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Bearing housing modal testing 190802

Resonance is more than critical speeds!

Pumps are subject to many sources of excitation. If you are lucky, you can avoid some of them, but some require attention and can lead to serious vibration problems. A chance for resonance should never be underestimated, especially if only critical speeds are considered, leaving out the individual components such as bearing housings, could be problematic.

Modal Testing of Pump Bearing Housings

API 610 requires maximum allowable vibration levels, which will not be met under resonance conditions of bearing housings. API stipulates that the bearing housing natural frequencies should be tested by means of methods such as impact hammer testing. The natural frequencies should have ideally a 15-20% margin away from excitation sources including multiples of running speed and vane pass frequencies. Usually bearing housings have stiff designs, but pumps flexible couplings and pumps with high vane pass frequencies may trigger a resonance of their fundamental modes.

OEMs should take a note of this early on in the design stage by performing 3D FEA modal simulations of the bearing housing, including the full casing and the baseplate in the model to avoid costly issues down the road. However, the uncertainties regarding the boundary conditions and the unknown damping, can lead to deviations up to 20% from measurements.

Accordingly, KHE was called to perform modal impact testing on a centrifugal pump bearing housing. KHE performed the measurement per API instructions with the bearing housing mounted on the assembly and the pump not piped up.

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The outcome of the impact test was analyzed for Frequency Response Functions (FRFs) to identify the natural frequencies. KHE in its experience with such modal testing has identified several pitfalls to be avoided. Impact tests have a low repeatability, so one needs to make sure to take multiple readings and discard results with cohesion values below 95%. Know your vibration instruments and the interest range of measurement to excite all the applicable modes and capture at high enough resolution. Supplement your measurements with simulations to discard unrelated natural frequencies excited in nearby components.
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* FRF in DB scale is a good measure to identify the housing natural frequencies

In this case, the bearing housing had sufficient margin from the excitations, due to its stiff design. Supplementary FEA results, implied that the few low frequency modes related to the baseplate, would shift considerably after installation with grouting.
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