Heat transfer and fouling behaviour of Siemens PWR steam generators – long-term operating experience

Abstract

The evolution of heat transfer performance of the steam generators (SGs) is regularly monitored on the basis of plant data recordings in many Siemens pressurized water reactors (PWRs). For the oldest SGs operating experience covers more than 27 years. Heat transfer performance is quantified in terms of the fouling factor. It is correlated with the amount of iron ingress into the steam generator stemming from the feedwater/steam cycle. After all plants that have started under phosphate treatment have converted their feedwater chemistry to H-AVT treatment (all volatile treatment with hydrazine dosing to assure high pH-values), iron transport in these plants has drastically reduced and, consequently, the fouling increase has almost stopped. In plants which operate under H-AVT from the beginning, no significant fouling factor increase has been observed after up to 15 years of operation. Hence, H-AVT is the preferred feedwater chemistry treatment – not only from a corrosion prevention point of view but also from a heat transfer performance point of view. The thermal effects of chemical cleaning as well as the fouling behaviour specific to economizer-type SGs are also addressed in this paper.