Inadequate Maintenance and Inspection

On 13 Dec 1994, two massive detonations leveled portions of an ammonium nitrate plant near Sioux City, IA. The primary explosion allegedly occurred in defectively-designed titanium sparger piping inside the neutralizer vessel. Investigation however, revealed the explosion occurred because of unsafe plant operations and poor maintenance procedures. Specifically, the ammonium nitrate within the 18,000 gal capacity neutralizer vessel had become contaminated and made highly acidic. The operators then injected superheated steam directly into the ammonium nitrate in the neutralizer vessel.

On 22 Feb 1997, one of the arms on an amusement park ride became detached from the central pylon, allowing the passenger carriage at the end of the arm to fall to the ground. Detachment of the arm was found to have occurred as a result of fracture of the cast steel bearing cap and the retaining bolt, which then allowed the axle to move out of its housing. The cap material was a cast 0.25% carbon steel. The retaining bolt failed in the threaded region, with no evidence of fatigue. One of the reinforcing straps welded to the outer surface of the cap had also fractured as the result of fatigue. Evidence of a weld repair could be seen at the location of the fracture. The bearing cap, cap 2L, failed by fatigue initiated from the corners of the two threaded holes. One of the two reinforcing straps then failed by overload, while the second one became detached by fracturing through the welds. The bolt then failed by overload. The cracking in cap 2L was not an isolated occurrence. Cracking was detected at the same location in almost all the other caps on the same device. The cracking had been in progress for considerable time. To prevent the reoccurrence of such a failure, recommendations are given for a more-rigorous inspection protocol.