Failure analysis of the 350 MW steam turbine blade root

This paper presents an analysis of the cause of fracture in a steam turbine blade root. During the maintenance of a steam turbine, several L-2 blade roots with crack were encountered. In this work a visual inspection was combined with a revision of the turbine operation history. A turbine flow analysis was realized together with verification of the blade root dimensions and tolerances. Also, a finite-element analysis (FEA) of the blade group was carried out to calculate the natural frequencies. In the analysis, dynamic and static steam forces were calculated for the determination of the blade root stresses with mismatch in different contact areas. Finally, a fractography analysis on the root crack surfaces was realized. The results show that improper mismatch between the contact areas of the blade root and rotor fastening tree was the main cause of blade root fracture.