Diagnose This

[PUMPDESIGNER]

Go to the following page and examine the photographs.

http://www.irrigationcraft.com/diagnose_this_2.htm

What do you think caused the damage?

PUMPDESIGNER

 

[BobPE]

LOL pumpdesigner, you know what my answer is!!! LOL only this time, change "suction" to "discharge." Where do you find such nasty people that hurt their pumps like this????


BobPE

 

[Scipio]

Now that's pretty impressive - looks like someone decided to make the shroud out of a radial saw blade! Well, it looks like something I've seen on Star Trek, but on the off chance it isn't space termites, I'd have to say casing recirculation induced cavitation, either due to prolonged low-flow operation (was that pressure reducer on the suction side?) or improper shroud clearance. Any nobody heard anything? If the cavitation alone wasn't bad enough, I would think the chunks of weakened metal flying off the edge of the impeller at 3600 RPM would make a couple of bangs.

 

[hydrae]

pumpdesigner
Do you have pictures of the volute or diffuser?
I agree with BobPE
Hydrae

 

[PUMPDESIGNER]

I have seen tons of low flow cavitation damage on impellers.
What's with the wormhole pattern. Strange huh?

Almost like a swarm of termites got onto this thing. Weird.


PUMPDESIGNER

 

[BobPE]

pumpdesigner:

the holes are kinda the give away. In discharge cavitation, flaws or defects on the impeller surface that would normally not pose any concern at all in normal conditions become the focal point of cavitation damage. Once the damage starts it becomes a snowball effect, making holes like that. As the damage continues, energy in the flow allows cavitation to occur at other locations, possibly a spin off from the starting point of cavitation, or possibly triggered by the energy imbalances generated from the damage that had occured from previous cavitation.

Another fun post....pump abuse is a terrible thing...lol

BobPE

 

[longeron]

Let me see if I get this. Discharge cavitation is generally caused by recirculation from the O.D. of the impeller around to the suction side of the impeller. This can be caused by loose wear ring clearances and by lower than designed suction pressure. How is it that the damage to the impeller due to this kind of cavitation wouldn’t be of concern at normal operating conditions? Is it because as the pictures show the damage is on the discharge side of the impeller, that damage here doesn’t make it easier for the pumpage to flow back to the lower pressures on the suction side of the pump? Wouldn’t this increase the flow to the stuffing box which (on a single stage overhung pump with balance holes) would be at an intermediate pressure?

Next question. Discharge cavitation is caused by running the pump to the left of the BEP, and suction cavitation is caused by running the pump to the right of the BEP?

 

[PUMPDESIGNER]

The main puzzle to me is why this impeller is so very much different than any other impeller I have seen. BobPEs explanation makes sense and is logical, but does not explain why this impeller looks very much different than other impellers I have diagnosed as recirculation damage.
I put up new photographs with what I would consider "normal" type recirculation damage on the same page:

http://www.irrigationcraft.com/diagnose_this_2.htm

That is really why I posted this, to learn why the difference in the appearance of this impeller from others.

To start the fight, (sorry, I meant discussion), what if this impeller was full cut and riding very close to the cut water? Would that do it?

PUMPDESIGNER

 

[d23]

PUMPDESIGNER:

Look close at the hubs of the impellor. I can’t tell very well from the pictures, but there seems to be some “shiny” metal (may just be camera flash.) You have the impellor do you see signs of thrust wear? What kind of condition was the bearing in? This may be a chicken-egg question, but could or did your customer throttle the flow? Could bearing fatigue have been the beginning of the failure that appears to be leakage?

D23

 

[quark]

Huh.. I was not knowing so far that cavitation or corrosion can make such beautiful patterns to the envy of mathematicians. This is certainly of it's own class.

Damage at outer edge seems to be not related to the damage just below the outer edge. (It looks almost like somebody has done dynamic balancing act)

The MOC of the impellers in the two sets of picters seems to be different(CI and Bronze?). So I cannot take that as a clue or primary evidence.

I would like to agree with Bob. But what is about the damage just below the outer edge of the impeller? Bob, can you enlighten me?

Sorry for more questions. Holmes is dull today

Regards,

 

[PUMPDESIGNER]

Good point about the shiny area on forward outer edge.
The final act for this impeller was that the retaining bolt broke, the impeller slid forward, and rubbed where you see the shiny area.

I do not know operating conditions, motor or bearing information, etc. Totally new pump was installed.

I was tempted to think that the impeller bolt breaking is related, and may be, but who can know?

I am however definitely stuck on the pattern and appearance of those holes, very intriguing.

PUMPDESIGNER

 

[cponnu]

Following points come to my mind:

a. Gas porosities in the bottom impeller would have given way with some amount of recirlulation resulting in the damage observed;Outer peripehry is most likely area for this.
b. Possibly better material selection has resulted in lesser damage in the bottom impeller.
c. High fluid velocities ,but sufficient NPSHA has resulted in cavitation bubble collapsing in discharge area of the impeller( but not resulting in suction cavitation) ,more so on the front shroud where recirculation possibiites are more with clearances between casing and impeller being non-uniform and higher in magnitude compared to back shroud.
d. I would venture to consider the material selection, higher flow velocities-both Vr and Vi inside the impeller(high head might have necessiated this) and recirculation might have resulted in the damage of first impeller.