Risk Assessment of an L-0 Stage Blade Retrofit

The tie-lugs on a 46” last stage blade (LSB) operated in a power plant first commissioned in the early 1960s experienced a long service history of cracking problems. A root cause investigation was performed, which identified the third mode of the bladed disc as operating near resonance. The original configuration is a 4-blade group structure with two tie-lugs. The third mode is a group torsional mode in which maximum stress occurs in the outer lugs. Dynamic stress throughout the rotating stage was calculated using a FE model. Based on the results of the investigation features of a retrofit were identified to detune the resonant mode and change the brazed lug to a more reliable forged design. A probabilistic analysis was used to demonstrate how the risk of cracking problem would be dramatically reduced.