A procedure for the evaluation of residual life of steam power plants

Since the introduction of a deregulated energy market in Italy, new strategies in plants and systems management have been applied: strongly irregular and discontinuous operation of power plants has been requested in order to meet the users demand and produce energy mainly during peak hours, when the electricity price is higher. More and more often this strategy is asked to all power plants, also to those designed for base-load operation, as the (old) big steam power plants. This operation mode supplies greater money income in the short period, but is likely to cause lifetime reduction of the most critical components, due to creep and thermo-mechanical fatigue loadings and the consequent serious long-term profit losses deriving from the extra-costs associated with the unplanned maintenance and the unavailability of the plant if a failure occurs. In previous papers, the Authors have presented a technique based on creep and thermo-mechanical fatigue failure models aimed at the estimation of the residual life of the devices of power plants. The effects of rapid and frequent variation of temperature and pressure in the superheater of a steam plant have been evaluated also in economic terms. This paper will present an improved procedure, where the effects of welding, corrosion and erosion are considered, too. Indeed, in many devices, as the boiler of a coal fed plant, their presence accelerates the consequence of creep and fatigue on operating life. The procedure, on the basis of the historical data, predicts the residual life of the most critical components. It also permits to assume different future strategies for plant management and evaluate for each of them the residual life and the economic effects. An example of application on a real steam power plant will be presented, too.