Absolute vibration monitoring is perhaps the primary
method of machine health monitoring on steam turbines.
The type of transducer used is seismic (ie vibration of
turbine relative to earth) and can either be a
velocity
transducer or an
accelerometer.
The choice of transducer has been the subject of debate
for many years and often the final decision is purely
subjective. A number of factors however should be
taken into account.
The steam turbine is a fairly simple machine when
considering vibration signatures.The frequencies of
interest are normally from one-half to five times running
speed (broadly 25 to 300Hz). The unique high frequency
detection capability of the accelerometer is not often
used.
Vibration monitoring is nearly always in terms of velocity
or displacement and can therefore be obtained by an
accelerometer or a velocity transducer. Particular care
needs to be taken when double integrating an
accelerometer signal to provide a displacement
measurement. Problems usually occur below 10Hz when
double integrating and 5Hz when single integrating. In
the frequency ranges normally monitored on steam
turbines this is not a problem. These measurement
issues can be reduced by integrating the signal at source
rather than after running the signal through long cables
(ie having picked up noise on route). Accelerometers
with built-in stages of integration are available to perform
this task as discussed in the previous section.
Pedestal vibration is normally measured in the two axes
perpendicular to the shaft direction where the bearing is
under load, providing complete measurement coverage.
In some instances the thrust direction is also monitored
depending on turbine configuration.
Gas turbines demand high temperature transducers for
absolute vibration monitoring (>400º typ). For this
reason, a separate charge amplifier is normally utilised,
located away from the high temperature environment.
Difficulties can be encountered when monitoring the HP
turbine pedestals using accelerometers. The high
frequencies generated by steam noise can saturate the
amplifier electronics. Filtering the signal prior to the
charge amplifier will eliminate the problem but this must
be incorporated into the amplifier circuit of an
accelerometer with built in electronics.
In summary, the
velocity transducer is simple and easy to
fit to a turbine but has limited frequency and phase
response (not a problem in the range
10 to 1000Hz) and
requires periodic maintenance. The accelerometer on the
other hand requires more careful installation but can then
be left without maintenance.
The velocity transducer has the advantage over the
accelerometer of being self generating and not requiring
any power supply. On the other hand, the accelerometer
has no moving parts and should not require frequent
calibration.
