Depends on pump design, bearing housing vibration and shaft vibration are measured and plotted in the FFT (Fast Fourier Transform) spectrum at each data point from minimum to maximum flow to ensure the vibration values are within acceptance range.
The bearing housing vibration is measured with velocity meter or accelerometer transducers (sensor). If the accelerometer is used, then the software makes a calculation and changes that to the velocity.
The velocity vibration acceptance range for the overhung and between-bearing pump is provided in Table 8 and for vertically suspended pump addressed on table 9 of API 610.
Based on the API 610 requirement the overall vibration for overhung pump shall be less than 3 mm/s (0.12 in/s) for overall frequency and 2 mm/s (0.08 in/s) for discrete frequency.
The API 610 requires out of preferred operating region the vibration level shall not be more than 30% of above-mentioned limits. Please note this only applies to performance test since the pump is tested in different flowrates.
In the mechanical running test, the pump will run on the rated flow, so this 30% extra limit is not applied to the mechanical running test.
Let's continue with an example
Following FFT plot refers to the bearing housing vibration of centrifugal pump performance test in X, Y and Z axis
X Axis Vibration @ Pump Bearing Housing
The overall vibration is 1.1817 mm/s and it is less than 3 mm/s so the overall vibration test result is OK.
The left side table indicates max amplitude is 0.8686mm/s at 29.167 hertz (synchronous vibration). The 0.8686 mm/s is less than 2 mm/s so the result of test for discreet frequency is OK
Note: The synchronous vibration means the amount of vibration amplitude at the shaft RPM, in this example, the pump shaft speed is 1750 rpm which equals to the 29.167 HZ). We usually experience high spike at shaft 1X, 2X, etc. frequencies regarding the mass of unbalance on the rotor.
Y Axis Vibration @ Pump Bearing Housing
The overall vibration is 1.6863 mm/s and it is less than 3 mm/s, so the overall vibration is OK.
The left side table indicates max amplitude is 0.9516 mm/s at 29.167 hertz (synchronous vibration). The 0.9516 mm/s is less than 2 mm/s so the result of test for discreet frequency is OK
Z Axis Vibration @ Pump Bearing Housing
The overall vibration is 0.5529 mm/s and it is less than 3 mm/s, so the overall vibration is OK.
The left side table indicates max amplitude is 0.1586 mm/s at 29.167 hertz (synchronous vibration). The 0.9516 mm/s is less than 2 mm/s so the result of test for discreet frequency is OK
So above FFT plot verified for rated flow. The pump manufacturer provides FFT plots for each flowrate, and all of these plots shall be verified. As per API 610, the pump shall be subjected to the performance test at least for 5 data points. So you need to review one set of the above-illustrated plot for each data point.
The shaft vibration is measured by proximity probe and acceptance range based on API 610 is 50 μm (2.0 mils) for overall frequency and "0.33 times overal vibration" for discreet frequency.
Let's continue with an example. See following FFT plot for shaft vibration.
The overall vibration was measured as 16.83 μm, and it was less than 50 μm, and so the result is OK. The vibration at the discreet frequency of 68.75 Hertz was measured as 2.55 μm which is less than (16.83 x 0.33 = 5.55 ) and the result of test for discreet frequency also was OK.
This was for one axis; two more plots shall be verified to complete assessment for this data point.
As mentioned in above, at least 5 datapoints (5 different flowrates) shall be taken so totally 5 sets of FFT plots (3 plot for each set) shall be received and verified for the pump vibration testing at performance testing part.
After successful completion of the performance test, the pump is subjected to the mechanical running test for 4 hours (at the rated flow) and pump vibration testing data are collecetd in real time basis. The pump manufacturer provides these plots for review after completion of the test. The plots shall be reviewed and checked aginst acceptance range.
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