Resonance occurs when a system vibrates with a larger amplitude at a specific frequency, and it is more than critical speeds.
Case study: Modal testing of pump bearing housings
Area of pump requiring testing
Pumps are subject to numerous excitation sources, some of which can be avoided — 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.
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 ideally have 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 with flexible couplings and with high vane pass frequencies may trigger a resonance of their fundamental modes.
Case study: Modal testing of pump bearing housings
Bearing housing modal testing process
OEMs should take note of this early in the design stage by performing 3D finite element analysis (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, KnightHawk Engineering (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.
Case study: Modal testing of pump bearing housings
Frequency Response Functions in decibel scale (dB) are a good measure to identify the housing natural frequencies.
The outcome of the impact test was analyzed for Frequency Response Functions 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%. Having knowledge of vibration instruments and the interest range of measurement is important to excite all the applicable modes, and capture at a high enough resolution. The measurements can be supplemented with simulations to discard unrelated natural frequencies that are excited in nearby components.
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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