Long time lurker, first time poster! I apologize in advance for the length of this post.
I'm having some challenges determining the root cause(s) of some vibration on an 800 Hp, 1200rpm, center hung, forced draft fan & motor for one of our clients. Both fan and motor have sleeve bearings and the fan is directly coupled via grid style coupling. The motor has been recently serviced after it had failed on site, and the fan bearings were also removed and confirmed to be in proper operating condition (or so I'm told). No proximity probes are installed in any of the bearing housings, so the only data I have is what I collected with standard accelerometers.
Yesterday, I was asked to collect vibration data as the fan was fired up and run through different operating loads. Motor speed is constant, and load is adjusted by opening and closing the dampers. What I observed was somewhat reasonable vibration levels, albeit a little higher than I'd ideally like, from 0% up to 75% load, predominantly 1X running speed along with very minor harmonics. However, once they hit 75% I started to notice visible, non-periodic axial pulsations that were quite evident right at the coupling, as both motor and fan shaft appeared to be shifting axially. My best guess for this issue at the moment is that the air flow becomes turbulent under this load condition and thus the fluctuating pressures cause the axial movement as the fan draws fresh air from the one side exclusively.
After the axial movement was observed load was reduced down to 50% and I continued to monitor vibration for another 30 ish minutes as the equipment continued to come up to operating temperature. As things continued to warm up the vibration amplitudes continued to increase, predominantly on the motor. The final set of readings I collected showed amplitudes upwards of 0.5 in/s at 1X in the horizontal axis on the NDE of the motor. Phase readings across the face of the DE of the motor and fan bearing showed the phase being out by 60-90 degrees as I went around from 0 to 9 o'clock positions. With this information, and the developments in the spectral data (increasing harmonics and large increase in some 2X and 3X) I am inclined to believe that the thermal growth of the motor is causing misalignment, especially considering the fan bearings are liquid cooled and the motor bearings have no cooling whatsoever.
Now I understand that does seem like a heck of a lot of vibration from a bit of misalignment caused by thermal growth, but there is one more issue I believe to be contributing. As I was walking around the equipment collecting readings I noticed the vibration I felt where I was standing change drastically depending on where I stood. The motor and fan are supported by a raised concrete base, and this base is sitting on what I believe is further reinforced concrete, as there is a definitive cut line a few feet around the whole perimeter. As I stepped from inside to outside the cut line there was a notable reduction in vibration. When I measured vibration inside the cut line at the corner nearest the NDE of the motor I recorded 0.037 in/s peak amplitude at motor running speed. I then collected a reading just outside the cut line (~3 in away) and not only had the peak amplitude significantly reduced down to < 0.01in/s, the phase also lagged by approximately 60 degrees compared to just inside the cut line. This leads me to believe that the concrete foundation may be compromised (near the NDE of the motor) resulting in some structural instability, possibly as a result of the previous failure. I'm inclined to believe that this instability is allowing the vibration caused by the misalignment to run rampant, particularly in the horizontal axis.
I'm hoping to get some input in regards to the conclusions I've drawn regarding the axial movement, misalignment/thermal growth, and possible structural instability. Does it seem like I'm heading in the right direction? Or am I completely out to lunch/missing something completely obvious? I've included full spectral data (starting with 0% load, 20%, 50%, 75%, then back down to 50% for the last 3 readings as temperature increased), along with a couple pictures of the general layout of the equipment.
Any and all feedback is greatly appreciated!