VIBRATION SEVERITY
Figure 23.12 illustrates the severity of vibration depending on displacement, frequency, and velocity.
Vibration Diagnostic Chart. Table 23.1 illustrates a vibration diagnostics chart. It includes comments about the amplitude, frequency, phase, and features of each vibration cause.
A CASE HISTORY: CONDENSATE PUMP MISALIGNMENT
Problem
Machinery diagnostic program baseline data were collected for a power station’s conden- sate pump. The data showed that levels recorded at the inboard pump axial and horizontal positions exceeded alarm values defined for these points.
Test Data and Observations
Velocity spectra from the inboard pump bearing showed the majority of the vibration was at 2X rotational speed in the axial direction and at 1 times rotational speed in the horizontal
direction. Misalignment between the motor and pump was suspected, and a coupling inspection was performed. The coupling was found to be out of alignment, as the following readings indicate: parallel side to side was out 0.009 in; parallel top to bottom was out 0.018 in; and the motor was 0.018 in lower than the pump. Two of the four upper bearing housing bolts were also discovered to be loose.
Corrective Actions
The coupling was realigned to the following tolerances: 0.002 in parallel side to side and
0.006 in parallel top to bottom. The motor also was raised to a position 0.002 in lower than the pump. New bolts were installed on the upper inboard pump bearing housing.
Final Results
A new set of baseline vibration spectra was collected for this equipment at a later date. The data showed that alignment greatly reduced the vibration levels. Inboard pump axial vibra- tion dropped from 0.42 to 0.09 in/s. The inboard pump horizontal vibration likewise was reduced from 0.18 to 0.12 in/s. Similar reductions were observed at other motor and pump measurement points.
Conclusion
Had this misalignment condition continued unabated, it is very likely that the pump and motor bearings would have been damaged. This, in turn, could have led to a catastrophic failure of the condensate pump, consequently derating the unit. It took two workers 12 h to align the coupling. If the bearing needed replacement, it would have taken 3 or 4 workers about 48 h to complete the job. In summary, correcting this condition before the bearings or coupling was damaged saved the station many worker hours as well as the expenses of new parts and lost production.