Page 358 - Steam Turbines Design, Applications, and Rerating

332 Chapter Sixteen

by 10 percent (at constant efficiency). Even if the turbine efficiency were
reduced by 5 percent (0.5 percent to 1.0 percent would be more common),
the overall increase in power would still be about 4.5 percent.
More complex design changes can be employed when necessary. If
the flow path components need to be replaced, the entire flow path may
be redesigned for the higher power and flow. This allows the rerate to
be accomplished while maintaining the best possible turbine efficiency.
Rerates are also done when the steam conditions vary from the origi-
nal design values. For example, it might be determined that the savings
in maintenance costs for dropping boiler pressure and temperature out-
weigh the slight reduction in plant output. The turbine can be operated
at the reduced conditions without modification, but it may be desirable
to redesign the unit for the new inlet conditions during the next outage.
Thus, the maximum amount of turbine efficiency is retained and the
reduction in plant output is minimized.
Turbines can be refurbished every 5 to 15 years, depending on the
severity of the operating environment. During this time interval, since
the turbine was designed and commissioned or since the last outage,
technological development will have taken place. These advancements
may be in the areas of new technologies, new materials, improved man-
ufacturing tools, and/or improved manufacturing processes. This
enables users to decide whether to replace worn components in kind or
take advantage of the technological development and incorporate
state-of-the-art components. An updated flow path will increase tur-
bine performance, which will result in more power, reduced fuel con-
sumption, or some combination thereof. Typical areas of technological
development include seals, airfoil design, stage design (reaction, area
ratios, or other flow path refinements), and increased-area final stages
(taller blades in the last row).

16.1.1 Brush seals and labyrinth seals
Steam turbines require a method of preventing steam from escaping
from the casing at the region of the rotating shaft. Due to severe tem-
perature and pressure conditions, many of the sealing methods used for
pumps and compressors have not been practical for steam turbines.
Until recently, the most common methods employed labyrinths or close-
clearance carbon bushings. In reality, however, these components do not
serve as true seals; they act only as throttling devices to minimize leak-
age. A typical carbon ring seal is illustrated in Fig. 16.2. Depending on
design and operating conditions, substantial steam losses can be
expected. Even a small steam path leads to steam-cutting action. Steam
cutting describes an escape-jet action whereby high-velocity steam
rapidly and exponentially causes erosive wear of the bushing bore.