The Case of the One Million Gallon Welding Failure
Water inlet pipes and inlet collar plates contained defective welds that failed on the tank bottom. The tank was originally emptied for interior re-coating and minor improvements. Minor improvements to the tank included welding a pipe extension to each inlet pipe to modify the inlet flow and create silt-stops above the tank bottom. The weld failures occurred as the tank was re-filled with water after a cleanout. The water in the tank emptied rapidly through the openings and created voids under the tank. The failed welds on the inlet pipes were repaired a second time before refilling the tank.
The cause of the collar failure at the inlet pipe collars was defective welding at the ¼“ plate edge surface of the failed collar weld and defective butt welds in the pipes that extended the inlet above the tank bottom. The collar-to-bottom fillet welds failed all-around the collar. The butt welds for the inlet pipe extensions exhibited mis-match, internal discontinuities and incomplete weld penetration.
The failed collar-to-bottom fillet weld all-around the collar was unsupported underneath the tank. The fillet welds fractured as a result of the increased load from water inside the tank. There were voids under the tank bottom and the voids increased in width and depth due to the leaking water. The weight of the rising water in the tank applied a uniform load to the collar-to-bottom fillet weld cross-section which ultimately resulted in fracture of the fillet weld all-around the collar. The butt welds at the inlets were to heightened silt-stops and were installed on each inlet pipe. The butt welds exhibited discontinuities which included mis-match inside the pipe, slag and porosity and incomplete penetration
The Lessons Learned:
The collar-to-bottom fillet welds all-around the collars fractured as the tank bottom moved downward due to insufficient ground support beneath the bottom. The inlet pipe butt welds that created the silt-stops were defective and leaked all the water under the tank. The leaked water created voids in the ground support beneath the tank bottom; allowed the tank bottom to move downward; and caused the collar fillet weld to fail. The steel for the inlet pipe extensions, welding consumables and workmanship were not in accordance with the AWWA Standards.