Page 84 - Steam Turbines Design, Applications, and Rerating

Bearings for Mechanical Drive Turbines 65

3.1.4 Lubrication-starved
tilting-pad bearings
While the advanced tilting-pad bearing designs with evacuated hous-
ings have been successful in reducing the operating temperature of the
pad by 10 to 20 percent, care must be taken when implementing the
design. The wide-open end seals and housing drains result in low hous-
ing cavity pressures, often below 1.0 psig. Conversely, conventional
flooded housing designs have typical housing pressures that range
from 5 to 15 psig for a 20-psig oil inlet. These low housing pressures
can lead to oil starvation and subsynchronous vibration if improperly
applied.
Figure 3.17 illustrates one misapplication example in which open-
end seals and open housing drains will not work in a high-speed bal-
ance vacuum. Even though the inlet oil is introduced to the housing
through a spray bar at approximately 20 psig, the housing cannot
maintain a positive pressure, and the oil immediately atomizes, result-
ing in oil starvation. This manifests itself as a subsynchronous rotor
vibration.
The solution is to use dummy end seals with a reasonable clearance
and to temporarily block the open housing drains. Dummy end seal
clearances of approximately twice the bearing clearance will produce a
reasonable housing pressure of approximately 10 psig. In this case, the
inlet oil exiting the spray bars will not atomize even in a high-speed
balance vacuum bunker.
Another example of the misapplication of the evacuated housing
design is depicted in Fig. 3.18. In this case, the oil is introduced with a
single housing hole between each set of tilting pads at the pad’s axial
centerline. The pad length-to-diameter ratio is 1.0, resulting in a rela-

Figure 3.17 High-speed balance vacuum pit oil atomiza-
tion resulting in subsynchronous vibration. (RMT, Wells-
ville, N.Y.)