FIELD MEASUREMENT RECONCILIATION FOR COMBINED CYCLE HEAT RECOVERY STEAM GENERATOR MONITORING

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 SH1 respectively). 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 exergies), 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. INTRODUCTION Combined Cycle Power Plant (CCPP) performance analysis is an important topic for the large diffusion of this kind of systems worldwide. CCPPs, in fact, have the highest efficiency between the large size power generation systems. Nowadays the effect of electricity market in Europe leads to a flexible operation, in which CCPP components are stressed by high frequency of start up and shut down (even more than one per day). Additionally fast load ramps are often requested to support the grid stability. Such operating method has an impact on the components performance and structural integrity. The correct evaluation of the efficiency and thus performance degradation of the whole system and its subsystem is a valuable method to drive action of condition based maintenance and in general to optimize the plant operation. Several analyses can be conducted to monitor the HRSG functionality: performance analysis, structural / chemical analysis. Procedure for the HRSG performance assessment was presented by Cafaro et al. [1,2] focusing on the evaluation of diagnostic indicators for real applications; the effect of the GT degradation on HRSG and cycle performance was discussed by Zwebek and Pilidis [3]. On the other hand, evaluation of material stresses during transient are mandatory in the new market scenario. Today’s analytical techniques to assess transient behavior and the associated stresses and creep or fatigue damage can be used in a combination with off-line analysis and on-line monitoring to better quantify the consequences of this flexible operation, Bauver and Decoussemaker [4] presented assessment techniques, inspection methodologies and monitoring tools focused on HRSG; chemical monitoring of the HRSG and the main corrosion mechanisms are presented by Dooley [5]. Long term monitoring is a mandatory issue for companies working in the energy production market, also manufacturers exploit their deep knowledge of power plant system to create