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Subsynchronous vibration multistage high pressure centrifugal pump

Hello fellow maintenance engineer,

I would like to share about high vibration problem in multistage high pressure pump, and hopefully you guys might help in resolving this problem

This pump is turbine driven boiler feed pump, with 2 plain bearings on drive end side, and 1 thrust bearing on non drive end side to handle high axial load. The pump is directly coupled and the turbine speed is varied throughout operation according to operation load.

We recently replaced all pump bearings and mechanical seals upon customer's request due to high vibration for the past year. But the high vibration still occur after those minor overhaul

There is high peak on vertical side at 0.77X and 1X. While on horizontal side shown 0.5X, 0.77X, 1X and also BPF peak. Attached the spectral data

My first thought is the pump might have worn clearance in wearing, but when I plot the operating data to pump curve the pump is still operating within the curve. Since then, I suspect balance drum might be over-clearance and this causes rotor instability because I haven't check the clearance of balance drum. And also I suspect there might be resonance on rotor, but my concern is this pump already operating on its designed speed at 5345 rpm and have been operating for about 8 years. Do critical speed could change in time?

I would appreciate if you have any suggestion for resolving this problem


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