Page 71 - Steam Turbines Design, Applications, and Rerating

52 Chapter Three

nal loads. Volute loads in pumps, mesh loads in gearboxes, and asym-
metric loads in steam turbines are examples of external loads.
The vibration attenuation for any steam turbine rotor/bearing sys-
tem is significantly influenced by the journal bearing design. A good
bearing design will provide sufficient bearing damping for vibration
suppression plus operate at reasonable metal temperatures. A poor
design will lead to vibration problems or high-temperature operation,
both resulting in reduced bearing life and increased downtime.
Steam turbine journal bearing design has evolved over the years
from primarily fixed-geometry bearings to tilting-pad bearings. Fixed-
geometry or sleeve bearings have the annoying property of creating an
excitation force that can drive the turbine unstable by creating a sub-
synchronous vibration. This phenomenon usually occurs at relatively
high rotor speeds and/or light bearing loads. This inherent bearing-
induced instability can be completely eliminated with the tilting-pad
bearing.
Further enhancements in tilting-pad steam turbine bearing design
over the years include switching almost exclusively to four pad bear-
ings loaded between pivots. The tilting-pad axial length has also
increased from around 50 percent of the journal diameter to typically
75 percent of the journal diameter. Both of these enhancements pro-
vide increased effective damping for vibration suppression for most
steam turbine designs.
Most recently, reduced temperature tilting-pad bearing designs have
evolved to allow larger-diameter bearings to run at faster surface
velocities. These designs utilize the cool inlet oil more efficiently,
thereby reducing the operating metal temperatures.
The following sections address all of these advancements, with the
advantages of each design feature clearly outlined. The disadvantages
of these advancements, when applicable, are also addressed.

3.1.1 Fixed-geometry journal
bearing stability
Fixed-geometry or sleeve bearings have the annoying property of
creating an excitation force that can drive the rotor unstable by creat-
ing a subsynchronous vibration. This phenomenon usually occurs at
relatively high rotor speeds and/or light bearing loads, or, more gener-
ally, at a high Sommerfeld number. The Sommerfeld number S is
defined as follows:
S = (μNLD)/(60W)(D/C d ) 2
where μ= average fluid viscosity, lbf⋅s/in 2
N = rotor speed, r/min
L = bearing length, in