Page 355 - Steam Turbines Design, Applications, and Rerating
P. 355

Chapter
                                                             16








                                        Rerates, Upgrades,

                                           and Modifications












            Typically, steam turbines are designed for long, useful lives of 30 to 50
            years. Periodic replacement of steam turbines’ primary working parts
            may be required, due to the damaging effect of the environment in
            which they operate. Many parts are custom designed for each particu-
            lar application, but standardized components are also used to facilitate
            delivery and minimize cost. Therefore, a steam turbine can be effec-
            tively redesigned several times during its useful life. These redesigns
            are usually referred to as rerates and upgrades, depending on the rea-
            sons for doing them. A typical cutaway view of a turbine is shown in
            Fig. 16.1. For example, the flow path components can be freely
            redesigned while the basic structure of these turbines can remain
            essentially unchanged. Steam turbines, because of the unique design,
            manufacturing, and operating environment in which they exist, can be
            reworked at regular intervals to match the unit’s performance with the
            owner’s changing needs.
              Changes in hardware in an existing turbine may be required for
            many reasons, which can be categorized into four distinct needs: effi-
            ciency upgrade, reliability upgrade (including life extension), rerating
            due to a change in process, and reapplication and modification of exist-
            ing turbines for a use different from that of its original design.
              Efficiency upgrades might be required and desirable for older tur-
            bines. As a turbine is used and ages, it experiences loss of performance
            as a result of change in the nozzle and bucket throat areas and blade
            profile. Deterioration of finishes always happens, and clearances may
            also increase.
              Depending on a user’s specific needs, a steam turbine can be rerated,
            upgraded, or modified. Possible reasons for these changes include

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