Comparison Among Thermo-Mechanical Fatigue Criteria in The Context of Long-Term Power Plants Optimisation

When optimising power plants production schedules to be submitted in the deregulated market, it is necessary to consider all the technical features of each component of the plants. In this new market scenario a 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. So, the relation existing between the time-dependent thermodynamic operating parameters and the corresponding cyclic stresses and strains produced in the plant components should be evaluated, in order to estimate the in-service life of a component, provided that the operating conditions of the plant are known or have been assumed. The final aim is to investigate the impact of different power plant management strategies, which may lead to a reduced service life in a long-term perspective, even though they satisfy the plant technical constraints and the market needs in the short time. In this paper some well known models will be considered and applied to a suitable range of materials. The life equations for each model will be determined together with a brief description of the material parameters involved. Then predictions based on the theoretical models will be compared with experimental results taken from the literature and obtained on specimens tested in laboratory and made of typical steels used in power plants superheater manufacturing.