Unusual cavitation on impeller outer diameter tip 1

[mojtababalaj]

We have got a failure of a Sundyne pump model LMV-331. It had an unusual pattern that we have never seen before. Cavitation damage is evident on the impeller tip. The pump had been running for about 3 days since the start-up of the unit. The pump tripped off on high vibration and mechanical touch.
How did you resolve the cavitation problem? Why may this failure happen?
Has anyone else experienced similar cavitation as I have described?
I am pleasure if someone explain physical reasons of this failure (tip vortex cavitation).

M. Balaj

 

[JJPellin]

I have seen cavitation damage at the tips of the vanes on Sundyne impellers. Each time, I found that they were operating the pump at very flow flow rate or even running it dead-headed. The damage was attributed to discharge recirculation caused by operation below minimum flow. Vortices form at the vane tip because of low flow.

The last example where I saw this damage was in a de-superheater pump in one of our cokers. They inject hot condensate to de-superheat steam. The condensate quill plugged off and they were not getting flow. Our cavitation damage was on the front of the vanes very close to the tip.


Johnny Pellin

 

[JJPellin]

On closer examination of the attached picture, I see that you are also getting cavitation damage on the front (pressure side) of the vanes close to the tip and at the ID of the balance holes. All of these suggest operation below minimum flow with associated recirculation cavitation.

Johnny Pellin

 

[DubMac]

Looks like are experiencing discharge recirculation. Look up old Warren Fraser (from Worthington) paper. Unlike suction recirculation or classic cavitation, this will always show on pressure side of impeller.

 

[Rasoul1250]

Dear JJpellin:
what is your suggestion to over come this problem (cavitation problem at tip vanes)? does it solve by considering the minimum flow line?

 

[Rasoul1250]

Mojtaba: does your pump have an Inducer? can you ever see cavitation between vanes of the inducer? specially close to the impeller eye?

 

[mojtababalaj]

Thanks all so much for the replies.

Rasoul1250: The pump has an inducer. But the cavitation damage is not seen for this part.It is noticeable that the pump had been running for about 3 days.

M.Balaj

 

[JJPellin]

The solution is to run the pump at a higher flow rate. If you can increase the process flow, that would help. If you have a minimum flow line that you can open to increase the total flow through the pump, that will help. As a bare minimum, the flow needs to be greater than the manufacturer's recommended minimum stable flow from the datasheet. But, for a high speed Sundyne like this, I would recommend operation above 75 percent of best efficiency point flow in order to have good reliability.

The manufacturer may claim that you can run this pump down below 20 percent of BEP. But, look at the test curves, they probably don't show values below 40 percent of BEP. I would absolutely stay above 50 percent of BEP and try to stay above 75 percent, as noted above.

Johnny Pellin

 

[Vazhoor]

The cavitation at the impeller tip is due to discharge cavitation - you are running off the curve - too far to he right.

 

[Artisi]

Vazhoor: would you like to quantify your theory regarding operation of the pump off the curve to the right.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[Strong]

I agree with Vazhoor that Cavitation on discharge side of impeller indicates excessive flow (beyond BEP) causing flow separation from the vane surface. The collapsing vapor bubbles may impact the vane tip, but can also go harmlessly downstream. Of course any conjecture should be verified by pressure and flow measurements to see what is really going on. At the very least vibration (acceleration above 200-Hz) or contact ultrasound could be measured on pump case near cutwater while changing flow rate to indicate changes in Cavitation correlated with pump performance. A microphone for airborne audible sound or ultrasound can also be used effectively.

Walt

 

[bimr]

Agree that this looks like the typical damage to be expected with discharge recirculation caused by operation at low flows. Why not post the pump performance curve and operating conditions?

 

[1gibson]

I vote low flow as well. The NPSH curve for a pump with a diffuser is bowl shaped, increasing at low flows as well as high flows. Just one of the drawbacks of using a diffuser. It's too easy to write off all cavitation as a high flow problem, that's just not the case every time.