Rotor Life Prediction and Improvement for Steam Turbines Under Cyclic Operation

The evolution of the energy market is leading to a general increase in demand for cyclic operation and rapid startup capability for steam turbines utilized in power utility plants. As a consequence, turbine manufactures must optimize designs to minimize transient stress and make available to plant operators the necessary understanding of the impact of operating conditions on parts life. In addition, if continuous duty operation is not economical for an existing plant, operators considering switching to the cyclic mode need to take into account the cost associated with reduced maintenance intervals and parts replacement. This paper presents the methodologies applied to assess and optimize steam turbine rotor life. The discussion stems from the case analysis of a 60 MW steam turbine that was operated almost uninterrupted for 10 years in a combined cycle plant and was then expected to switch to cyclic operation with approx 250 startups/year. The effects of different rotor geometries on transient thermal stress/strain conditions are presented along with the consequences of startup sequence modifications for rotor life vs. on-line time. The discussion is supported by modeling details and results from transient thermomechanical FEM analyses. The possibility of a simplified approach in the form of approximate models for the analysis of such behavior on a project basis is also addressed.