Dynamic analysis and experimental study of vertical pumps

Abstract

When a horizontal pump is in operation, the shaft is submitted to its own weight, and a stable position is naturally found. It is not the case for vertical pumps. As there is no load to stabilise the rotor inside the bearings, this one can move in a chaotic way in the available clearance, resulting in dangerous vibration amplitudes. In order to avoid that problem, it is necessary to better understand the behaviour of a shaft inside fluid film bearings. On the other hand, the whole pump must be modelled to take into account all the factors influencing its eigenfrequencies. Doing so, it becomes possible to modify these until they get far enough from the speed range of the pump.

This topic is a major concern of Ensival Moret Belgium (EMB), famous pump manufacturer, located at Thimister-Clermont. Suggested by its Research & Development department, this work takes place in the Rotordynamic Project, initiated a few years ago. It has two main objectives. The first one is a deep understanding of the MADYN 2000. This software, specialised in the modelling of turbomachinery, is the main program used by the R&D department to model the vertical pumps. It is then necessary to clearly identify the possibilities and the limitations of that software. The second objective is to predict the critical speeds of the pump in operation, in order to be able to make some modifications if needed.

To achieve these objectives, experimental tests have been performed. Firstly, impact tests have been carried out on a vertical pump in a suspended configuration. It enabled to highlight the discrepancies between the MADYN model and the real pump, free from any external connection. Secondly, vibration measurements have been taken on four different pumps, but with similar geometries, in operation. Thanks to the MADYN software and other modal analyses of 3D models, it has been possible to determine the origins of most natural frequencies in the frequency range of interest. As there was still no clear explanation for some of them, propositions of improvement have been made about the numerical model or other kinds of measurement to carry out on vertical pumps.