Page 359 - Steam Turbines Design, Applications, and Rerating

Rerates, Upgrades, and Modifications 333

Figure 16.2 Conventional carbon ring seal. (Saudi Aramco)

Over the past decades, the industry has witnessed development of
brush seals and retractable seals in addition to increased use of spring-
back seals. Better seal performance increases turbine performance by
ensuring that more steam passes through the flow path, generating
useful work. New end gland seal systems return steam to the turbine
rather than sending it directly to a gland condenser. In most cases,
sealing improvements can be retrofitted into an older design.
Leakage through labyrinth seals can be reduced by up to 80 percent
by integrating brush seals with the usual stationary labyrinth seals, as
shown in Fig. 16.3. These types of seals reduce gaps, thereby decreas-
ing interstage leakage without rotor rubbing. If not properly selected,
however, unwarranted rotor vibration can occur.
The brush seal consists of bristles that are angled slightly with
shaft rotation. They can tolerate some deflection and still spring back
to their original position. Brush seals can be incorporated between the
labyrinth teeth as well as at the ends. Figure 16.4 shows brush seals
in a heavy metal retainer. For higher pressures, brush seals may
require pressure balancing to avoid excessive downstream deflection.
Note that because of the angled bristles, some brush seals may not tol-
erate reverse rotation. General Electric’s advanced compliant brush
seal design is based on gas turbine and aviation engine technology.
Experience shows that this design, when properly installed, reduces
leakage by about 70 percent. See Fig. 16.5 for variation of brush seal
designs.