Failure Analysis of Once-Through Steam Generator (OTSG) External Piping Welds

Once-through steam generator external piping has experienced unexpected severe degradation at welds near a bend. Failure analysis has been conducted to identify the metallurgical root cause for the degradation so that mitigating measures could be implemented. In addition to the metallographic analysis, chemical, phase analysis and computational fluid dynamics (CFD) simulation of the flow patterns have been conducted. The CFD flow patterns have been linked with the erosion factor toward erosion-corrosion. The root cause for weld failure is found to be erosion-corrosion, most severe at the 6 o'clock location Weld-1 and 12 o'clock location Weld-2, where the condensed water had the maximum velocity.