(119c) Explosion From a Smoldering Silo Fire

Many organic granular materials are susceptible to self-heating when stored in silos. Under the right environmental conditions self-heating can lead to spontaneous ignition and smoldering combustion. This is a case study of an explosion that occurred in a grain storage silo with a smoldering fire. Although the hazards of combustible dust are relatively well recognized, the explosion hazard presented by a smoldering fire is less well known. There were two explosions in this incident: a primary explosion involving carbon monoxide and smoke generated by the smoldering fire, and a secondary explosion fueled by combustible dust. The explosion caused four injuries and significant property damage.

The subject facility stored wheat grain and grain products in a concrete elevator. One of the operators discovered the fire by detecting a distinctive burnt odor in the access tunnel. Several silos were examined before it was determined which silo was the source of the odor. Subsequent investigation the next day revealed several unmistakable signs of combustion: burnt odor, light smoke, elevated temperatures measured by an optical pyrometer, some charred and discolored grain product material, and elevated levels of carbon monoxide inside the silo. That evening one of the operators opened the roof hatch to measure the temperature of the silo contents and noted glowing coals on the surface of the grain product. Unsure of how to proceed, the facility owner attempted to suffocate the fire by preventing the intrusion of air into the silo. No attempt was made to consistently and effectively monitor the progress of this action.

Eventually a grain salvage company was hired to remove the grain product from the silo. This was a process that took several days. During this time the rooftop hatch and the tunnel hatch were often both open creating a chimney effect which accelerated product combustion. Water was frequently sprayed on smoking material from both the top of the silo and from the tunnel. On the day of the explosion, small flames were observed at a vent. The tunnel hatch was closed but the rooftop hatch remained open. The explosion occurred half an hour later. Almost two months had elapsed after the discovery of the fire.           

This paper discusses the causal factors for the explosion and the lessons learned for effectively monitoring and responding to a smoldering fire in a silo. The potential explosion hazard of the carbon monoxide and smoke generated by a smoldering fire is emphasized.