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Chapter 3 SUMMARY OF ON-SITE INVESTIGATIONS In the Appendix detailed information is given on six explosion investigations conducted by members of the investigation subpanel. It is useful to point out how the previously discussed methodology was used in these investigations. Recapitulating, the major steps of the investigation methodology include physical site examination of pieces and parts of the elevator complex; determination from rubble, wreckage, etc. of the direction and origination of blast and combustion waves; and interrogation of witnesses. In the six incidents described, all of these steps were taken. In all cases the interviewing of witnesses played an important role in determining the probable scenario. On-site investigation was important in all except the second incident in which the cause was immediately obvious upon interviewing. In the incidents 1, 3, 4, and 5 the direction of blast and combustion waves also played an important role in determining probable sequence of events. In the first incident, considerable site investigation and interviewing were necessary before the investigators were led to the conclusion that the ignition resulted from an electrical equipment failure in one of the bins. Although the precise cause of the initial propagation of the explosion is not fully deducible, the end result follows the assumption of a primary explosion in a loading bin connected directly to other portions of the facility. In the second incident, the major explosion was definitely attributable to the use of firefighting procedures that stirred up dust and generated an explosive atmosphere. The initial fire was probably caused by a hot light fixture. In the third incident, a light fixture also was the probable initial cause of a fire in the top of the headhouse cupola structure. The explosion that occurred after firefighters had arrived at the scene was inevitable because of the state of housekeeping of the building. The second and third incidents were fairly easily identifiable events. However, in the fourth incident, considerable interviewing was necessary to ascertain the events that led to the explosion. The primary cause was a failing belt; however, only after detailed investigation was the whole story obtained. The single bucket elevator in the complex had been damaged so that choking and stoppage had occurred. There may have been a loose bucket 13
14 on the belt. Jogging procedures apparently caused a friction fire that burned through the belt. The main explosion occurred in an inaccessible concrete legwell through which the bucket elevator passed. The explosion force propagated upward into the headhouse and downward into the tunnel. Because the concrete legwell shattered early in the explosion sequence, very little pressure was transmitted to the bulk of the elevator itself. The top of the headhouse was damaged because it did not have enough vent area to relieve the explosion pressures developed from the legwell directly below the headhouse. However, little damage was done in the tunnel as the explosion force had diminished because of rupture of the main headhouse walls. In this investigation an apparent discrepancy in the events just prior to the explosion was later explained when a hospitalized witness gave a different version of what had happened and refuted earlier testimony. This illustrates the importance of persistence in interviewing to validate the most probable scenario if it cannot otherwise be verified. In the fifth incident, site investigation soon revealed that the initial explosion probably occurred in the headhouse itself or was communicated into the headhouse from the legwell. The initial explosion occurred someplace in the bucket elevator casing and propagated into the headhouse proper. The head pulley showed signs of scorching and evidence that the belt had caught on fire and broken. This was not confirmed until the wreckage was removed from the boot area several weeks afterwards. In this instance the communication from the headhouse to different bins through open spouting led to additional damage. However, the direction of propagation from leg to headhouse to other parts of the structure became fairly obvious early in the investigation. The ruptured bucket elevator confirmed the logical ignition source. In the last event, witnesses confirmed that a belt had broken. The site investigation simply confirmed the expected propagation of blast and combustion wave damage after the leg and casing exploded on the bucket elevator. In summary, the logical methodology described for the investigation of grain elevator explosions can be successful. Observations at the sites and interviews with witnesses are the principal mechanisms to determine cause. No investigation can be considered complete until all of the facts coincide and essentially verify the sequence scenario. Major inconsistencies must be resolved, or the cause will fall into the unknown (unverifiable) category.
15 REFERENCES Brasie, W. C., Guidelines for Estimating the Damage from Grain Dust Explosions, Proceedings of the International Symposium on Grain Dust, Kansas State University, Manhattan, Kansas, October 2-4, 1979. National Fire Protection Association, Explosion Venting, Bulletin No. 68, Boston, 1978. National Materials Advisory Board, The Investigation of Grain Elevator Explosions, NMAB Report 367-1, National Academy Press, Washington, D.C., 1980. National Materials Advisory Board, Prevention of Grain Elevator and Mill Explosions, NMAB Report 367-2, National Academy Press, Washington, D.C., 1982a. National Materials Advisory Board, Pneumatic Dust Control in Grain Elevators: Guidelines for Design Operation and Maintenance, NMAB Report 367-3, National Academy Press, Washington, D.C., 1982b. Verkade, M., and Chiotti, P., Literature Survey of Dust Explosions in Grain Handling Facilities: Causes and Prevention. Report IS-EMRRI-2, Energy and Mineral Research Institute, Ames, Iowa, 1976.