Evaluation of Two Different Cavitation Photo

[Zartosht1963]

Dear All;
Would you please review the two impeller which works on the same application, (but the cavitation effects on the impeller blades are different one in leading and the other in trailing edge) and specify the differences and the main reason for it?
The pumping media is drinking water which contain chlorine (for disinfection).
The impeller material is cast iron.

 

[EdStainless]

First, you forgot the photos.
Second, how over chlorinated was this? And what is the max temp that you see?

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P.E. Metallurgy, Plymouth Tube

 

[Zartosht1963]

Dear Friend;
I tried to attach the files but unfortunately it was not successful.
The chlorine content is about 0.9 t0 1.1 ppm.

 

[EdStainless]

I can't get the link to open.
1ppm continuous sounds high to me.

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P.E. Metallurgy, Plymouth Tube

 

[Zartosht1963]

Dear Sir;
Please find attached the files for your kind attention.

Best Regards

 

[Zartosht1963]

Next Photo

 

[Artisi]

Quick look, more likely corrosion erosion, not cavitation.

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.)

 

[JJPellin]

Both of these look like cavitation damage to me. The first one is further up the vane. I could not tell if the damage in the first picture was originating on the visible side of the vane or from the backside. The damage in the second picture seems to originate from the backside (pressure side) of the vane. I would classify both as cavitation damage. The location where the bubbles collapse is where the damage will occur. A slight difference in the location just suggests that one may be running at a higher flow rate than the other. Or, one may have slightly lower NPSHa than the other. In any case, the pump should be analyzed for suction head. Verify that there is a sufficient NPSH margin across the full range of operating conditions. Verify that the pump is being operated within an acceptable range around the best efficiency point. Check for suction restrictions such as fouled lines, blocked suction strainers, dropped valve gates, etc.

Johnny Pellin

 

[Zartosht1963]

Dear Johny.
Thank you for your clarification. I have to add some explanation which may be helpful.
1- The first set of picture is relevant to new impeller which has been fabricated by 3D scan of existing damaged impeller, array of the best vane for all six vanes of impeller. The new fabricated impeller installed on the pump. after four month of operation we have taken to photo to evaluate the possible damage which was supposed to happen.
2- The second set of picture is relevant to old impeller which has been damaged during its operation.
3- It seems some thing is changed as it can be seen from the starting point of damage.
4- in both cases the back side of vane are remained corrosion is initiated from visible side of impeller.

 

[JJPellin]

On closer examination of the pictures, I agree that the damage appears to have originated on the visible side of the vane. This suggests that the cavitation may be a result of suction recirculation cavitation which would typically be a result of operation at very low flow. The necessary analysis is the same as I suggested above. Suction recirculation cavitation cannot be suppressed with a high NPSH margin. I would consider evaluating the pump suction energy as defined in “Pump User’s Handbook” [Bloch and Budris, Fairmount Press, Inc, © 2004]

The change in the location of the damage may be a result of changes to the impeller pattern as you described above. But, the root problem is still likely operation at low flow relative to BEP.


Johnny Pellin

 

[Zartosht1963]

Thank you for your prompt reply.
The pump is installed inside a Municipal Water Treatment Plant. In such a plants, the pumps always are running on full load capacity the demands will be compensated by storage tanks which are available in distribution side.
The backside of vanes are almost smooth. We have studied the available NPSH. It is on boarder side but I can not find any solution to increase it.
We have changed the material from GG-25 (Original impeller material) to GGG-40 now (which you can see the corrosion issues on that- first photo).
I am trying to change the material to Nile-resist or Bronze. But the change of impeller weight is one of the major issues.
Would you please explain more about "suction recirculation cavitation", the main parameters and how to verify?
I have fabricated the new set of wearing rings (Impeller/Casing sides, but it seems the pump casing wearing ring seat is somehow worn and may be it could be a source of such recirculation?

 

[JJPellin]

The link below is to an article that does a very good job of describing suction recirculation cavitation. In review of this article, I found that it contradicts my point above about the location of the damage. This article suggests that damage from suction recirculation cavitation would tend to develop on the underside of the vane rather than the visible side. This would not match the damage you have seen and may allow you to rule this out as a root cause. If the pumps are never allowed to operate at low flow relative to BEP, this would also tend to rule this out as a cause.

Based on your comments, I still don’t understand where the pumps are operating relative to the best efficiency point. “Full load capacity” suggests that the flow is not lower than BEP. But, is it much higher than BEP? Does the pump have splitter vanes or something similar in the suction area to prevent pre-rotation of the fluid entering the impeller eye? Pre-rotation can encourage cavitation even if an adequate NPSH margin is present.

Simply converting to a material with better resistance to cavitation damage does not seem like a good solution. It would be better to stop the cavitation rather than just installing an impeller that can withstand the cavitation.

http://www.waterworld.com/articles/...pump-tips-techniques/how-to-avoid-damage.html

Johnny Pellin