Subsynchronous vibration problems were experienced on two reactor charge pumps during on-site commissioning, which led to large whirl amplitudes, making the units automatically trip. A number of possible mechanisms of excitation were investigated, and the problem was identified to be associated with the influence of the fine annular clearances in the machines on the stability of their rotors. The problem was rectified by making modifications to the annular seal configuration. Following an experimental test program, the machines were successfully recommissioned. A comprehensive analytical investigation into the stability of the rotor for varying seal configurations was carried out in parallel with the experimental test program. Agreement between analysis and test data was found to be good.
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