Chloride Contamination of the Water/Steam Cycle in Power Plants Part IV: Association with Tubesheet Dealloying as Evident during a Condenser Re-tubing

摘要

This paper is a continuation of a series of publications on investigations performed at the Shand Power Station (SaskPower) to determine a hitherto unfamiliar source(s) of chloride contamination to the water/steam circuit. The first showed that current popularly known causative factors were not individually or cumulatively responsible root causes of the chloride ingress. The second provided evidence from overhaul inspections and sample test results that the chloride ingress emanates from a cooling water source, but through two different (gaseous and water-borne) pathways, each of which is distinctly different from the common condenser tube water leaks. The gaseous ingress pathway was identified as the likely main source of the contamination. In the third paper, computational analysis and plant operational data were used in support of this idea. The working hypothesis was that chlorinated compounds with significant vapor pressures (e.g. free chlorine species HOCI and Cl2, chloramines NH2CI, NHCI2 and NCI3, trihalomethanes (THMs), haloacetic acids, HCI, etc.) ingress in gaseous forms into the condenser hotwell through weak seals of tube-to-tubesheet joints. Apart from some THMs (i.e. CHCI3 and CBrCI2), which are likely to be lost significantly through air ejection and deaeration processes, NH2CI, HOCI and NHCI2 were noted to be potential significant contributors of chlorides to the water/steam cycle from the re-circulating cooling water. Dezincification was implicated as the corrosion mechanism occurring from the attack of these oxidizing chlorinated compounds on weak naval brass tube-to-tubesheet joints, thus creating a pathway for the vapor of these relatively volatile inorganic solutions to enter the shell side of the condenser. This fourth paper first provides a brief literature background and follows up with pictorial evidence of the dezincification process and results of more detailed laboratory analyses of samples collected during the condenser tube removal and re-tubing periods in support of the above hypotheses.