Page 356 - Steam Turbines Design, Applications, and Rerating

330 Chapter Sixteen

Figure 16.1 Cutaway view of a steam turbine. (GE Energy)

increasing power output, optimizing performance, improving reliabil-
ity, reducing maintenance requirements, solving operating problems,
extending equipment life, and, finally, replacing the turbine, either in
whole or in part, as a result of catastrophic failure or normal wear, or
to rectify problems found during an inspection. A decision can be made
regarding possible action, bearing in mind clearly defined goals and
taking into account any constraints of the existing equipment and eco-
nomics. Needless to say, the success of upgrades and rerates depends
on effective coordination between the user and the manufacturer.
Experience has shown that reliability may decrease after many years
of service. For a typical unit, the forced, unplanned outage rate can
increase 3 to 4 percent every 10 years. To arrest this trend, reliability
improvements can be made by utilizing new and improved components
that may also improve performance. Utilization of new-generation com-
ponents, such as buckets and nozzles, will aid in an upgrade.
Process change may also require the turbine to be modified, rede-
signed, or rerated to accommodate changes in process parameters such
as new throttle, extraction admission, and/or exhaust conditions. An
existing turbine can also be reapplied at a different location or for a dif-
ferent application. Many reviews are required to assess whether a
given turbine may be reapplied and what modifications are necessary.
(See Table 16.1.)