Pump Natural Frequency
Pump Natural Frequency
(OP)
I have a velocity/CPM peak vibration spectrum plot of an Aurora fire pump with 6 impellers. The pump operates at 3550rpm and weighs 2275lbs.
How can I determine the natural frequency from the spectrum plot? It shows consistent spikes at 2X,3X,4X,5X and 6X(BPF) and there is a high spike at 1xRPM. At approximately 2500CPM there is what I think is the system's natural frequency. Is this the natural frequency or can it just be cavitation? Is it possible to accurately determine the natural frequency from this plot?
Thanks in advance.
~Bob
How can I determine the natural frequency from the spectrum plot? It shows consistent spikes at 2X,3X,4X,5X and 6X(BPF) and there is a high spike at 1xRPM. At approximately 2500CPM there is what I think is the system's natural frequency. Is this the natural frequency or can it just be cavitation? Is it possible to accurately determine the natural frequency from this plot?
Thanks in advance.
~Bob





RE: Pump Natural Frequency
An ideal test is with the unit shut down. Impact it with a hammer and measure the response. Try impacting at different points and orientations to excite each mode of resonance you are concerned about.
If the pump cannot be shut down it might also be possible to get a running test while impacting and a running test without impacting and compare the difference.
Worst and most unreliable test is a running test without impacting which I think you have. It is often true that on a log scale the noise floor will rise at the point of resonance, but not always.
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RE: Pump Natural Frequency
RE: Pump Natural Frequency
Just wanted to ask why are you trying to find the Fn of the pump? Is the pump bouncing around and you are trying to dampen it or are you going to attached a unit onto the pump and you wanted to see what if the Fn of the pump is the same as the attached unit?
Go Mechanical Engineering
Tobalcane
RE: Pump Natural Frequency
Regards
Dave
RE: Pump Natural Frequency
Go Mechanical Engineering
Tobalcane
RE: Pump Natural Frequency
Try this, you need a key-Phaser and a acc or vel meter. Track 1XN Amplitud and Phase against the key_phaser during run up/down. From this data you can se if you have a resonace pass att 2500rpm Check also phase information if you have a 180degres phase change. From this data it can be hard to know if it a rotor or a stator reconas. A bently proximeter can give you answerto determ if its a rotor resonance.
RE: Pump Natural Frequency
The purpose of the test was to see what has been causing a low grinding sort of noise when the pump is operating at 3550 rpm. We mounted the accelerometer on the spindle bearing and plotted the peak vibration spectrum. Nothing looked out of the ordinary except for what I thought was the natural frequency at approximately 2500CPM. Can the "disturbance" in this area be an electrical problem because it is around the 60Hz (3600CPM) area? Can the 60Hz be considered a forcing frequency that might show up like that on the spectrum?
Please excuse my inexperience with this, I am relatively new to this and am working with a co-worker who is also relatively new.
~Bob
RE: Pump Natural Frequency
RE: Pump Natural Frequency
I don't think your 2500cpm vibration is likely caused by electrical origin.
great comments vanstoja. It does sound like the most likely cause would appear to be cavitation, especially if your 2500cpm vibration is spread out over a broad range of frequencies a few 100 cpm wide (broadband).
Maybe you can try analysing the available NPSH compared to required on your pump curve. Also try changing fluid system conditions and check for change in noise. For example slightly throttling a suction valve and look for increase in noise. Also increasing flow through reducing system flow resistance or increasing pump speed should increase the suction pressure drop and increase cavitation.
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