According to investigators, a recent dust explosion in the Flagstaff Nestle Purina plant was caused by welders conducting hot work repairs on the facility’s grain elevators. The incident occurred on the afternoon of September 14, 2014, which was a Sunday when the plant had been shut down for the weekend for maintenance, including the hot work repairs that sparked the explosion.

According to the Maintenance Manager at the plant, all protocols were followed prior to starting the hot work. These protocols included:

  • Cleaning the area prior to the work, including dusting and vacuuming areas to remove dust;
  • Running the equipment without product in it to clean out internal areas of the grain elevators;
  • Lock out and tag out of all equipment in the area; as well as
  • Hot work permit procedures.

While the pre-cleaning of the exterior areas subject to the hot work probably prevented a secondary explosion from occurring in the process tower, it seems clear that the interior of the bucket elevators were not sufficiently cleaned of combustible materials prior to performing the work. Based on the witness statements, it is likely that the patch welding operation ignited material clinging to the interior elevator casing, and the subsequent hammering of slag eventually knocked this ignited material (and other fuel) loose, resulting in the explosion.

This incident highlights the importance of ensuring the interior of dust handling equipment undergoing hot work repairs is thoroughly cleaned of all dust deposits prior to performing the work. Simply running the equipment without product will not sufficiently clean the equipment, as even thin layers and small deposits of combustible dusts can be enough fuel to support a combustible dust explosion. NFPA 61, Standard for the Prevention of Fires and Dust Explosions in Agricultural and Food Processing Facilities, requires hot work permitting systems to ensure (among other things) that “all equipment [is] thoroughly cleaned of combustible material and oil residues, and any combustible linings [are] removed.” Had this requirement been implemented as part of the hot work permitting process in place at the facility, the dust explosion in the grain elevator would never have happened.

The US Occupational Safety and Health Administration (OSHA) considers the grain handling industry high hazard because of the risk of fires and explosions that can result from the accumulation of combustible grain dust. The explosion at the Purina Facility injured four contract workers, blew out several metal doors, and caused an estimated $2-3 million in damages. According to the investigation report, “all factors were present that are necessary for an explosion to occur. Fuel (grain dust), oxidizer (air), confined area (grain elevator shaft or elevator), dispersion of fuel and oxidizer within its explosive limits, and an ignition source (welders, grinders, tools) all occurred simultaneously to cause this explosion.”  If the fuel had been removed from the equipment prior to the work, as required by NFPA 61, one of the required elements would have been eliminated and the explosion would have been prevented.

The Flagstaff Purina plant employs approximately 250 people and produces 1,000 tons of dry dog and cat food each day. According to Flagstaff Fire, this was the first incident they’ve ever had at the plant in its 40 years of operation.