Evaluation of Pedestal Stiffness Variations in Steel Supported Structures for Steam Turbines

This paper is intended to present potential considerations for engineers and scientists to assess bearing pedestal degradation for a category of Low Pressure (LP) steam turbines. Structural degradation is assessed primarily comparing measured rotor frequencies of the nominal and the degraded configurations because pedestal stiffness plays an important role in the determination of rotor system natural frequencies. An electrical shaker was applied to obtain frequencies for the components tested. The results indicated that among the two rotor modes (U and S modes that are defined in Sect. 2) tested, S-mode frequency corresponding to the conical mode consistently indicated strong correlation to pedestal degradation. Additionally, measured pedestal stiffness data corresponding to the S-mode complemented these findings. 72 shaker tests were performed to date on LP bearing steel pedestal designs of different manufacturers and the conclusions derived for pedestal degradation are the same regardless.