The Imperial Sugar manufacturing facility housed a refinery that converts raw cane sugar into granulated sugar. A system of screw and belt conveyors, and bucket elevators transported granulated sugar from the refinery to three 105-foot tall sugar storage silos. Granulated sugar was then transported through conveyors and bucket elevators to specialty sugar processing areas and granulated sugar packaging machines. Sugar products were packaged. A four-story building surrounding the silos contained packaging machines for the sugar products. Granulated sugar was also bulk shipped in railcars and tanker trucks in the bulk sugar loading area.
The U.S. Chemical Safety and Hazard Investigation Board (CSB) determined that the first dust explosion initiated in the enclosed steel belt conveyor located below the sugar silos. Steel cover panels installed on the belt conveyor less than a year before the explosion allowed explosive concentrations of sugar dust to accumulate inside the enclosure. An unknown source ignited the sugar dust, causing a violent explosion. The explosion lofted sugar dust that had accumulated on the floors and elevated horizontal surfaces, propagating more dust explosions through the buildings. Secondary dust explosions occurred throughout the packing buildings, parts of the refinery, and the bulk sugar loading buildings. The pressure waves from the explosions heaved thick concrete floors and collapsed brick walls, blocking stairwell and other exit routes. The resulting fires destroyed the packing buildings, silos, palletizer building and heavily damaged parts of the refinery and bulk sugar loading area. The CSB investigation identified the following incident causes: 1. Sugar and cornstarch conveying equipment was not designed or maintained to minimize the release of sugar and sugar dust into the work area. 2. Inadequate housekeeping practices resulted in significant accumulations of combustible granulated and powdered sugar and combustible sugar dust on the floors and elevated surfaces throughout the packing buildings. 3. Airborne combustible sugar dust accumulated above the minimum explosible concentration inside the newly enclosed steel belt assembly under silos 1 and 2. 4. An overheated bearing in the steel belt conveyor most likely ignited a primary dust explosion. 5. The primary dust explosion inside the enclosed steel conveyor belt under silos 1 and 2 triggered massive secondary dust explosions and fires throughout the packing buildings. 6. The 14 fatalities were most likely the result of the secondary explosions and fires.
This paper presents the CSB investigation and discusses important management practices to prevent similar combustible dust expolsions.
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