To help answer this question, NFPA has released NFPA-652 and 654. A combustible dust is defined as a finely divided combustible particulate solid that presents a flash fire or explosion hazard when suspended in air or the process-specific oxidizing medium over a range of concentrations. Basically, what this means is when the dust is in the air and its concentration is enough to cause a flash fire or propagate a deflagration or explosion if exposed to a spark or heat source, then it’s considered combustible.
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Posts about Combustible Dust:
Dust Collection Systems: Rotary Valves and NFPA-69
When you have a combustible dust, not only does your dust collector require to be protected against a deflagration, but the airlock must contain the event. NFPA-69 allows the use of a rotary valve as long as the following: