DIAGNOSE THIS
DIAGNOSE THIS
(OP)
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.
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





RE: DIAGNOSE THIS
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
RE: DIAGNOSE THIS
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
RE: DIAGNOSE THIS
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
RE: DIAGNOSE THIS
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
RE: DIAGNOSE THIS
Don't give it away too quickly. Give us all time to think about it.
I guess your back.
Richard
RE: DIAGNOSE THIS
Please advise the impeller material, casing material, etc. Was erosion observed in the casing as well?
Richard
RE: DIAGNOSE THIS
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!!!
RE: DIAGNOSE THIS
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
RE: DIAGNOSE THIS
-pmover
RE: DIAGNOSE THIS
pmover - Would not bad casting have caused entire impeller to go away?
PUMPDESIGNER
RE: DIAGNOSE THIS
What does the suction screen look like? could it pick up sand?
Hydrae
RE: DIAGNOSE THIS
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
RE: DIAGNOSE THIS
PUMPDESIGNER
RE: DIAGNOSE THIS
RE: DIAGNOSE THIS
you are so patient lol....
BobPE
RE: DIAGNOSE THIS
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
RE: DIAGNOSE THIS
Is the pump discharging into a common header along with other pumps?
Please describe the pump: vertical sump type, end suction, waste water??
RE: DIAGNOSE THIS
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
RE: DIAGNOSE THIS
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,
RE: DIAGNOSE THIS
hydrae - Creative guess. We have seen the results of iron bacteria and mostly that is a deposit issue with little or no damage caused to iron.
Wear ring was bronze, no damage visible. We see bronze wear rings frequently on pumps with iron impellers and no galvanic problems.
I'm ready to give it up if you guys are. The reason I stuck with this was because that impeller sat on my desk for about 1 day with me thinking the problem was cavitation, and yet that impeller just does not look like cavitation, I could not relax. Then I guess at the problem and had someone go to the site and check which verified I was correct. Easy solution too.
PUMPDESIGNER
RE: DIAGNOSE THIS
BobPE
RE: DIAGNOSE THIS
The pattern of the damage was gradual, the damage appeared to be rather even in many areas (but not the back side of the impeller). It appeared that something was attacking the impeller other than an a turbulent process which cavitation is. The only thing left is chemical attack.
I had someone go to the site and look at the system. There was a little tube someone had installed into the pump intake line. That tube ran to a chemical storage tank. We inquired and found out that the owners had been sold a fertilizing system where very dilute fertilizer was constantly being pulled into the system. We called the guy that sold them the system and he said (as most people will say when questioned), "we have had no problems".
Anyway, here is what I think I know.
1 - The nitrates and/or phosphates were very dilute.
2 - The impeller is susceptible more to chemical attack because of its relative high velocity, whereas the volute experiences less velocity.
3 - After the eye shroud of the impeller deteriorated sufficiently serious recirculation began, which caused the front side of the impeller to be attacked more by the recirculating water. It is possible that the front of the impeller was thinner than the back side explaining why the front only is missing. However I think that the front is missing because the water behind the impeller would be far less turbulent than the water on the front side under heavy recirculation conditions, and that additional turbulence exacerbated the chemical attack.
Am I correct that high velocities and high turbulence around impellers can exacerbate or multiply the effects of chemicals?
If so, then how does that process work? How can higher velocity increase the effects of chemicals.
PUMPDESIGNER
RE: DIAGNOSE THIS
You know how I am, so I have to comment....
we pump stuff like that and worse all the time so I would find it hard to believe that the type of primary damage was caused by chemical attack. I think one of the keys is that the impeller exhibits non uniform holes. Chemical attack would be strictly uniform as a function of the mechanism and would not form holes, leaving other metal surrounding the holes more intact.
I hear arguments all the time about this velocity thing. You know what discredits this, look at the other areas of the impeller like the suction, they are under high velocity as well, and no one that prescribes to this thoery can explain why damage did not occur there. Also look at the diffuser section, again high veloity, but no damage. So velocity can not be a contributing factor in chemical attact in this case.
