Failure of a Seamless 2-1/4Cr-1Mo Hot-Reheat Pipe Bend: Acoustic Emission Testing and Fitness for Service of Other Steam Pipe Bends

In April 2008, after 113,230 hours of operation at Belledune Generating Station Unit #2 in New Brunswick, Canada, a seamless ASME SA-335 grade P22 (2 1/4Cr-1Mo) steel hot-reheat steam pipe was found to be cracked longitudinally through the presence of a steam leak. The crack, shown in Figure 1, was about 760 mm (30 inches) long and on the extrados of a long-radius bend. No catastrophic rupture occurred of the pipe, which had an outside diameter of about 950 mm (37.5 in.). The particular bend in question was made by hot bending after furnace heating and slowly cooling in air. Prior to the unit being returned to operation, the cracked bend was replaced with a newly fabricated bend. As reported in a companion paper [1], a detailed root cause failure evaluation indicated that overheating the pipe in the furnace at the time the bending was performed produced an extremely large grain structure and was likely a key factor in the creep cracking degradation observed. Further, the failure evaluation indicated exceptionally low creep ductility for the material in the failed bend. Metallurgical examinations of the other 16 hot-reheat and main steam pipe bends in the unit showed a range of atypically large grain sizes, although not as large as for the failed bend. This paper describes the methods and findings of a study to assess the suitability for continued service of the other primary steam pipe bends at the station; and to determine the appropriate actions, such as additional inspections, for maintaining safe operation of these piping systems. This study included detailed assessments of the hot operating stresses in the bends (including those due to pipe moment loads and ovalization effects), creep crack growth assessments using the EPRI BLESS software, and acoustic emission (AE) testing on four bends remaining in service.