Restoring steam turbine dimensions, tolerances and efficiency

Steam driven turbogenerator units with size up to nearly 2 GW provide large fraction of the electric power in the world. Steam turbines may show good reliability, but their disturbances tend to be associated with significant cost so that preventive and corrective maintenance is generally easily justified. However, it is often not simple or convenient to implement repairs or replacements when unexpected damage is found at the time of opening the casing. This is because many turbine components are large in size or unique by design, and possibly only available from a limited number of suppliers and even then with a long lead time. Fortunately alternative measures can be often taken, and two types of steam turbine damage are considered here: steam erosion of inner casings and other stationary components subjected to flowing wet steam, and steam leaks of the main flange surfaces. From the user point of view, it may matter that much of the manufacturers' knowledge on turbines resides far away, partly because unlike some decades ago, the number of manufacturers in the world is now small. Together with low number of comparable machines, this may to some extent limit the local availability of expertise and experience for successful maintenance. It helps however that many features such as materials and operating conditions in steam turbines are not particularly daunting in the required knowledge accumulation, and that today the manufacturers may agree to offer help in the turbine maintenance regardless of the original manufacturer. Challenges may appear when outsourced turnkey maintenance implies shipping of major turbine sections or components to factory, to correspondingly extend the shutdown period because of the geographic distance involved, even if the maintenance project were to stay within schedule. It is demonstrated in the paper that successful on-site repair is often a viable and attractive alternative to conventional full repairs or replacements.