Synthèse des observations d'érosion-corrosion recueillies en centrale sur des circuits en vapeur humide

Summary of erosion-corrosion observations made in power stations on damp-steam circuits

EDF - Direction des Études et Recherches

Abstract

From the HP turbine up to and including the separator feed heaters, there has been considerable erosion-corrosion of carbon steels in the PWR stations under observation; it has affected about 35 % of surfaces with losses of thickness locally attaining 2.7 mm/10,000 h. This erosion-corrosion has been stopped at Chooz by morpholine treatment of the secondary circuit together with the use of finned elbow pieces. At Tihange, treatment with morpholine has caused a spectacular reduction in erosion-corrosion. Compared with ammonia treatment, total content of iron in the drain water from the separators has been reduced by a factor of 11 ; this value is strongly representative of erosion-corrosion phenomena. At Doel, despite the use of ammonia treatment, erosion-corrosion has been limited by the total demineralization unit and the use of finned elbow joints. At Fessenheim, one unit has been treated with ammonia and the other with morpholine. The former showed extensive destruction by erosion-corrosion, while in the latter there was repassivation after the adoption of morpholine as the treatment reagent. Samples of different grades of steel were placed under severe exposure conditions facing a flow of damp steam in various PWR stations. Useful information was obtained after 40,000 hours of test: - steels with low chromium content (Cr = 1 %) appear to be on the threshold of the appearance of erosion-corrosion phenomena, though these steels stand up well when used for elbow joints ; - steels containing about 2 % chromium are unaffected by erosion-corrosion and make it possible to avoid the use of austenitic steels, which are subject to corrosion under tension. While finned elbow joints improve the flow, the experiment with "laminar" elbows, of circular or elliptic section, divergent convergent, did not give the expected results. An attempt was made to reduce the humidity of the steam at the HP output by preseparation of the liquid phase using experimental equipment of the water-trap type. While 30 % of the water is recovered with this equipment, the slight reduction in the liquid film cannot prevent the electrochemical process of erosion-corrosion in a unsuppressed electrolytic environment. High-speed driers seem to be a solution to the problems of erosion-corrosion for the future in PWR stations; they would result in total elimination of humidity in the HP exhaust, and consequently remove ail problems of erosion-corrosion, whatever steels or treatments are used. The Studies and Research Division of Electricité de France has developed such devices : they are a special type of centifugal separator, consisting of separation cells placed in parallel in the linkage piping immediately after the HP turbine. Two experimental units have been installed since 1980 in the Le Bugey station. The humidities attained at the output from them are less than 0.1 %, thus bringing us back to the classical conditions in the steam turbines of fossil-fuel power stations, and eliminating all erosion-corrosion problems. If the test results were confirmed, the very substantial reduction in the size of the separators should make it possible to lower the cost of the drier-feed heaters, and in general that of the machine room.