DIAGNOSE THIS 2

[PUMPDESIGNER]

Please go to the following link:

http://www.irrigationcraft.com/diagnose_this.htm

Please examine the photograph and make your diagnosis.
We pondered the situation for a long time before guessing correctly what the cause was, which we then verified.
However we were never able to figure out exactly how the damage was inflicted in detail, it remains a mystery to us.

Thank you.


PUMPDESIGNER

 

[BobPE]

pumpdesigner, you are getting skimpy on your information that you usually provide us!!!! LOL

My guess would be high flow induced cavitation (suction cavitation) most likely caused by a demand that can be varried by the pump user. I envision a sprinkler system where the owner is adjusting the spray heads visually to get an effect he desires?

it looks like there is also some non cavitation damage possibly caused by the loss of the missing impeller material.

give us some more info, pressures, flows, pump type, stages, what the pump was servicing, fluid, sump design?

you are holding far too much back.....LOL

BobPE

 

[dadfap]

pumpdesigner,

coupla questions;
1. where is the water coming from
2. have the water characteristics change noticably
3. did u take a big stick to end user for destroying pump?

all the best

dadfap

 

[PUMPDESIGNER]

You made me feel bad BobPE. But I learned something from you again, you are pretty self confident in posting without info knowing full well you were poking in the dark and easily could be wrong.

Yeh, I held back but not to mislead or mess with you guys.
I have always been mystified by this impeller because even though I did figure out the problem (but not easily), I never understood the destructive process and how it worked. That is what I was hoping you guys would teach me.

FURTHER INFO is:
Fresh water lake in South Florida, water temp ranges 70-85 degrees F. year round. Water appears to be clean, clear, and "healthy".

PUMP - NS = 9270, Nss = 1772, NPSHa exceeds NPSHr by 9.4 feet worst case, 3550 rpm, 30 horsepower.

I stared at that impeller for days in my office, thinking probably severe NPSHa defficiency. Then I observed that the damage was extremely uniform throughout the diameter of the impeller. Cavitation would have a lot of trouble making this damage pattern. This made me suspicious and I had someone go by the job to check something, which then revealed the problem.

PUMPDESIGNER

 

[BobPE]

pumpdesigner:

you know I love pumps and the associated hydraulics!!! Every picture tells a story. That impeller pic tells a story too.

I was just busting you on witholding the info, I like when you post these challanges and you can't give it away right from the beginning!!!

I await the rest of the info you have...and your diagnosis...

Take care,

BobPE

 

[Kawartha]

PUMPDESIGNER,

Don't give it away too quickly. Give us all time to think about it.

I guess your back.

Richard

 

[Kawartha]

PUMPDESIGNER,

Please advise the impeller material, casing material, etc. Was erosion observed in the casing as well?

Richard

 

[mekanicles]

pumpdesigner,

I most guess that the damage was cause mainly by a misalignment on the pump shaft. At least that what I can guess from the picture. It looks like there is some wear in the shaft housing (upper section). Misalignment can cause cavitations and vibration stresses causing, as many other issues, pressure drops similar as discribed.

Only my guess, look forward to hear the veredict!!!

 

[PUMPDESIGNER]

dadfap - There was someone in particular that should have been "sticked" for sure.

Kawartha - Cast iron volute and impeller. Case was in good condition. That is the real stinger, how that impeller could "disappear" and yet the case is in good condition. But then that was the primary clue. OOOOPS!

PUMPDESIGNER

 

[pmover]

bad pour/casting of impeller at mfg.
-pmover

 

[PUMPDESIGNER]

mekanicles - Pump was operating smoothly, just low pressure. Mis-alignment would have mechanically destroyed the pump quickly, this impeller did not get to this state quickly.

pmover - Would not bad casting have caused entire impeller to go away?

PUMPDESIGNER

 

[hydrae]

Pumpdesigner
What does the suction screen look like? could it pick up sand?
Hydrae

 

[Kawartha]

Richard,

I think hydrae may be on the right track. Was the suction located in a sand bar or sandy beach? Can't explain the casing being in good condition. One would expect some erosion there as well. What type of casing - volute or diffuser?

Richard

 

[PUMPDESIGNER]

hydrae - Intake screen was in clear water, clean screen, no sand. Kawartha is correct that volute would have been damaged by sand also but was not. Volute type no diffuser.

PUMPDESIGNER

 

[lilliput1]

Was the impeller not properly keyed into the shaft? The key was forced in with part of the impeller not fully seated on the shaft. The half of the closed impeller then eventually sheared off at the shaft. This sheared half remain lodged in the casing while the other half half rotates, grinding the rotating impellers on the split half.

 

[BobPE]

pumpdesigner:

you are so patient lol....

BobPE

 

[PUMPDESIGNER]

lilliput1 - What you describe sounds very noisy. Only symptom was low pressure, which they could not figure out and therefore they called us for help.
The photographs show no mechanical damage such as scraping, egged shaft hole, damaged key way, etc. We have seen many impellers come loose, total destruction every time within short period, very noisy, and all that would be predictable with 3550 rpm and 30 horses yoking that loose stuff around.

Notice how the damaged vanes are gently worn to sharp edges. Process had to be gradual.

PUMPDESIGNER

 

[Kawartha]

PUMPDESIGNER

Is the pump discharging into a common header along with other pumps?

Please describe the pump: vertical sump type, end suction, waste water??

 

[hydrae]

Pumpdesigner
Iron bacteria?
The bacteria eats a layer on the non moving parts and develops a coating, but the impellor slings the protective layer off on every pump start. The closed portion of the impellor is thinner than the face so it ate away completely.
Hydrae

 

[quark]

Actually the vanes are tappered to the inner edge and that is why I thought of suction cavitation at first. But it is denied in the webpage and here as well.

Is there a wear ring at the suction which might have caused galvanic corrosion? A shot in the dark with eyes shut will not make any difference if you don't have other options

What do the Ripleys say?

Regards,