Assessment of stresses and residual life of plant components in view of life-time extension of power plants

The deregulated energy market requires strong, irregular and discontinuous operation in order to meet the user demand and produce energy mainly during peak hours, when the electricity price is higher. The increasing number of plants powered by non-predictable renewable sources and of cogenerative units has amplified this need. For this reason, over the last decade new more flexible strategies in thermal power plants and systems management have been applied, which ensure greater income in the short term, but likely cause a lifetime reduction of the most critical components, due to thermo-mechanical fatigue, creep and corrosion. A procedure aimed at evaluating this extra cost related to a flexible operation, and at assisting the management decision about power plants’ operation and maintenance scheduling has been implemented by the Authors. The procedure, on the basis of the historical data, predicts the residual life of the most critical components, considering the effects of creep, thermo-mechanical fatigue, welds, corrosion and oxidation, as a function of the past and the forecasted operation strategy. The core of this procedure is the simulation tool, able to evaluate the variation of the most important thermodynamic parameters versus load during the transitory periods and then to estimate the creep and fatigue stresses on plant devices. In this paper the model for the analysis of a combined power plant will be presented. It permits to simulate the load variation of the plant also during start-up and shut-down. The velocity of these variations is taken into consideration too. Then, the stresses of the most critical components are calculated and the related damage evaluated. Finally, the residual lifetime can be estimated and the consequences on the long term profit of the plant assessed.