In-service degradation of hot gas path components were evaluated for life extension purpose in a combined cycle power plant. In this paper only results for stage 1 bucket evaluation are discussed. The study included a detailed thermal and structural analysis of the hardware simulating typical operation of the TEPCO power plant, together with nondestructive and destructive examination of the parts condition at various hot gas path inspections. The analytic predictions of critical locations compared well versus actual, and the damage trends were tracked to quantify the damage evolution in the hardware. The destructive tests also verified temperature gradients based on γ microstructure measurements at near-surface, and internal locations.
Microstructure and mechanical properties of the base metal and coating for the service exposed conditions were compared versus new buckets for baseline referencing. Also, the material properties for the service exposed condition were compared versus the original material assumptions used in the design of the original part, which also included approx. 60,000 hrs of uninterrupted creep testing. Knock down factors were established for the original materials design data in order to establish the life extension capability, and design margin beyond its specified original design life.
Life extension capability was determined using cumulative damage considerations along with the thermal-mechanical stress and strain predictions and adjusted materials properties, both of which were unique for TEPCO’s operating profile.