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

 

[d23]

PUMPDESIGNER:

Few more questions:

Is the chemical(s) in the tank water soluble chemicals? At what rate were they being injected? What is the vapor (NPSHa) pressure of the chemical?

D23

 

[PUMPDESIGNER]

scalleke - OOOHHH, learned something!!! Thought I might get through the whole year learning nothing, then whammo! Understood immediately your analyses of the back shroud "structural component" concept. Always wondered about that. We use a motor with a thrust bearing, sucker must have worked huh?

d23 - There you go again, metalurgist smetalurgist, you got to be kidding! Sheltered world of yours, they got any job openings there? Like the idea though.

Air probably did enter pump a few times, but I don't know how much could have entered through that opening. I don't think air can cause too much damage.

Do not know anything about the chemical, just that it was a dilute water soluable fertilizer. Probably Llama crap or something.

Check out their site, no drawings. Go to this page to see a photograph. The top photograph "E, F, and G" Motor is not the same, but the pump is about right.

http://www.griswoldpump.com/cent/cent.htm

No diffuser, just plain ol end suction, close coupled, horizontal centrifugal.

PUMPDESIGNER

 

[BobPE]

pumpdesigner:

I think if you give us flow and pressure at BEP and operating flow and pressure you could help us. As I stated in one of my previuos posts, this operating info probably dosnt exist (please confirm this for us). What is the pump curve BEP? maybe we can look at the demands and sum up how he operated....

BobPE

 

[PUMPDESIGNER]

BobPE - Much further back in the discussion you mentioned that you have had experience with irrigation. You are correct to be supsicious of Irrigation, which is a step-child industry where almost anything goes. Original design should always be suspect, as also we should always suspect how the system is operated and how it has been modified. I cannot give definitive answers to those questions. I'm just gonna sit here and wait for the next purchase order and fix it again, perhaps I'll ask for a system survey, which they will refuse to pay for and they will just fix it again. I try to maintain high discipline and really help people, but they refuse most of the time.

I put up the pump curve on the same page:

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

Pump "should be" running 300-400 gpm.
How did you know it has no flow meter? LOL, ain't in great to know ahead of time how things are?

You made me think of something. I recall visiting the guy who bought it in July, 1995. He looked at me and smiled while he told me the pump never seemed to run out of water, he was thrilled with it. At that moment I immediately knew that could be a problem; flat curve pump, people that figure if the pump gives it up why not take it - bad combination. I told him to stay in limits, but ...

Also I noticed an error on my part, 20 hp not 30 hp as I stated above.

There is some cavitation damage on the impeller, located at the base of what vanes are left, but chemicals still stick in my mind as playing an important part.

PUMPDESIGNER

 

[d23]

PUMPDESIGNER:

I thought my little pump world had flat pump curves. Boy was I wrong!

D23

 

[PUMPDESIGNER]

scalleke - I just carefully re-read one of your statements earlier. "cavitation damage takes place where the pressure starts to increase. that is one of the reasons why I co not believe we are looking at a cavitation problem here, the high pressure zones on the impeller are not damaged (unless if a higher res. picture could show otherwise)."

That statement of yours above is the main reason why I could never accept cavitation as the only cause. Cavitation damage occurs where the pressure increases causing voids to collapse, which is usually a specific area and not the entire impeller.

However, the following statement of yours puzzles me.
"As the impeller gets damaged the discharge pressure decreases but also the NPSHR goes up."

Why does NPSHr go up?

I cannot account for the results of a destructive process such as this, specifically how the impeller works at each stage of its degradation. At some point the impeller data no longer resembles the original impeller data, including NPSHr. But the following should be considered: as impeller performance degrades less water moves out against system resistance, which then causes NPSHa to increase, and NPSHr to decrease because the flow rate is moving more and more to the left.


PUMPDESIGNER

 

[PUMPDESIGNER]

BobPE - I will confess that I hold an opinion formed on my own and not read anywhere I know of. I believe that the shock waves created by the collapse of cavitation voids are in fact the exact cause of the damage we attribute to cavitation. I read material from a grad student in a government funded project on cavitation. He claims the following: Pressure in the bubble reaches 22,000 psi just before disappearing, plasma is created, shock wave moves out, light is emitted from the plasma. If that stuff is even remotely true, then that would in fact be the source of the damage we frequently see.

Now your statement from above:
"...shock waves from cavitation do not cause the damage, they are a seconday effect that you hear commonly called pumping rocks....."


PUMPDESIGNER

 

[zdas04]

That grad student is the most quoted individual since Bernoulli. Think about the size of the voids (milimeters), and think about how you could take a 100 psig (for example) chunk of liquid and increase the pressure 220 times in a nanosecond. Can't happen. A bubble containing 22,000 psi even for a fraction of a nanosecond? Can't happen. I also contend that the plasma and flashes of light had more to do with the mushrooms he smoked for lunch than with reality.

Cavitation isn't magical or super-natural. A void space is at a low pressure, when the bubble colapses the liquid enters at sonic velocity. A mass traveling at Mach 1 has some considerable energy to impart ("f" really does in fact equal "ma&quot and the sound of pumping rocks is mostly sonic booms.

David

 

[PUMPDESIGNER]

zdas04 - Have you any source for your information? I have sketchy stuff, doctoral papers and stuff, but not stupid people, and widely accepted in the physics realms. New technology allows them to investigate into the phenomenae more deeply.

zdas04 - Do you believe those shock waves cause the damage we see occuring from cavitation?

BobPE - I think that cavitation can be considered not just the creation of the voids, but the implosion also. Then I can say that cavitation does do the damage, and I think it is the shock waves that do the damage.

