Long term monitoring and diagnostic of power plants is a permanent challenge for the energy companies. In particular with the increase of flexible exercise (e.g. daily start-up and shut-down cycles, part load operations) the definition of proper diagnostic indicators becomes mandatory. Different monitoring strategies were developed, implemented and tested for the main machineries of combined cycle power plants(e.g. Gas Turbine, Heat Recovery Steam Generator, Steam Turbine, Pumps) to prevent fault or failure or to plan/evaluate the maintenance activities.This work focuses on the first principles health assessment of the Heat Recovery Steam Generator(HRSG). At first a brief analysis about the relationship between the global HRSG efficiency and the GT net power is presented. From an initial global study the attention has been addressed to a single component analysis focusing on the section that involves the first three heat exchangers in the HRSG gas path(named SH2, RH and SH1respectively). This choice has been done for two main reasons: firstly these heat exchangers are the most important in terms of quality of energy recovered (higher temperatures means higher exergy), secondly this section has the highest number of measurement points which allows redundancy in energy balances. This second point is the key for the implementation of an effective validation phase based on Data Reconciliation and Gross Error Detection in order to improve the accuracy of the results and show the effectiveness of such techniques in the power plant monitoring. Several input set of data have been preprocessed identifying steady-state conditions and then analyzed and compared to find the optimal subset of measurements giving the best accuracy of the results.
