High pump vibrations... 1

[MGC1]

I have been experiencing high vibrations in our one of our auxiliary boiler feed pumps since a recent rebuild. It is an Ingersoll Dresser 2WX8A horizontally mounted multi stage pump, with a Schrodahl ARV. The vibrations are normal for the pump, in the .450 in/sec range when the pump is in the recirc mode, once the pump reaches full load and the recirc flow drops to zero, the vibes jump up to .750 in/sec. The normal discharge pressure is between 1150 and 1250, flow is around 50 -60 KPPH. The recirc flow is 68 gpm at its max and the vibes move up when the main flow gets above 50 KPPH and the recirc gets below 5 gpm.

The alignment has been checked and rechecked, today we tried adjusting pipe hangers to decide if pipe strain was a factor. Any other areas to look at to determine why the vibes change so much at the high load?

Thanks,

 

[electricpete]

Let me guess - the predominant vibration is at blade pass frequency ?

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[electricpete]

Also, is it a constant speed pump or variable speed?

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[MGC1]

I do not have the particular frequency, I am using a simple hand held vibration probe.

The pump is a single speed, electric motor for a driver.

Thanks,

 

[JJPellin]

Without frequency data it is difficult to say. But, since it was recently rebuilt, the obvious options might be balance, run-out, alignment and axial position of the rotor. You indicated you checked the alignment. I would also suggest checking the following:

• Run-out on the rim of the coupling hub should be less than 0.003".
• Run-out on the face of the coupling hub should be less than 0.003”.
• Thrust float will depend on the thrust bearing configuration. For a ball bearing, you want at least 0.002" and no more than about 0.008". The pump manufacturer may specify a tighter range. For a tilt pad thrust bearing, it could be as little as 0.008” or as much as 0.015” depending on the bearing manufacturer’s recommendation.
• If you are not certain that the rotor was axially aligned to the volutes or diffusers, it might be a good idea to verify this.
•If the discharge pressure is normal, it can’t be running with reverse rotation. But it is an easy thing to check.
•If there is a suction strainer, I would check to make sure it is not plugged up.

Any more details about what was found during the overhaul could help us understand what may be the problem.


Johnny Pellin

 

[Tmoose]

Might be able to get a signal out of the hand held device and feed it into the sound card of a PC and toss it thru a freeware FFT program to get the frequency content.

 

[Rob45]

A simpler version of Tmoose' suggestion is to just record noise near the pump and analyze that with Cool Edit 96.

 

[baw1]

I don't fully understand the issues, but if the pump was pulled off of its base for the rebuild, have you remounted it exactly the same way is was before, with fully torqued bolts to its mounting surface or however it was mounted?

 

[electricpete]

In the absence of vib frequency data (which you really should try to get), my guess would be blade pass frequency vibration because you mentioned that it changes with flow. It can be affected by the clearances and gaps, which might have been altered during the rebuild.

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