From our friends at Powder Bulk Solids, the "Ask the Expert" Blog, and Shawn McCorkle with Oseco, a question about keeping smaller dust collectors inside and avoiding mitigation:
Explosion Venting/Suppression Q&A
Answered July 26th, 2010 by Expert:This is a tricky question. NFPA 654 (220.127.116.11) requires that protection be provided for air material separators that have an explosion hazard. Section 18.104.22.168 requires that air-material separators shall be located outdoors unless it is has explosion protection per 22.214.171.124 (venting through duct, suppression, innerting, containment, flameless venting) or if the volume is less than 8 cubic feet. Therefore it can be located indoors but still must be protected. The problem is that the volume is too small to vent or suppress. So the way I would interpret it is that it would have to be built to contain or innerted. I think that the best solution is to direct this question to NFPA 654 for a clarification.
This was a very interesting Question. In our business, the business of helping to protect industrial conveying and dust collection systems from fires and explosions, this question comes up often, "How can mitigation be avoided?"
There is no simple answer. I would first review the process, and in some cases challenge the motive of the question. Presumably people want to avoid mitigation due to cost factors. I would submit that mitigation of risk is less costly than loss of life or other injury to personnel, production, or company reputation. These factors must be included in your decision making process.
However, having said that, realize that many factors affect the operation of an air-material-separator, and many factors affect mitigation. Some of these factors include size, volume, strength and location of the vessel, along with combustibility characteristics of the material, air-to-fuel ratios, existing safety systems, applicable current regulations and standards, etc. You can reduce risk, and you can reduce probability. Consider mitigation as insurance.
These are all factors to be considered within Process Safety Management. - Jeff Nichols