POWER March 2015 - 18

Advanced Bearing Technology
Eliminates Subsynchronous
Steam Turbine
Vibrations
A facility's steam turbine ranks at, or
at least near, the top of the list of vital
power plant equipment. Without it, the
thermal energy in pressurized steam can
not be converted to rotary motion, which
is required to generate electricity. That is
why it is imperative for a plant's steam
turbine to operate flawlessly.
Abnormal vibrations are a good indication
that something's not right. If ignored,
the problem causing the vibration
will frequently worsen, and in a turbine it
could result in damage to blades or other
internal components. In extreme cases,
catastrophic failure of the equipment can
occur, endangering personnel and costing
millions of dollars to repair.
Commissioning Hiccup
Doosan Škoda Power understands that abnormal
turbine vibration requires action.
The company has more than a century's experience
manufacturing steam turbines and
has invested in research and development
to be an international leader in the delivery
of advanced clean energy technologies.
For one of its power generation customers
in Scandinavia, Doosan Škoda Power
engineered a 46-MW steam turbine as part
of a combined cycle system for generation
of electricity as well as heat recovery. During
the initial commissioning process, the
turbine experienced rotor instability that
prevented the drive train from operating
at full load. High subsynchronous vibrations
forced a trip in turbine operation at
just 27 MW versus the rated 46 MW.
Changes to the bearing clearances and
configurations mitigated the vibrations
but were not able to eliminate them completely.
Doosan Škoda Power decided to
contact Bearings Plus, a Waukesha Bearings
business, for a damper solution.
Assessing and Solving the Problem
Bearings Plus performed a system-level
rotordynamic assessment of the turbine,
which evaluated the rotor, bearings, and
seals. The cause of the vibrations was
confirmed to be a flexible rotor (caused
by the large span between the bearings)
combined with steam whirl forces in secondary
sealing locations.
The steam turbine's original five-pad
18
1. Abnormal vibrations identified. The waterfall spectrum shows subsynchronous vibrations
at 30 Hz with the original five-pad tilt pad journal bearings. Courtesy: Waukesha Bearings
rocker pivot tilt pad journal (TPJ) bearings
were designed with asymmetrical oil
film stiffness to try to accommodate the
rotordynamics of the combined cycle system.
However, the rotor flexibility and destabilizing
steam whirl forces resulted in
a negatively damped system and, consequently,
strong subsynchronous vibrations
at about 30 Hz (Figure 1).
For a solution, Bearings Plus suggested
soft-mounting the rotor system on TPJ bearings
with trademarked ISFD technology. In
contrast to the original design, bearings
with this integral squeeze film damper technology
provide low-stiffness and high-effective
damping to maximize the damping ratio
and eliminate subsynchronous vibrations.
How It Works
The ISFD design is manufactured through
electrical discharge machining. Integral
" S " shape springs connect an outer and
inner ring, and a squeeze film damper
land extends between each set of springs.
Bearing pads are housed in the inner ring
(Figure 2). The unique design allows for
high-precision control of concentricity,
stiffness, and rotor positioning. It produces
superior damping effectiveness by
separating stiffness from damping.
While a conventional squeeze film
damper (SFD) experiences a dynamic stiffness
from the damper film that is dependent
on amplitude and frequency, in the
ISFD design, the stiffness is defined only
by the springs. This allows for good predictability,
and precise placement of critical
speeds and rotor modes, regardless of
vibration amplitudes and frequencies.
Whereas damping in a conventional SFD
is generated by squeezing in the damper
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POWER | March 2015
film and governed by circumferential film
flow, the segmented ISFD design prevents
circumferential flow and absorbs energy
through the piston/dashpot effect. Flow
resistance at the oil supply nozzle and end
seals controls ISFD damping.
Both the stiffness and the damping of the
ISFD design are optimized for the application
through a rigorous rotordynamic analysis. For
the steam turbine, because steam whirl was
one of the root causes of the subsynchronous
vibrations, the analysis of the ISFD solution
paid careful attention to modeling destabilizing
seal forces and stage forces.
A damped eigenvalue analysis without
those forces showed a better stability margin
by a factor of 12 with the ISFD design
compared to the original bearings. With the
destabilizing forces, the ISFD solution maintained
a high stability margin. The combination
of low stiffness and optimum damping at
2. The ISFD design. This four-pad tilt
pad journal bearing utilizes integral squeeze
film damper technology. Courtesy: Waukesha
Bearings

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POWER March 2015

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