Life assessment and inspection of a hot reheat turbine bifurcation

Abstract

A life assessment of a hot reheat bifurcation has been undertaken. Surface breaking crack-like indications were detected in the bifurcation weld. Subsequent replication showed that this was due to significant creep damage in a number of material zones within the weld. This damage was removed by hand dressing until it had been reduced to an acceptable level below which good material remained with minimal damage; this was confirmed by additional replication. A finite element analysis was undertaken based on the elastic rupture reference stress multiplied by weld redistribution factors which quantifies stress redistribution within the weldments. The results indicated that creep damage would be evident on both the external surface of the saddle region and on the inside surface at the crotch position. Additional analysis based on a RIKS procedure and an elastic-perfectly plastic analysis confirmed the results of the rupture reference method. A full re-inspection of the bifurcation branch was recommended before another 10,000h of service was completed as a consequence of this study. At the next inspection recurrence of the damage was found, and internal inspection confirmed the analysis results with damage also present on the internal surface at the crotch position. Based on these findings replacement of the bifurcation has been recommended. Crown copyright © 2011 Published by Elsevier Ltd. All rights reserved.