Eccentricity - Allen-Bradley 1444-TSCX02-02RB User Manual

Measurement Definition Chapter 4

Eccentricity

Used in steam turbine monitoring, Eccentricity is a measurement of the
amount of sag or bow in a rotor. It can also provide indication of a bent shaft.
This measurement is used by the operator during startup to indicate when the
machine can safely be brought up to speed without causing rubs or damage to
the seals.
Steam turbine rotors are long shafts, supported at the ends, with heavy loads in
between. So when shut down (cold and not rotating) the weight of the rotor
causes the shaft to bow over time. If the machine is then brought to speed, the
imbalance that is caused by the bow could damage the machine. Startup
procedures are provided to bring the turbine to speed slowly, and temperature,
so that rotors have time to straighten out on their own. Key to this process is
monitoring the amount of bow (eccentricity) so that a machine can be safely
started.
The eccentricity measurement is similar to the common overall measurement
in that it is the measure of the difference between the maximum and minimum
peaks in a signal. However, a normal overall measurement is measured by
sampling rapidly while continually updating the minimum and maximum
values and calculating the difference. This process is done without
consideration of shaft rotation. So, when the shaft is spinning rapidly, the
measure could span multiple revolutions, and when spanning slowly can be
measured from less than one revolution.
For eccentricity measures, this latter case result in a misleading reading. As the
measurements are made, the overall value grows and shrinks depending on the
position of the shaft relative to the sensor. To solve this problem the
eccentricity measure can be defined so that it is made on a per revolution basis,
regardless of how long that revolution takes to complete. For this reason, a
tachometer can be associated with the eccentricity measurement.
When using a tachometer for eccentricity, the minimum pk/revolution (RPM)
parameter is used to define a speed where the measurement method transitions
from the "single peak per revolution" method to the normal fast sampling
method.
Rockwell Automation Publication 1444-UM001D-EN-P - June 2018 187