Page 202 - Steam Turbines Design, Applications, and Rerating

Rotor Dynamics Technology 181

quency at slightly less than one-half of the running speed frequency.
When unusually high-vibration amplitudes have been encountered in
the past, the vibration signal has been analyzed for its frequency con-
tent to determine if the high vibration was subsynchronous and, there-
fore, related to an instability mechanism. This type of problem has
been identified as a bearing oil film instability caused by improper
loading of fixed-arc bearings. The problem has always been resolved by
redesigning the bearing to increase its eccentricity or, in some cases, by
installing tilting-pad bearings. With the extensive application of tilting-
pad bearings over the past several years, this type of problem has not
occurred in well-designed equipment.
In the context of normal, acceptable vibration behavior, the total
vibration signal includes many frequency components, both syn-
chronous and nonsynchronous. Generally, the synchronous component
related to rotor speed has the largest amplitude, depending on the
degree of rotor balance. In the past there has been little concern
regarding the frequency spectrum and the identification of the various
components. This has resulted in very little documentation, for tur-
bines in service, in the form of spectrum analyses that define exactly
the subsynchronous components. The total, unfiltered, vibration signal
is always somewhat greater in amplitude than the filtered, syn-
chronous vibration amplitude, and subsynchronous vibration may ac-
count for some of the difference. When a classic instability occurs, the
total vibration is several times larger than the synchronous vibration
amplitude.
Vibration limits are usually specified as the total amplitude of the
unfiltered vibration signal. This is compatible with present control
room vibration readouts that are unfiltered. In factory tests, more
emphasis is placed on the synchronous vibration component that can
be reduced to very low levels by precision balancing while other fre-
quency components are unaffected. Since the final vibration limits are
unfiltered, any sizeable subsynchronous vibration components that
occurred during factory tests would be apparent regardless of the
degree of balance. This is also true of field installations, where the total
vibration signal is monitored.
The presence of subsynchronous vibration components can be
detected by vibration meters, which display only the total unfiltered
amplitude, because of the fluctuation in the reading. The synchronous
amplitude and its multiples are usually fairly steady in amplitude. But
when subsynchronous vibration is superposed, it causes periodic rein-
forcement and cancellation of the total steady signal that produces the
amplitude fluctuation, or bounce, in the amplitude reading.This type of
behavior has been observed on many turbines in the past.
In recent years, signature analysis has evolved as a possible indica-
tor of the condition of rotating machinery. Frequency spectra of vibra-