Vibrational prediction of a waste water pump

Sammanfattning: An off-market waste water pump is suspected to have high vibrational velocity levels. These vibrations are sought after at the outer surface located at the upper bearing position of the pump. It has then become of interest for trying to predict these vibrations by using the commercial FEA software COMSOL. The aim is to quantify to what extent it is possible to predict the vibrations numerically by comparing to experimentally measured data. The ‘root-mean-square’-value or the RMS-value of the vibrational level is used to quantify the predictions. The proposed method for vibrational predictions separates the rotating structure and non-rotating structure of the pump. The rotating parts include shaft, bearings, rotor and impeller, which may be modelled in the Beam-Rotor module in COMSOL. The non-rotating structure includes the rest of the pump excluding the discharge connection and sliding bracket. Furthermore, the non-rotating structure is simplified where the drive unit of the pump is replaced by a homogenous hollow cylinder while the pump housing remains intact. The next step is to couple the rotating structure to the non-rotating structure. This may be done by using ‘moving foundation’ at the upper and lower bearing position in the Beam-Rotor module, where appropriate displacements may be added. However, it is found necessary to find a way to replace the non-rotating structure with a simplified version to avoid having to solve its large structure in time-domain. This has meant using a modal method with appropriate boundary conditions on the non-rotating structure to quantify the displacements at the upper and lower bearing position, which may then be inserted to the ‘moving foundation’ in the Beam-Rotor module which in turn produces the bearing forces that displaces the non-rotating structure. Two different constraints are used, namely, one symmetric constraint and one asymmetric constraint. When using a low water level covering the pump, it is found that the RMS-value for the symmetric constraint at the flow rate 0.5