Page 203 - Steam Turbines Design, Applications, and Rerating

182 Chapter Ten

tion signals are obtained, usually with a real-time analyzer. This
instrument performs a Fast Fourier Transform analysis of the total
signal from which plots of vibration amplitude vs. frequency can be
generated. By periodic monitoring and comparisons with baseline or
reference plots, changes in the frequency spectra can be identified, and
vibration trends can be defined. Changes in vibration at frequency
multiples of the rotor speed may be attributed to misalignment, loose
connections, or rotating dissymmetries such as buckets, gear teeth, and
impeller vanes.
Changes in vibration at frequencies not associated with rotor speed
may be attributed to changes in the external excitation or to responses
of the stationary and rotating structures at any of their natural fre-
quencies.
Quite often, real-time analysis spectra will show low-amplitude
vibration at partial frequencies of rotor speed. Because of bad experi-
ences in the past with classic instability, which also occurs at partial
frequencies, many operators and consultants become concerned when
they observe the presence of partial frequency vibration components.
These vibration indications tend to be erratic and fluctuating (some-
times disappearing), but they remain limited and low in amplitude in
contrast to classic instability vibration, which rapidly increases to very
high amplitudes.
Many different types of rotating machinery (steam turbines, gas tur-
bines, compressors, gear sets, and generators) have exhibited limited
amplitude, subsynchronous vibration with no deleterious effects in
operation or reliability. This vibration has occurred in machines with
different types of bearings, fixed-arc and tilting-pad. Many mechanical
drive steam turbines in successful operation for many years have been
involved in signature analysis programs. The analyses revealed that
their frequency spectra exhibited limited amplitude, subsynchronous,
vibration components. It can be inferred that such vibrations existed
from the time the turbines went into service but were not documented
at the time because the necessary sensitive instrumentation and spec-
trum analyses were not available.
On the basis of such experiences and other observations, it was con-
cluded that limited amplitude, subsynchronous vibration is fairly com-
mon and does not affect reliable operation in any way.
An important distinction must be drawn between an unstable sys-
tem and a stable system. Unstable systems are self-excited and exhibit
the characteristics of negative damping. The exciting force is created
by shaft motion and is normal to the direction of shaft deflection. When
the shaft is deflected, the destabilizing force increases proportionately.
This causes the rotor to whirl at its natural frequency with rapidly
increasing amplitude until physical contact occurs in close clearance