resonace at vane pass

[Ssampson]

I have a couple of 500 hp vertical pumps. Wastewater application, they are series centrifugal pumps. The pumps vibrate excessively readings are above 1.0 in/sec at 3 times turn speed. They have 3 vane impellers. Bump test indicates a natural frequency of the pump bearing case at 3 times turn speed.
How do I go about stiffening the pump case to change the natural frequency, or what would be the alternatives to stiffening.

 

[fredb]

Hi,

Here are some things that may be of help:

1. Dynamically balance all rotating elements of both pump & motor.
2. Investigate higher precision bearings to reduce shaft movement. (talk w/bearing Mfg.)
3. Increase the rigidity of the main support and the connection between the motor and the discharge column.
4. Change the stiffness of the discharge column to raise or lower the natural frequency.

A technical reference that may be of help is:

Vibration of Vertical Pumps, by A. Kovats
Trans. ASME, J. Eng. Power, ser. A, 84:195 (1962)

 

[Ssampson]

I guess I should have provided more info. Wish i could include a picture. There is 42 foot of shafting between motor and pump with 2 additional bearings and 3 separate flexible couplings (shim pack type). The vibration at turn speed is below .05 in/sec. suction and discharge pipes are encased in concrete (building is 4 stories 1st floor at ground level and 3 floors below grade
Sean M Sampson
Littlejohn-Reuland Corp
Ssampson@littlejohn-reuland.com

 

[fredb]

Hello Ssampson,

Thats a lot of shaft between driver and pump. The distance involved allows for a lot of eccentric shaft movement if they are not perfectly aligned to each other. I would spend my time making sure the alignment of the shaft between the pump and motor is near perfect and that the steady bearings are in top notch condition. Flexible couplings are not a good idea since they are typically used to compensate for mis-alignment.

fredb