High VPF Axial Vibration on the Vertical pump

You don't indicate where the measurements were made, machine speed and Hp, machine type and arrangement, foundation, etc.. Some causes of high VPF vibrations:

  1. Flow too high or low (well away from BEP)
  2. Resonant support structure (natural frequency close to VPF)
  3. Inlet flow distortion from debris, sand/silt, air vortex (also causes Cavitation)
  4. Inlet blockage (debris or damaged screen)

The vibration levels are somewhat low, so did they increase over time and that causes your concern?

Walt

You give no details, so you get a general answer. A resonance is possible as a cause of high cycle low stress or high stress low cycle fatigue. An impulse-response (impact) test can confirm or eliminate this hypothesis.

Rigid debris can jam the impeller vanes and cause an immediate structural overload and crack. A close inspection of the impeller, suction bell, volute/vanes for visible impact damage, and inspection of sump for debris can confirm or eliminate this hypothesis.

Walt 

Walt Strong posted:

You don't indicate where the measurements were made, machine speed and Hp, machine type and arrangement, foundation, etc.. Some causes of high VPF vibrations:

  1. Flow too high or low (well away from BEP)
  2. Resonant support structure (natural frequency close to VPF)
  3. Inlet flow distortion from debris, sand/silt, air vortex (also causes Cavitation)
  4. Inlet blockage (debris or damaged screen)

The vibration levels are somewhat low, so did they increase over time and that causes your concern?

Walt

Hi, i agree on what walt commented, please share the data when the pump run on its BEP, high axial means more forces taken by thrust bearing assembly. As i remember, i have a experience on high axial at CWP with the same speed and we found out that the inlet strainer is block by debris due to typhoon came.

regards,

 

Vibration Analyst2811 posted:

Thanks for your quick response.the rpm of this motor is 200 and on the dismantle we found the crack on the impeller blade.my question is there any possibility of resonance actually and what is cause of resonance of there is? 

Motor RPM 200?, are you using accelerometer pickup.? 

"on the dismantle we found the crack on the impeller blade.my question is there any possibility of resonance actually and what is cause of resonance of there is? "

If crack is caused by high cycle low stress fatigue, then there may be a natural frequency excited by hydraulic pulsation at vane pass frequency +/- 1xSS. This would require an impulse-response (impact) test to confirm natural frequency.

There can be other causes or contributors to the impeller crack.

Just to confirm the OP; I assume by axial vibration you mean in vertical direction, because the shaft is vertical; correct?

Walt

Have you check that the setting for rotor position is correct or not? How about normal vibration in steady stage and how much VPF in this?

I agree with Walt that more information would be needed such as:

-  Spectrum.

- Drawing and measurement point.

- total free movement of rotor without bearing and how do you set the rotor position? is it a half of this value?

- How about flow, temp, pressure of this pump and performance curve respectively.

The VPF when coast down not talk too much as at that time, the flow is not stable and far from BEP.

"Place a strain gauge on a blade and what do you see?  Not vane pass!"

Bill you are correct, since strain gage is on a rotating coordinate system. That is why I said to look for natural frequency at vane pass +/- 1xSS; not at vane pass. The OP was looking for a natural frequency on impeller and not on stationary structural component!

Walt

 Thanks for all of you. We have done the operation test by closing the discharge valve partially and we absorove the vibration decreasing as the valve closing we suspect may the pressure of the lower bearing doesn't have enough pressure so while we decrease the flow the pressure increase and the pump stabilize.

"We have done the operation test by closing the discharge valve partially and we absorove the vibration decreasing as the valve closing we suspect may the pressure of the lower bearing doesn't have enough pressure so while we decrease the flow the pressure increase and the pump stabilize."

It is not obvious how bearing pressure would change and affect vane-pass pulsation. I am not saying you are wrong, it does not fit with my pump and hydro-turbine experience. The reduction in pump flow may have moved the operating point closer to the BEP, that is typically the operating point with the lowest vane-pass pulsation amplitude. Hydraulic performance measurements and pump performance curve would be needed to confirm this.

Walt

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