Resonance in Cooling Tower Pump Suction

[RJB32482]

We had a cooling tower pump motor fail on us recently. Testing indicated severe impacting on the outboard and inboard bearings. Maintenance had asked for engineering support to investigate if the suction piping configuration is causing a resonance issue for this pump.

Any thought on what to look at on the suction piping to see if its causing a resonance issue in the piping?

Also, I read an article that stated NPSH available calculation is different for cooling tower sumps because of dissolved air. Anyone have the technique to calculate NPSH with dissolved air in the water?

 

[StoneCold]

My money would be on poor alignment of the motor to pump shaft, not harmonics. If the pump was being chewed up then it might be the piping but for the motor I would go for alignment of the two shafts.

Regards
StoneCold

 

[rzrbk]

I am not aware of a difference in NPSH calcs due to dissolved air, but in many cases, cooling tower suctions will have a ‘submergence’ requirement that exceeds NPSHr. Submergence will be the distance the suction line has to be submerged to avoid air entrainment into the suction. This can sometimes be reduced with baffles or strainers. (may not apply unless this is a new installation)

I think piping resonance of a magnitude that damages the bearings would be very rare. I would suspect instead:
-air entrainment mentioned above
-cavitation/recirculation (check running point versus Best Efficiency Point)
-maldistribution of flow to either side of the impeller (assuming this is a between bearings, double suction pump) This can be caused by an elbow in plane with the pump shaft just before the suction. In double suction impellers, this can cause more flow to one side of the impeller, with resultant vibration and axial shuttling of the impeller. Since the motor bearings died first I suspect that the damage was to the thrust.

If you still suspect resonance is multiplying a vibration in the pump, I would look at the baseplate for voids, check pump/motor for soft foot, or do a bump vibration check on the motor.

 

[Artisi]

At this stage without more information I'm with StoneCold, probably mis-alignment causing excessive load on the motor bearings.

 

[MJCronin]

RJB...

I feel you problem may be a change in NPSHa, but this can be a very complex problem with many possible causes.

There is barrels of information about NPSHa problems associated with cooling tower pumps, including the entrained air syndrome that you have mentioned...

How about some more specific details ?

- Pump type, style, rating, motor type
- How old are the pumps ?
- Has the local pump vendor/rep said anyting about this ?
- Submergence, and/or NPSHa and NPSHr
- What is the emperature of the sump water ?
- How much does the cooling tower basin level vary.....


Does the pump operate within the NPSHr envelope defined by the manufacturer ?

This is a useful link about Cooling tower pump vibration:

http://turbolab.tamu.edu/pubs/Pump21/P21pg018.pdf

-MJC

 

[25362]

Try Accounting for dissolved gases in pump design by Mao J. Tsai, Chemical Engineering, July 26, 1982, p.65 +. It has an example on how to calculate NPSHA for cooling water.