Development of titanium 3600rpm-50inch and 3000rpm-60inch last stage blades for steam turbines

Abstract

Reduced exhaust loss by an increase in the exhaust annulus area improves the thermal efficiency of steam turbines, such as 1000MW-class turbines. To cut this loss, the 3600rpm-50inch and 3000rpm-60inch last stage blades have been developed, getting one of the world's largest exhaust annulus areas in the turbines. Three main problems are associated with long blades: large centrifugal stress, high Mach number flow, and low rigidity. To reduce the centrifugal stress, titanium alloy was applied for the blade material; it has higher specific strength than steel. Turbulent flow analyses were utilized to clarify the complex flow phenomena including shock waves. Variations in the flow passage area formed between blades were designed in accordance with Mach number which minimized losses caused by shock waves. The continuous covered blade structure, in which blades are interconnected with shroud covers at the tip and tie-bosses at the mid-span, increased the rigidity and vibrational damping.