Secondary Dust Explosions: The Hidden Hazard
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 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.
Secondary Dust Explosion Incidents
Three dust explosions occurred in 2003. They demonstrate how many different types of production and manufacturing can produce a deflagration risk, including a secondary dust explosion. In a North Carolina plant, the application of a polyethylene coating to rubber caused accumulation of the dust. As the material dried, dust formed and accumulated above the work area. The work area was clean. But a layer of dust 1/4″ 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 before they circulated 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. So, 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.
Aluminum dust from scrap processing fueled the explosion in Indiana. The dust collector in this case was the source of the explosion. It did not have explosion vent panels, 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.
NFPA Recommendations
The National Fire Protection Association, which establishes many of the codes and standards for handling potential fire hazards 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. Accomplish this by collecting dust at the source so there’s no accumulation 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. Dust control throughout the 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.
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