Time for a new thread?


PUMPDESIGNER

 

[zdas04]

Pumpdesigner,
No one has done more theoretical and empirical work on cavitation than the US Navy (they really hate the noise when a submarine propellor cavitates, and they have a lot of pumps in steam plants). When I was in Nuclear Power School (lo, those many years ago) there was some confidential work (height of the cold and Vietnam wars) presented on what cavitation is and what the damage mechinism was. By now that stuff should have been made at least partially public. I'll try to look at work today and see if I can find any of the old references.

The bottom line of all of it was the momentum transfer at the end of the bubble collapse was the energy source for all cavitation damage. No plasma, no thousands of pounds of pressure, no funny lights.

David

 

[BobPE]

Pumpdesigner:

No flow meter??? LOL I won't tell you how I knew there was no flow meter on this application!!! LOL As an engineer, you should point out the correct pump to the owner...and define the operating range so that if they choose to damage the pump, you can be justified in charging them to repair it.

Cavitation damage is quite unique. Do a search on the Fluid Hydraulics forum and you will see 2 discussions on the subject that are really good.

By the way, are you leaning towards my answer as cavitation damage??? or am I reading wrong????? LOL....

this was a great post.....

take care...

BobPE

 

[PUMPDESIGNER]

Don't start picking on the irrigation guys now. LOL
You are correct. Verbal warnings don't get it, a pump curve with limits marked would be better.

Not sure, but my final best guess is that both cavitation and chemical did the damage. There is some obvious cavitation, but overall impeller condition shows strong chemical damage as indicated by the smooth and even deterioration pattern especially on the vanes, which left them appearing like they were eaten away smoothly, not as cavitation would do by tearing away the metal roughly in specific areas.

PUMPDESIGNER

 

[Scipio]

Don't over-estimate the usefulness of marking the pump curves with limits either. One of my professors in university always said, "No matter how idiot-proof you make something, sooner or later someone comes along and makes a better idiot."

As a result, I have to admit my first thought when I saw that impeller was somewhere along the line, someone installed an open-faced impeller and nobody will admit to it - I still have a hard time buying that the entire front shroud just 'dissolved', either by chemical attack or cavitation, without one or two good-sized chunks of steel being blown out through the pump discharge, or at least damaging the cutwater.

 

[PUMPDESIGNER]

I too was amazed at the front shroud disappearing.
However the manufacturer of this pump (Griswold) does not manufacture even one single pump with an open impeller, therefore it would be impossible for this to have been an open impeller.

I suppose that fact also kept bothering me to the point where I would figure chemical damage. No way could cavitation just make the front shroud disappear, had to be chemical. Cavitation would eat holes all through the shroud, but could not make it disappear completely.

PUMPDESIGNER

 

[ischgl99]

This guess may be out in left field, but are these pumps by chance being run by VFDs? I read an article on the ABB website about bearing and shaft currents caused by VFD operation of motors. Basically, a current path is formed in the shaft that causes errosion of the bearing races and it seems possible that the current path could be going through the impeller. By having fertilizer in the water, this increases the conductance of the fluid and possiblity of electolytic corrosion of the impeller.

The link to the article is:

http://abb.com/global/abbzh/abbzh25...C1CD4E660D29D77685256CB7004BC905!OpenDocument

Select the bearing currents paper. If you have trouble with it, email me and I will send it to you.

Mike Bensema

http://www.dutchmenservices.com

 

[ccfowler]

Is it really certain that this is truly what's left of the original closed impeller? It is very difficult to believe that any combination of deterioration phenomena would result is such perfectly uniform disintegtation of the closed impeller that no damage from debris chunks from the front shroud (and attached portions of vanes) would not be present (or stuck in downstream passages).

Is it possible that someone replaced the original closed impeller with an open one from another pump series? I would want to verify that the remnant is consistent with the dimensions, vane configuration, and materials of the original impeller.

 

[scalleke]

Are we able to come to a consencus or do we leave it at this?

Pumpdesigner, as the inlet eye of an impeller gets damaged because of cavitation. Bits and pieces will get removed. a section of the inlet vanes can be removed. consider that the inlet eye of the impeller is opened up. In fact the way the liquid travels in in the pump will get longer. Liquid pressure will be able to drop deeper than it did when the impeller was complete. Larger pressure drop in the pump = more NpshR. = more cavitation, even more damage to the inlet vanes ........ the whole impeller can be eaten away.

That does not seem to be the case here though. The front shroud would have let go and catastrophic failure would have been the result.

With the front shroud disappearing over time like this and everything going so smooth my vote goes for electro - galvanic corrosion.

Best regards.
Scalleke

 

[PUMPDESIGNER]

mbensema - No VFD, Direct Across the Line starting only.

scalleke - Truth by concensus, I like that. We can all vote and that will be the truth of the universe. Just kidding, but you bring up an excellent point. I think the majority would go chemical as the cause at this point.

BobPE probably sticks with cavitation, perhaps a little chemical in there too? I just love putting words into people's mouths, especially when they are ornery.

I lean to chemical for majority of visible damage even though there is some obvious cavitation damage at the discharge side of impeller, which is common enough. The most convincting facts are the overall smoothness and evenness of the erosion whereas cavitation is highly erratic and damage is not smooth or even.

PUMPDESIGNER

 

[BobPE]

LOL pumpdesigner....NO NO chemical here....I know better....LOL

and you didn't tell us about the discharge side cavitation damage.....hmmmmmmmmmmmm......

Was there a hole in that impeller pic? I think there was....maybe it was a smooth hole...LOL

I know one thing, I will not be out of a job anytime soon LOL

BobPE