Abstract
The paper describes a vibration investigation undertaken as part of the commissioning of a 12 M.V. steam-turbine driven blower. The set which was mounted on a flexible steel foundation. suffered severe vibration of the exhaust-end pedestal of the turbine. Early attempts to cure the vibration had failed. To avoid further delays it was decided to make a theoretical model of the machine and its foundation and to develop this until it agreed with measurements taken at site. Having proved the theoretical model it was then to be used to try out suggested modifications to the foundation to see if a solution could be found. The first stage of the work was to make site measurements of the machine's vibration pattern over the speed range of the machine. Recordings were made and analysed of the motion of various points on the shaft, bearing housings, casings and supporting structure in sufficient detail to demonstrate the validity of the theoretical model's predictions. The theoretical model used finite element techniques to model the shaft and the foundation structure. The shaft, mounted via hydrodynamically-lubricated bearings in the structure, was spun over the speed range of the machine and the response to unbalance of the shaft and structure computed. It was found that considerable detail was required in the idealisation before the predictions finally agreed with the site test results. When this agreement was obtained a modification involving a central column under the centre bearing pedestal was put into the theoretical model. It was predicted that this modification would reduce the peak vibration by 78 percent. When the real column was fitted to the foundation the vibration level was found to have been reduced by 75 percent.
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