What are the emands like as I hnted to in my original post? This guy was using this pumps as an adjustable tool to acomplich many jobs it seems. I would think, like Quark started to agree, that this would be the root of the problem.
I agree that all hell broke lose when the impeller selfdestructed.
But none the less, this was a fun one.....
BobPE
RE: DIAGNOSE THIS
I'm having some problem understanding your explanation for this damage. I guess BobPE is also.
I think what you are saying is that the chemical attack occured because of the high velocity, and only in specific areas. Are you making a comparison to erosion-corrosion, where the liquid passing the metal surface continuously erodes the protective oxide layer. If that is the case then as BobPE suggested the impeller back shroud should also be corroded since velocity and inertia force is very high in this area also. Even the volute casing, especially the cut water (also a high velocity and high inertia location) should show signs of this damage.
I like the chemical attack theory but have trouble with the lack of damage to other parts of the pump.
RE: DIAGNOSE THIS
I bet that they keep 'ol BobPE around even if he doesn't do anything just so that he can scare the bejeebeees out of the younger engineers and keep them on the straight and narrow.
Kawartha, hold me up and I'll talk to him.
Anyway, I understand the concern about why damage is so uneven on the impeller and very little damage on the volute.
However, explain how this impeller continued to pull cavitation as damage progressed, and long before it got to the station we see in the photo. Severe cavitation would eat away until the pump's ability to create low pressure at it's eye was just insufficient to continue creating cavitation.
Also, Since then we came across this guy one other time. This chemical guy beats his competitors on price because he does not have to install an injection pump. I spoke with him and this is exactly what he said: "OK, so our stuff does eat up impellers, I'll grant you that. But replacing an impeller every 2-3 years is still cheaper than installing an injection pump".
I spoke with him in a friendly way. He had just installed a fertilizing system on another pump station of ours. A Parks Department technician called and said that they had low pressure. Our dealer (different dealer this time) investigated and eventually pulled the pump apart and found the same thing. But this time we knew right off what the problem was.
Best I can figure is that a stream of chemical in the intake line damages one area of the pump but then as the impeller is damaged that stream hits different areas.
PUMPDESIGNER
RE: DIAGNOSE THIS
what your missing is how cavitation occurs in pumps, and a lot of people do miss it....its not an easy concept...even for me sometimes lol
As the impeller wore away, the pressure went away, lowering the pump curve, but the system curve stayed the same. As such as the impeller wore, the operating point continued to move more and more to the right of its curve (or what was left of its curve) which at 3550 rpm could still induce cavitation. This continued until the owner could not take the pumps operation anymore. I could tell from the picture that it was an irrigation pump based on the damage, thats how good I am at this.
Think beyond the pump to help you figure what no one on the oners side was telling you. The pump was in a lake, the water was free, the demands were real, so....use as much water as possible from the lake...this is a typical response in systems like this. In operating like this the owner routinely cavitated the pump albeit at varying levels of demand over BEP based on the pic.
You chemical vendor sees this damage and associates it with the chemical. this is a natural response since all his clients do the same thing with their pumps, by operating them off BEP.
I feed strong oxidents to pump suctions to pleae owners, pumps are great mixers, so thats not the solution. When they hydraulics are correct, the oxidizers eat the impeller and pump uniformly. You almost had it in realizing that the damage was only restricted to the impeller. That was an important piece of information you gave. To me that confirmed the story the picture told....
I am not the creepy engineer you may think, you would be surprised lol. I have seen this before and I learn from experience. I am also very well educated in hydraulics. I dont scare my junior engineers LOL....But I do make them learn how to do this stuff and they enjoy it as much as me.....
and don't get me wrong, these are great posts!!!! But you have to be commended too....in posting these you have to give you opinion just like the rest of us.....
engineers can run, but they can never hide....
BobPE
RE: DIAGNOSE THIS
The impeller being the only damaged part is trouble I admit.
However, as the impeller degraded it became less able to push water out, which lowers the suction pressure, which makes the cavitation go away. At least that is the picture I get over time.
If you are speaking of turbulence causing the rest of the damage I will not say that cannot happen, but turbulence cannot cause the type of damage that cavitation can because cavitation creates shock waves which can eat metal whereas turbulence (from bizarre shape of damaged impeller) cannot.
I don't think you too tough, just making fun.
I was very busy today so I have not had time to ponder your stuff yet. I just posted so that you would not think you won easily.
Still think I'm right.
Sorry.
Laugh out Loud like you always say.
PUMPDESIGNER
RE: DIAGNOSE THIS
However I do not believe we are facing cavitation here. Why, because the damage is too smooth. This is water that would normally eat it's way roughly through anything. Secondly cavitation starts after the inlet of the impeller and the impeller is more eaten away at the beginning than at the end. If this would be cavitation more of the leading edge of the blades would be left over. And to throw on some moree arguments: cavitation happens where pressure increases, and the compression side of the blades is not more damaged than the opposite side.
Is it possible that the fertiliser causes the water to become an electrolyte and that we are looking at galvanic corrosion here? It would be logical that the material would be carried away with the electrolyte water that exits the pump.
Have a great day.
Scallake
RE: DIAGNOSE THIS
Don't confuse my statements with turbulance...I am not talking about that....shock waves from cavitation do not cause the damage, they are a seconday effect that you hear commonly called pumping rocks.....
scallake, that pic didnt look smooth to me, but smooth or rough, cavitation does many different things to pumps, and varying the flow characteristics of the pump will cause cavitation to occur in may places over and over again, making smooth damage if varried enough, making holes if flow conditions dont vary much....
pumpdesigner....you poseted this hear for a reason...I hope its not to prove your right, but rather to learn...If you take anything from what I said, open your mind and think outside the box (or pump in this case) its very rarely the pump that is broken, its the system that is misunderstood...your picking up on the fertelizer input was a good thought, but look at the industry of pumps, we move diluted and non diluted fertilizers all the time with normal iron pumps....that simply cant be the reason.
BobPE.....LOL.....
RE: DIAGNOSE THIS
I cannot get by your experiences with oxidants, got me there. But if those impellers of yours were failing would you know? Do you have long term relationship? Hey, just a thought now, you don't need glasses to you BobPE? LOL
I agree with scalleke. That impeller is smooth on the 15 cm scale, rough on the 1 mm scale. Cavitation damage is usually localized.
I am also concerned about the Ns of 9270. Could that negate the NPSHa over NPSHr of 9.4?
Pump is used strictly for irrigation. Not used for hose bibs, or anything else. Perhaps I should send someone out there now and take a vacuum reading. But then if they call with same problem and there is no chemical system, I will be able to release quickly on this to cavitation.
I still get stuck on the process over time, cannot see this impeller causing cavitation as it went away. Even if cavitation did the initial damage, I think that the impeller eventually got to the point where it could not move enough water out the discharge to continue creating sufficient low pressure at the eye to cause vaporization. If you believe however that turbulence caused cavitation then perhaps I could go there. Something like recirculation cavitation?
How 'bout both. I could write a book on "Cavitation Exacerbated Chemical Attack"
I'll let you guys write the introduction. I could go down into the history books for ...
Anyway, stick with me, I'll get there if you don't wear out.
PUMPDESIGNER
RE: DIAGNOSE THIS
Back on June tenth you said
"NPSHa exceeds NPSHr by 9.4 feet worst case" did you measure or calculate that?
How complicated is the inlet piping, such as, a compact street ell tight to the inlet or a strainer that was never cleaned?
Is there a valve on the suction line the operator would throttle to get more chemical draw?
