Class 2 - "Finely Divided" dust clarification

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quickquestion

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NFPA 499 - 3.3.3* Combustible Dust. Any finely divided solid material
that is 420 microns or smaller in diameter (material passing a
U.S. No. 40 Standard Sieve) and presents a fire or explosion
hazard when dispersed and ignited in air.


Does this mean?:
a) 99%+ of my material must be less than 420 microns
b) 1%+ of my material must be less than 420 microns
c) the weighted average of the materials must be less than 420 microns
d) something else

Do you have a source for your interpretation?
 
quickquestion said:
NFPA 499 - 3.3.3* Combustible Dust. Any finely divided solid material
that is 420 microns or smaller in diameter (material passing a
U.S. No. 40 Standard Sieve) and presents a fire or explosion
hazard when dispersed and ignited in air.


Does this mean?:
a) 99%+ of my material must be less than 420 microns
b) 1%+ of my material must be less than 420 microns
c) the weighted average of the materials must be less than 420 microns
d) something else

Do you have a source for your interpretation?
Click to expand...
Basically, it is ANY amount that "...presents a fire or explosion hazard when dispersed and ignited in air."

The rest of the document helps you determine if an installation or process might fit the discription.
 
Thanks for the quick answer. Your answer implies 3.3.3 has essentially the same definition as "combustible particulate solid" in NFPA 68 and 499.

I think there are two requirements to the definition:
1) Any finely divided solid material that is 420 microns or smaller in diameter (material passing a U.S. No. 40 Standard Sieve)
AND
2) presents a fire or explosion hazard when dispersed and ignited in air.

If my material (for which the dust is combustible) is generally between 420 microns to 1000 microns (of course you could find some "dust" due to attrition) and is coming out in a well ventilated bagging spout example similar to the 499-Figure 5.8(i), does my material meet the first part of the definition?

If yes, then the definition is the same as "combustible particulate solid" and that any material (regardless of size) for which its dust is combustible is included in the definition of "combustible dust." If size doesn't matter, why is it mentioned in the code?

If no, what about material generally between 150 and 1000 microns?

Thanks for your help. I'm attempting to become educated enough to be sure I can ensure buy in.
 
FWIW, I spoke to two others today:

One guy says that you run your materials through a 420 screen, run test on the fines, and if the fines are combustible, then the whole thing is considered combustible.

One guy says use the same definition as NFPA 654.

Thanks.
 
The definitions are indeed similar. The conceptual difference in the documents though is that NFPA 499 addresses the prevention of ignitions created by electrical equipment; NFPA 68 is a set of response techniques for fires and explosions that occur regardless of the ignition source.

Within the context of NFPA 499, ?dusts? greater than 420 microns do not present a fire or explosion hazard by electrical equipment ignition sources. That is not necessarily true in the scope of NFPA 68.

If the material is greater than 420microns, it is outside the scope of NFPA 497. If the particle size is from 150-1000 microns, several criteria from Chapter 4 need to be considered.
 
Both of them are correct. The open item is whether you have a Class II environment within the Scope of NFPA 499.
 
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