What is the Value of a "Bump Test"?
What is the Value of a "Bump Test"?
(OP)
My knowledge of vibration analysis is relatively nil. I have replaced four old vertical centrifugal pumps with new pumps and 1200 r.p.m. motors. The installation is on a reinforced concrete slab over the pump wet well. The manufacturer's rep checked everything out before startup and blessed the installation. Upon startup the two inboard units exhibited quite a bit of vibration, whereas the two outboard pumps were good. On the two inboard units we uncoupled the motors from the pump and ran them uncoupled and they shook badly. Next, we swapped motors between outboard and inboard units and ran them uncoupled; again the inboard unit ran rough whereas the outboard unit was okay. We then had a vibration analysis contractor come and check things out. He ran a "bump test" and pronounced the problem being the natural frequency of the vibrating units was very near 1200 (cycles?)the same as the motor r.p.m. and the mating surface of the motor and discharge head moved in the direction it was weakest - sideways to the flow- as the head has a large opening for access to the coupling and shaft seal. He said a possible fix would be some way to strengthen or stabilize the weaker axis of the discharge head. I can't just go and make modifications to this new unit as I also have to deal with the pump vendor and manufacturer to keep things in warranty. Is the "bump test" to determine the natural frequency a good tool in this case? Any suggestions?





RE: What is the Value of a "Bump Test"?
Another sign of resonance you can look for is "directionality".... machine vibrates much more in one direction than the other.
There can be other more subtle indicators observed during a coast-down test.
I would recommend that you monitor vibration while you install some very temporary bracing (2x4's against adjacent structure?). If it's resonance you will likely see a dramatic change. If machine is running at or slightly below resonance frequency, then the vib will likely decrease as you stiffen it and increase the resonance frequenccy. I suspect it will provide you with intuitive proof that a permanent brace will acomplish the same thing.
RE: What is the Value of a "Bump Test"?
RE: What is the Value of a "Bump Test"?
RE: What is the Value of a "Bump Test"?
RE: What is the Value of a "Bump Test"?
Some thoughts on technique.
We do quite a bit of bump testing on rotating machines. Mostly we use 2x4's (wood). No calibrated hammer. Sure there may be a theoretical curve which tails off toward higher frequencies and you may beyond flat part. It doesn't matter to us... we are looking just for frequency, not for magnitude (other than shape of the response spectrum).
Most times the resonant frequencies is easily distinguished in the time waveform. We also capture a spectrum. We like to use a high resolution spectrum. Not because we want to distinguish the frequency precisely better than 10 or 20 cpm... but because it provides a means to clearly differentiate resonant peaks from background machinery peaks. Resonant peaks show up like a mountain with gradual slope while background machinery vibration shows up on high-res spectrum as a super-thin vertical line. With lower frequency resolution the background machinery vib peaks become broader and harder to distinguish from resonant peaks. Sure we can also do comparison test without banging but I like the high-res trick.
Of course we test with the machine shut down. I have seen someone try to do a test with the machine running. Their analyser allows capturing two spectra... one while bumping and one while not bumping... then the two spectra were digitially subtracted to attempt to remove the influence from running machine. Results were not pretty and not accurate. I think the signal (bumping) to noise (machine vib) ratio was too low.
I have heard there are more exotic techniques which evaluate some coherence between the force function of an instrumented hammer and the resulting vibration... mathematically separate out the stuff that is not correlated to the impacts.
Sorry it has turned into a ramble. Someone else's turn now...
RE: What is the Value of a "Bump Test"?
RE: What is the Value of a "Bump Test"?
The open (____) shape of the pump volute can be responsible for most of the flexibility in vertical pumps. I tried to push for epoxied on thickeners for a few troublesome pump volute housings, but never got the chance to try it. I had made some simple FEA models that made me think it would work slick.
RE: What is the Value of a "Bump Test"?
A question for the group - Would you trust a bump test with machine shut down in this situation?
RE: What is the Value of a "Bump Test"?
RE: What is the Value of a "Bump Test"?
Another way would be to use many many hits at non synchronous intervals and do time domain averaging on the resulting waveforms. I haven't done this but it should work.
Cheers
Greg Locock
RE: What is the Value of a "Bump Test"?
I have seen cases where the modes shifted enough to tell you that everything is safe in static condition, and get a vibration issue when the machine is running.
To get enough force, you can as well use a cable attached to the structure and cut it when it is sufficiently tensed. Intrument it with a force transducer (to monitor your static input) and measure response synchronously (watch out for the cable flying away, this may be hazardous), then read time signals as well as transfers and coherences to know where your mode is.
Maybe simpler : if you can increase pump speed gradually, then record acceleration at various location synchronously with pump speed and build the Campbells.
Thomas.
RE: What is the Value of a "Bump Test"?
RE: What is the Value of a "Bump Test"?
I personally have never heard any discussion of gyroscopic effect being important for determining resonance of vertical machines. That doesn't mean it's not important, just makes me wonder.
My general impression from testing of our eight 3500hp 324rpm power plant circ water pumps/motors... we have done many simple bump tests with pump off (no water), with 2x4 (not instrumented hammer) and not considering any gyroscopic effects. Our results I believe have been good based on the fact we the machines we find with highest vibration have resonance closest to running speed.... and we can predict decrease in vibration by adding stiffening. We try temporary bracing prior to adding permanent bracing.
This weekend by adding bracing we reduced a 324rpm motor from 32 mils peak-to-peak to 2 mils peak-to-peak. Simple bump test peak went from 330rpm to 420rpm.
RE: What is the Value of a "Bump Test"?
Cheers
Greg Locock
RE: What is the Value of a "Bump Test"?
RE: What is the Value of a "Bump Test"?
http://www.mts.com/nvd/Software/pdf/300225-01TransientPost.pdf
Not a very interesting example because there are no obvious resonances in what looks like a strong 2nd order excitation, probably from a 4 cylinder engine knowing the source of the data.