Flow Accelerated Corrosion in HRSGs A Method to Quantify the Probability of FAC Occurrence

ABSTRACT Flow-Accelerated Corrosion (FAC) has become a source of failures in Low pressure units of HRSGs. Research in past 20 years has indicated that FAC is a very complex phenomenon. It is influenced by many variables, some among them are : pressure, temperature, pH of the water, Chrome content of the metal, the dissolved oxygen content in water, the velocity of steam and water etc. The matter of FAC was discussed in detail in a joint publication by Electric Power research Institute (EPRI), Electricité de France (EDF) and Siemens. However the methodologies presented to understand the phenomena and to find the solution are very complex. Considerable expertise is needed to arrive at a good solution. This paper combines the work done both empirically and experimentally and gives a simpler method to estimate the metal loss due to FAC in an HRSG given the design and operating conditions. The user should then be able to decide whether to proceed further with more complex and comprehensive investigations, install a monitoring system or make some changes in the design or the operating methodology. Since the most prevalent occurrence of FAC in an HRSG is in Low pressure (LP) evaporators, the discussion and solutions are focussed on LP evaporators only. At the end of the paper a tentative action plan is also described to handle the FAC related issues in HRSGs