Evaluation of thermo-mechanical fatigue stresses and residual life in the superheater of a steam power plant

In the deregulated market scenario wider power generation flexibility with respect to the past is needed; on the other hand, frequent changes of the operating conditions may reduce the life of the most critical components, such as steam heaters or turbine blades. Fatigue failures produced by cyclic thermal and/or mechanical stresses will be considered in this work. In a previous paper [1] some well-known models have been considered and applied to a class of materials used for superheater pipes. The life equations for each model have been determined and the most suitable model has been identified. Its predictions have been compared with experimental results taken both from the literature and by testing specimens made of typical steels used in power plants superheater manufacturing. An economic comparison between two different strategies of plant management has also been performed. In this paper the impact of thermo-mechanical fatigue in the superheater pipes of an actual coal power plant will be determined. The effect of welding, erosion, corrosion and pipe bends, steep temperature variations will give a more precise estimation of the in-service life of a superheater under cyclic thermal and mechanical stresses. The work aims at developing the on-line real-time computation and analysis techniques in order to optimise the plant operation over time, assess components state, plan its production schedule, and forecast the expected performance degradation. This estimation is based on creep and fatigue failure models, applied at component level, which continuously check the damage produced by cyclic thermal and/or mechanical stresses. In this way it is possible to determine how the present operation choices may influence the residual life of a component and of the whole plant and how to manage the plant under design and off-design conditions.