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Monday, August 22, 2011

Category » Dr. Gerd Mayer « @ Ask The Experts

From Poweder and Buld Solids "Ask The Experts" Blog, here is a great seires of questions about dust collectors and combustible, answered by Dr. Gerd Mayer with Rembe, Inc.

Category » Dr. Gerd Mayer « @ Ask The Experts

Explosion Venting/Suppression Q&A

  • If you have an existing dust collector with no provisions for handling explosive/combustible dust and you test your dust and find out it is combustible, what are the issues to consider in determining if the system can be modified to handle explosive dust or if it needs to be replaced with a new system?

    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.
    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.
  • If an existing older dust collector is collecting dust that can explode, is it better to upgrade the existing dust collector with explosion protection or buy a new one that already has the explosion protection built in?

    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.
  • This question pertains to dust collectors that are 8 ft in diam. and 11 ft high. Currently our dust collector bags are at the same elevation as the explosion vents. We are considering a modification - increasing the air to cloth ratio by increasing the length of the bags, hence increasing the height of the dust collector housing. Are there any regulations that require the explosion vents to be mounted lower than the bottom of the dust collector bags?

    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.
    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.”

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