Page 103 - An Introduction To Predictive Maintenance

Machine-Train Monitoring Parameters 93

effective control range results in extreme turbulence within the fan, which causes a
significant increase in vibration. In addition, turbulent flow caused by restricted inlet
airflow, leaks, and a variety of other factors increases rotor instability and the overall
vibration generated by a fan.

Both of these abnormal forcing functions (i.e., turbulent flow and operation
outside of the effective control range) increase the level of vibration; however,
when the instability is relatively minor, the resultant vibration occurs at the vane-
pass frequency. As it become more severe, the broadband energy also increases
significantly.

A narrowband window should be created to monitor the vane-pass frequency of each
fan. The vane-pass frequency is equal to the number of vanes or blades on the fan’s
rotor multiplied by the actual running speed of the shaft. The lower and upper limits
of the narrowband should be set about 10 percent above and below (±10%) the cal-
culated vane-pass frequency. This compensates for speed variations and includes the
broadband energy generated by instability.

Running Speeds
Fan running speed varies with load. If fixed filters are used to establish the bandwidth
and narrowband windows, the running speed upper limit should be set to the syn-
chronous speed of the motor, and the lower limit set at the full-load speed of the motor.
This setting provides the full range of actual running speeds that should be observed
in a routine monitoring program.

Shaft Mode Shape (Shaft Deflection)
The bearing-support structure is often inadequate for proper shaft support because of
its span and stiffness. As a result, most fans tend to operate with a shaft that deflects
from its true centerline. Typically, this deflection results in a vibration frequency at
the second (2X) or third (3X) harmonic of shaft speed.

A narrowband window should be established to monitor the fundamental (1X), second
(2X), and third (3X) harmonic of shaft speed. With these windows, the energy asso-
ciated with shaft deflection, or mode shape, can be monitored.

5.3.3 Generators
As with electric-motor rotors, generator rotors always seek the magnetic center of their
casings. As a result, they tend to thrust in the axial direction. In almost all cases, this
axial movement, or endplay, generates a vibration profile that includes the fundamental
(1X), second (2X), and third (3X) harmonic of running speed. Key monitoring para-
meters for generators include bearings, casing and shaft, line frequency, and running
speed.