Could use your help troubleshooting a problematic condensate pump.
This is a 16x25 - 7 stage condensate pump in a coal fired power plant. The unit runs in parallel with 1 of 3 sister pumps.... never runs in solo condition. The pump is constant speed and runs at 20 Hz speed.
This pump was placed into service about 2 years ago, and it reported to continually increase in vibration amplitude (1x dominant) at the MOH position (upper motor bearing). When I was able to get data on the installation, I found that there was a dominant 1x peak at the top of the motor that was around .45 IPS peak consistently but that when the power plant decreased MW down to minimum load the vibration increased significantly to up around .7 IPS peak.
I did check for imbalance but there was approximately a 180 degree phase shift between the horizontal and vertical planes of both the upper and lower motor bearings.
When I did an ODS, I found that there was a torsional like motion on the motor. Based on my impact testing, I did not believe there was any resonant excitation taking place. So my thought was that as the power plant reduced MW's, the pump would reduce flow moving away from BEP, and the differential pressure across the impellers would considerably increase. If the rotor was in contact with any stationary components below, the rotor may try to take a dog leg shape which would then transfer the motion up to the motor resulting in the torsional like motion I saw in the ODS.
We pulled the pump and found that there were two impellers in contact with the bowls (lift was set too high) so I felt like my hypothesis was confirmed. The damage was cleaned up and the pump returned to service many months later.
When we put the pump back into service, I recorded vibration most of the day and through the night.... luckily the power plant cycled load so I was able to see both maximum and minimum MW performance.
I also repeated the ODS.
I found that the 1x amplitude at the MOH position was up around .55 IPS Peak and that the swing in power plant load had no effect on the vibration. The ODS data shows that the torsional like motion is gone, but there is a side to side rocking motion present.
Once again, there does not appear to be any contribution from imbalance as the phase shift from H to V at the top of the motor was approximately 150 degrees.
I tried to repeat the impact testing during this second visit, however my impact hammer wouldn't work... go figure.
After looking at the impact test data again, there may be a resonance in the horizontal plane of the motor.... I am hesitant however to blame this on resonance... I have attached a power point with a bunch of data... if you look at slide 10-12, you can see the large impact peak at 20 Hz.... I kinda wrote this off because there are 2 sister pumps running at 20 Hz but if you look at the phase spectrum on page 12 there does look to be a phase shift there of about 135 degrees (MOH Z).... I wanted other people to look at this because I don't want to start looking for resonance just to have something to blame this on.
Could you please review the attached data and let me know what you think? I truly appreciate the assistance.
Please let me know any questions you may have.