Page 80 - Steam Turbines Design, Applications, and Rerating

Bearings for Mechanical Drive Turbines 61

bearing damping and decreasing bearing stiffness. For this reason,
tilting-pad bearing L/D ratios have increased in recent years from a
standard of 0.5 to 0.75.

3.1.3 Advanced tilting-pad journal bearings
In recent years, the focus of tilting-pad bearing designers has been on
reduced-temperature designs. This has been driven partly by the strin-
gent rotor dynamic specifications that steam turbine manufacturers
must adhere to in order to sell their rotating equipment. Large journal
diameters result in less severe critical speeds. However, larger-diameter
journals result in high surface velocities, often exceeding 90 m/s (300
ft/s). Furthermore, for increased efficiencies, steam turbines are required
to run at even higher speeds, again increasing surface velocity. At these
high surface speeds, special care must be taken to cool the bearing.
Figure 3.11 illustrates a vintage pressurized housing design with oil
inlet nozzles and drain holes between each set of pads from the 1980s.
At the time, this was the best tilting-pad bearing available for cool
operation. As speeds increased, increasing the bearing oil flow often

Figure 3.11 Pressurized housing tilting-pad bearing
design. (RMT, Wellsville, N.Y.)