Degradation management in steam generators is possible with the help of research and development or a technical support programme. Strategies can be established by supplementing inspection and repair programmes based on operating experience in power plants. Management of ageing can be divided into the understanding, prevention, detection, monitoring and mitigation of ageing. A measure to systematically combine the management strategies is needed for steam generators which are widely used globally. An effective strategy could also be established and efforts to reduce duplication made through the cooperation of the equipment vendors and energy utility companies as shown in Fig. 7.1.

With the techniques developed to date, such as shot peening, rotopeen — ing and heating and temperature reduction of the hot leg side, it is possible to reduce the tensile stress inside steam generator tubes. These measures markedly postpone PWSCC initiation. Plugging, sleeving or changing the affected pipes is effective in terms of repair. Secondary water chemis­try control is the best defence against ageing damage on steam gener­ator tubes. Measures to expand the life of the steam generator include

PLAN

7.1 General structure of a steam generator ageing management strategy. (Reproduced with permission from the Electric Power Research Institute © 2008).

controlling impurities (chloride, iron and copper ions in the primary side) and oxygen (in the secondary side) and to prevent the accumulation of sludge. Ingress of chlorine-containing inorganics through condenser leak­age, resin releases from condensate polisher and make-up water, are fac­tors that compromise water chemistry control. It is proven that certain chemical additives (e. g. boric acid or morphine) decrease intergranular attack (IGA), SCC and denting of the tubes. However, it is not known whether such additives influence the equipment of other power plants (Morgan and Livingston, 1995). A continuous monitoring and control programme should also be followed to reduce impurities in the secondary water (Wood, 1990).

Denting of the tubes, fretting wear and erosion-corrosion can be detected through a normal in-service inspection before leakage occurs, whereas it is difficult to detect regional pitting corrosion and cracking (fretting-fatigue, stress corrosion and formation of intergranular) before leakage occurs. The fundamental cause of fretting is related to the design of the stream gen­erator. As a result, the most effective management option is dependent upon the design. In most cases plugging of the affected tubes is an effec­tive solution when damage is found in a particular part of a certain design. The occurrence of erosion-corrosion and corrosion fatigue is limited to once through steam generators (OTSGs), and management options vary depending on the characteristics of particular power plants (Morgan and Livingston, 1995).

To avoid maintenance cost increases, suspension of operation or reduc­tion of output, it has become possible to replace the existing steam gener­ator with one using corrosion resistant alloys (Alloy 690). As of 2011, over 100 steam generators have been replaced around the world. For most of the replaced stream generators, thermally treated Alloy 690TT has been used. The power plant Cook-2 used this alloy for the first time in 1989. With advanced methods and greater experience, it no longer takes much time to replace a steam generator. Developments in design and material allows newer steam generators to have a long service life. Crevices can be removed, allowing a steam generator to have low residual stress. New generator designs also have improved accessibility for secondary lancing and chem­ical cleaning (Morgan and Livingston, 1995). Improved corrosion-resistant materials for SG tubes include high temperature mill annealed Alloy 600 (Alloy 600 HTMA), mill annealed Alloy 690 (Alloy 690 MA) and Alloy 690TT. Alloy 690TT has only recently been used in new steam generators. Ferritic stainless steel is used for tube support structure.