An Inadequate Weld Repair and a Consequent Rupture of a Pressure Vessel

Abstract

The incident of concern involved a steam accumulator that was a component part of a steam generating facility. The accumulator ruptured during normal operating conditions that included an internal pressure of 120 psi (0.83 MPa). The rupture involved the complete fracture of a circumferential weld joining the bottom head to the accumulator cylindrical shell. An analysis of the fracture revealed the presence of a subsurface crack at the weld. This crack encompassed the entire weld line around the vessel and extended radially across nearly the full thickness of the weld. Using metallurgical analysis, this crack was determined to have been in place since the time when the accumulator was first manufactured. Eventually, a leak occurred at the weld line. The process to repair the leak failed to detect the condition of defect. The work of investigation also determined that the repair preceding the fracture was not conducted in accordance with the requirements of the National Boiler Inspection Code (NBIC). The condition of defect, which would have been detected if NBIC procedures had been used, was instead left in place and the accumulator, as a pressure vessel, ruptured as a result.