April 04, 2013– There have been several combustible dust accidents in industrial plants across the world, including the US. The last major one was at the Hoeganaes Iron Powder Facility in Gallatin, Tennessee in 2011, when three accidents occurred at the same facility within one year. Another major accident was the one in the year 2008 at the Imperial Sugar refinery in Port Wentworth, Georgia.
Why do combustible dust explosions keep on happening at various facilities? The reasons are many but a major one seems to be a lack of awareness that seemingly harmless ordinary materials such as flour, iron powder or sugar can cause horrific explosions. One always treats materials such as gasoline or hydrogen gas with abundant caution but sugar? Come on, what harm can sugar cause? Can it explode? Hard to believe for most people, but in fact yes! Dusts that are harmless in small quantities can be extremely hazardous when being handled in massive quantities.
A dust explosion can be catastrophic and cause employee deaths, injuries, and devastation of entire buildings. In many combustible dust explosions and fires, employers and employees were unaware that a hazard even existed.
Similar to the Fire Triangle concept that we have with flammable liquids, vapors and gases, we have the concept of an explosion pentagon in the case of combustible dusts. In addition to the oxygen, ignition source, and fuel (the dust), we have dispersion of dust particles and confinement (these five conditions form the five sides of a pentagon). If one element of the pentagon is missing, an explosion cannot occur.
Many times, An initial (primary) explosion in processing equipment or in an area where fugitive dust has accumulated, may dislodge more accumulated dust into the air, or damage a containment system (such as a duct, vessel, or collector). As a result, if ignited, the additional dust dispersed into the air may cause one or more secondary explosions. These can be far more destructive than a primary explosion due to the increased quantity and concentration of dispersed combustible dust. Many deaths in past accidents, as well as other damage, have been caused by secondary explosions.
To identify factors that may contribute to a explosion, OSHA recommends a thorough hazard assessment of:
• All materials handled;
• All operations conducted, including byproducts;
• All spaces (including hidden ones); and
• All potential ignition sources
One of the most important (but often overlooked factors) is housekeeping. The accumulation of a layer of even an inch of dust on a large surface, such as a conveyor, may be hazardous. This because this dust can easily blow into a dust cloud and get ignited, causing a dust explosion. The prevention of such dust collection on surfaces is very important.
There are now modern dust collection systems, that also prevent explosions and fires from occurring. You can think of using these systems, however the key is prevention. Mere prevention of dust layers and taking care of potential ignition sources, can go a long way in preventing dust explosions.
The Occupational Safety and Health Administration (OSHA) has released several resources such as posters and white papers that explain the hazards of combustible dusts.
For an excellent training program on Hazardous Area Instrumentation, that also covers dust explosions, please see here.