Dust explosions are bad to begin with, but what do you know about secondary explosions? There are many factors that determine how likely a particular type of dust causes a deflagration. When you think of combustible dust, do you think of the explosions you’ve heard about at grain handling facilities? Maybe you think of the explosive potential of aluminum dust.
Three dust explosions that occurred in 2003 demonstrate how many different types of production and manufacturing can produce a deflagration risk (http://www.fireworld.com/Archives/tabid/93/articleType/ArticleView/articleId/86899/DEADLY-DUST.aspx). In a North Carolina plant, the accumulated dust came from a polyethylene coating being applied to rubber. As the material dried, dust was formed and accumulated above the work area. Even though the work area itself was very clean, a layer of dust a quarter of an inch thick was enough to cause an explosion that killed six people. In this situation, a dust collection system in the production area could have captured the dust particles as they came off the material, before they were allowed to circulate through the facility.
The explosion in Kentucky was caused by combustible dust that resulted from a resin used to treat fiberglass. Workers were aware of the large quantities of dust, but cleaning processes often just caused more of the dust to become airborne, and it accumulated in the ductwork and in dust collection equipment. There were no safeguards in place to prevent a flame front from traveling through the ductwork or getting into the dust collector. An abort gate with spark or flame detection could have identified and stopped the fire from spreading, and dust collectors designed to stop deflagration fronts could have prevented the dust collectors from becoming sites of secondary explosions.
The explosion in Indiana was fueled by aluminum dust from scrap processing. The dust collector in this case was the source of the explosion. It did not have explosion venting, and instead of being directed safely, the explosion traveled back into the building and ignited dust in the ductwork. A secondary explosion occurred when dust accumulated on surfaces inside the facility ignited. A dust collector designed to isolate and redirect a deflagration could have prevented this accident.
The National Fire Protection Association, which establishes many of the codes and standards for handling potential fire hazards (http://www.nfpa.org/codes-and-standards/all-codes-and-standards/list-of-codes-and-standards) recommends that all dust collection systems should have explosion venting to redirect explosions and abort gates or other equipment to stop flame fronts from spreading. It also recommends improved housekeeping measures to prevent dust from accumulating, which may be accomplished by collecting dust at the source so it cannot accumulate in difficult-to-reach places.
It’s often this accumulated dust, hidden on high surfaces, in corners, or inside ductwork, that ignites to cause a secondary explosion that’s far more dangerous than the original one. Witness reports of dust explosions often include descriptions of a smaller explosion followed by one or more larger ones; this is secondary ignition (https://www.osha.gov/Publications/osha3791.pdf). Dust control throughout the entire facility, along with fire prevention equipment such as abort gates, spark arrestors, and explosion venting, can control a potential explosion and prevent a small fire from becoming a fatal disaster.