Category » Dr. Gerd Mayer « @ Ask The Experts
Explosion Venting/Suppression Q&A
If you are in a situation where you don’t know the strength of the dust collector and you have no way of finding out the strength, you will either need to have an engineering analysis done on the dust collector or replace the collector to be absolutely sure you are properly calculating the vent area. There is no way to calculate the vent area if you don’t know the strength of the dust collector, and effective vent area is the critical component to minimizing damage to people and structures should there be a combustible dust explosion in the dust collector. In that case, you may decide you are better served by replacing the dust collector but you will still need to equip the collector with the appropriate venting/suppression and isolation equipment as indicated above.
Note: Even if you have all the information about your dust collector, you may find that the strength of the dust collector is such that it is more cost effective to increase the strength of the collector to reduce the costs of equipping with explosion protection equipment or to buy a new collector and properly equip it to protect it.
Under typical circumstances where you have complete information about your dust collector, such as the strength of the collector, retrofitting should be no problem. In that situation, in accordance with NFPA standards 654, 68, 69, and perhaps other standards that specifically address your industry, a dust collector must be vented/suppressed and isolated ( the inlet always needs to be isolated; the clean air side must be isolated if it is a return-air installation). If the dust collector is inside, the dust collector might be vented through a duct to the outside, an indoor flameless vent can be installed or chemical suppression might be used. If the dust collector is located outside, explosion panels or flameless vents can be used depending on the proximity to other structures and people. NFPA standard 68 provides the method by which to calculate the required vent areas.
Per NFPA regulations, each enclosure containing a combustible particulate has to be vented or suppressed, no matter how old the enclosure is. In the case of a dust collector, if the reduced pressure (Pred) for the existing dust collector can be determined either by the manufacturer’s specifications or through a structural analysis, the old dust collector can be upgraded and still be used. In some cases, the cost to determine the Pred could be quite high, and the dust collector may need too much “strengthening” or other redesign—retooling doors that will not withstand explosions, for example, so that purchasing a new dust collector could be the more economic solution. Whether you retrofit an old dust collector or install a brand new one: either one of them has to include explosion protection and explosion isolation.
Explosion vent panels should be installed underneath the bottom of the filter bags as described in NFPA Standard 68, 8.7.1. If, for any reason, there is not enough space to install the panels underneath the filter bags, you may, per NFPA Standard 8.7.1 (2), install the panels along the dirty air wall if “… bags are either completely removed or shortened so that they do not extend below the top of the vent for a distance of one vent diameter from the vent. In addition, bags immediately adjacent to the vent shall be removed and the remaining bags shall be restrained from passing through the vent.” NFPA Standard 68, 8.7.1(2), 2007 edition.
Another acceptable explosion vent panel placement option, per NFPA Standard 8.7.1 (3): “…the bottom of the vent(s) is at or above the bottom of the bags,…and the row of bags closest to the vent are restrained from passing through the vent… For this case, the volume used to calculate the vent area shall be the entire volume (clean and dirty) below the tube sheet.”
NFPA Standard 68 gives you precise guidelines for how explosion vent panels should be installed. The goal of NFPA Standard 68, 8.7.1 is to prevent the bags from blocking the vent such that the bags might be “sucked through” the vent if there is an incident.