designing the explosion relief for a system containing combustible dust will want to know the Kst. Actually, they will want to know what range of Kst the material falls in, because specific values of Kst are not especially reliable. There are four Dust Explosion Classes:
- St 0 Kst = 0 bar-m/s No explosion Table salt, silica
- St 1 0 < Kst ≤ 200 bar-m/s Weak explosion Charcoal, sulfur, sugar
- St 2 200 < Kst ≤ 300 bar-m/s Strong explosion Cellulose, polyacrylates
- St 3 300 bar-m/s < Kst Very strong explosion Aluminum, magnesium
OSHA makes a point of noting that “any combustible dust with a Kst value greater than zero can be subject to dust deflagration.”
MEC and MIE, don’t address the strength of a dust cloud explosion. Instead, they address the likelihood that a dust cloud will explode. MEC speaks to how thick the dust cloud must be to ignite. A couple of guidelines are often offered: a cloud so thick that that one cannot see their outstretched hand, or a cloud that reduces the visibility of a 200-lumen light bulb, e.g. a 25 W incandescent bulb, to 2 m (~ 6 feet).
MIE speaks to how much energy must be in the ignition source to ignite the cloud. The MIE of any particular flammable vapor is typically less than 0.5 mJ; the MIE of hydrogen is 0.011 mJ. While the MIE of a specific sample of a combustible dust can be measured quite precisely, the MIE of a particular material is quite dependent on the morphology and moisture content of the sample, which can vary considerably. The MIE of combustible dusts, on the other hand, typically fall in the range of 1 to 100 mJ. The MIE of wheat starch, for instance, is 25 ‑ 60 mJ.
What About Hazardous Area Classifications?
Hazardous Area Classification, sometimes called electrical classification, does not depend on any of these properties. Is the dust combustible? If yes, then rules for hazardous area classifications apply. Specifically, Class II. If it is a metallic dust, then it is Group E. A carbonaceous dust? Then it is Group F. Anything else—sugar, flour, cocoa, sulfur—and it is Group G. The distinction between Division 1 (normally present) and Division 2 (not normally present), doesn’t depend on any of these properties, either. The best guidance for hazardous area classification remains NFPA 499, Recommended Practice for the Classification of Combustible Dusts and of Hazardous (Classified) Locations for Electrical Installation in Chemical Process Areas.
Dependence on the Dust
Unlike vapors, the flammability of which is typically characterized with flashpoint and explosive limits (lower and upper) and solely dependent on the chemical composition of the vapor, the combustibility of dusts is not dependent on the chemical composition of the dust. It is dependent on the particle size and morphology of the dust, as well as the moisture content of the dust. As a result, testing labs make much of the fact that values reported in the literature may not be representative of the material in your facility. In fact, values for chemically identical materials can range widely. The apparent conclusion is that rather than turning to the literature, you should have your own material tested.
This recommendation is contingent on supplying a “representative sample” of your dust. The challenge, of course, is assuring that your sample is representative. The standards talk about sampling plans and the need to document your sampling, but don’t give a clue on how to assure that your sample is truly representative. Hence, it seems likely that in the face of literature values, your sample is no more likely to be representative than any of the others. If literature values are available, there is nothing in the standards that prohibit their use. Only in the absence of literature values is testing of your own samples likely to be warranted.
Do Your DHA
If you haven’t done a DHA yet, its time. Don’t let the absence of a lab report on a sample from your facility keep you. If the material can burn or otherwise oxidize when it is in bulk, then it can be a combustible dust when it is in powder form. And if the powder is fine enough, it can burn or oxidize all at once, then it can explode. Assume that it can and proceed from there.
