Three (3) 400 MW turbine-generators operated by ATCO Electric share the same 33.5" Last Stage Blade (LSB) and 20.9" L-1 steam turbine bucket designs. In 1994 one of these turbines experienced a number of cracks at the tie-wire hole of the L-1 buckets. These cracks have generally been attributed to corrosion fatigue initiation, followed by high cycle fatigue propagation. Industry experience with the L-1 bucket design indicates this problem has occurred in a significant number of these machines operated for more than ten years. Because of the high potential cost of dealing with the fatigue damage as a maintenance problem, ATCO decided to select a new L-1 design from the several solutiosn offered to the industry. To improve the reliability of the L-1 rows, a new design offered by the original equipment manufacturer (OEM) was selected that eliminated the troublesome tie-wire hole and also offered continuous coupling by means of integral covers. The integral covers also eliminated tenon cracking associated with the original design. The new L-1 design provided a modest improvement to the stage efficiency by eliminating the tie-wire obstruction i nthe flow path while also offering an improved airfoil profile. This paper presents details of the L-1 management strategy and the technical evaluation program that ATCO undertook to qualify the new bucket design. In close cooperation with the OEM, features of the new L-1 were evalauted in terms of stresses, frequencies and fatigue life, then compared against the same information obtained for the original design. As a check, natural frequencies and mode shapes obtained in the analysis were compared with OEm telemetry test data. Aero-elastic dampig was calculated and mechanical damping was derived from telemetry data to ensure blade stability under all operating conditions. At present, the first new row of 20.9" L-1 buckets has operated satisfactorily since 1998.
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