MSI was commissioned by a pump OEM to perform an FEA analysis of a future pump assembly to predict the structural natural frequencies of 5-stage vertical turbine pump (VTP) system.  By taking a system-level approach and performing the analysis at both the high water level (HWL) and low water level (LWL) station conditions, MSI was able to predict a potential resonance problem with the 2nd below-ground pump bending mode within the 1x pump RPM range at the LWL condition.  The pump manufacturer was then able to make design changes before building the pump to avoid installing potential resonance and vibration issues.

Based on MSI’s FEA models of the full pump assembly, including the pump, motor, stiffness from the piping, and station foundation, modeled in full 3D as shown in the image below, it was predicted that the pump structural natural frequencies would all be greater than ±20% from the 1x pump running speed range in the LWL conditions, as shown in the table.  However, when considering the internal and swept water effects of the same pump at the HWL conditions, the 2nd below-ground bending mode of the column was predicted to be only 13.2% from the 1x pump running speed range which violated the separation margin requirements of the specification as well as the recommended Hydraulic Institute separation margins of ±15%.  A plot of the potentially problematic 2nd below-ground bending mode shape is shown in the animation.  MSI was able to communicate this potential vibration issue to the pump OEM before the pump was built so that design changes could be made to avoid installing vibration problems.

The following images show models of the 5-stage vertical turbine pump assembly evaluated at LWL conditions, as shown in A, and at HWL conditions, as shown in B. Volumes shown in green indicated submerged pump components.

Water Level

The following table lists of modes and separation margins for the structural modal analysis of the 5-stage vertical turbine pump assembly at the low water level (LWL) and high water level (HWL) conditions.

5-Stage Vertical Pump Structural Modes: Assessment of Water Level Effects
Pump Speed (RPM):  1240-1755
Description Low Water Level: Baseline FEA Predicted Natural Frequency Separation Margin from 1x Pump (20.7-29.6 Hz) High Water Level: Baseline FEA Predicted Natural Frequency Separation Margin from 1x Pump (20.7-29.6 Hz)
Hz CPM % Hz CPM %
1st Below-Ground Column Mode Parallel 6.93 416 -66.5 6.63 398 -68.0
1st Below-Ground Column Mode Perpendicular 6.95 417 -66.4 6.65 399 -67.9
Motor Rocking Mode Perpendicular 13.7 822 -33.8 13.7 822 -33.8
Motor Rocking Mode Parallel 14.1 846 -31.9 14.1 846 -31.9
2nd Below-Ground Column Mode Parallel 36.5 2,190 23.3 33.5 2,010 13.2
2nd Below-Ground Column Mode Perpendicular 37.4 2,244 26.4 34.3 2,058 15.9

As you can see, a LWL analysis indicated adequate separation margin between the all modes considered, but the HWL analysis indicated potential issues with the 2nd below-ground column mode for the pump.

The LWL analysis indicated negligible forced response at 33.5 Hz, which was the 2nd Below-Ground Column Model Parallel for the HWL analysis.

2nd Below-Ground Column Mode is not evident at 33.5 Hz using Low Water Level analysis

2nd Below-Ground Column Mode response at 33.5 Hz is evident with High Water Level analysis

Check out these related case studies to see how the foundation and suction/discharge piping affect your pre-installation analysis as well.

If you would like to discuss pre-installation analysis for rotating machinery with an experienced MSI Engineer, contact us.

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