Hydrae
RE: DIAGNOSE THIS
you stated that the impeller is causing cavitation, this is where you may be stuck. Nothing in a pump causes cavitation, its the system that the pump is connected to that causes cavitation.
You would have seen discharge cavitation (recirc cavitation) on the pump caseing, cutwater, and discharge. You looked at this part of the pump from what I can tell of your posts, and you didn't see this damage. So that can't be it.
I can discredit most cavitation enhanced chemical attack. I just commissioned a replacement pump where a leading national consulting firm claimed corosion attack. It turned out to be cavitation damage from an improper pump selection. They still believe it was corossion attack, but the new pump (exact materials as the pump that was replaced) is not showing damage now.
You stated the pump was for irrigation (now how did I know that???? LOL), sprinkler heads? mobile or fixed? golf course? farm? I will make another bold statement, there is no flow meter on this system, maybe a pressure gauge on the discharge, but none on the suction. I can't tell you how many of these systems I have designed fixes for....Ask the owner how he uses the system the pump feeds...you will get a wonderful story....and it will tell you why the pump is damaged, trust me!!!!
As for my oxident pump systems, yes I keep in touch with them, they all good return clients of mine...the pumps dont cavitate...the system they are working in will not let them. they do corrode away, in about 1.5 years. From casing perforations.....it seems this part of the pump is the cheapest produced part, the impellers are made in a better process....the client is ok with this dmage becuae I told them it would happen.
No, i dont need glasses, i eat my carrots!!!! LOL
BobPE
RE: DIAGNOSE THIS
NPSHR is a figure that will tell you how much the pressure in a pump decreases before it will start to increase. and yes 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).
The issues that cause pressure to decrease in a pump are design related, Shape of the impeller surface finish of the parts dimensioning of the waterway etc. These are all under the control of the pump designer/manufacturer. this makes that NPSHR is a designer manufacturer issue.
The issues that determine NPSHA related are all under the control of the system designer.
Both NPSHR and NPSHA are equal contributers to cavitation problems. I have seen several cases where there was ample NPSHA and there was cavitation damage in pumps.
I do keep on believing that we are seeing a corrosion problem here. For one thing the extensive material removal would never happen gradually in case of cavitation and the unit would have devellopped vibration issues over it's lifetime.
Has the problem been solved by relocating the fertiliser feed?
Should we not focus our research on the reason why the front of the impeller is completely gone and why the backshroud is still intact?
Scalleke
RE: DIAGNOSE THIS
we are focusing on why the impeller disappeared....so far everyone but me thinks the fertilizer did that.... If a pump manufacturer designes a pump to cavitate in its applied operating range, I think they will be out of business rather quickly....
I am really not into pump design, its kinda boaring actually if you ask me...I am into hydraulics....much more exciting.....
NPSHr is not a contributor to cavitation, its just a known constant...NPSHa contributes to cavitation and it is a variable. And as you hinted, NPSHa is a system thing...system things cavitate pumps....
I guess I must be looking at a different picture than everyone, that has to be it....LOL
BobPE
RE: DIAGNOSE THIS
You are right we are focusing on why the impeller disappeared my question is just why is only one portion of the impeller gone. The portion of the impeller that is gone is the front shroud only, the back shroud remained intact even if there are erosion marks.
I do question the design of the pump! Wat about the recirculation flow that took place at the front side of the impeller? Was there a clearance at the tip of the impeller that was too small? That created low pression on the front side of the impeller?
What make and type of pump was this? How is it built?
What are the operating conditions both on the suction and the discharge side? And Bob, you are right we should know more about the construcion and funtioning of the irrigation system? I do not believe this was cavitation but I would not rule that possibility out though.
I'l try to eat some more carrots, maybe that way I can get a better look at the picture.
Scalleke
RE: DIAGNOSE THIS
I was joking about the cavitation enhanced ...
BobPE, you are correct to be suspicious about the conditions, and I cannot put you at ease there because they "do things" I agree.
I have not heard anything about this pump in probably two years. I will inquire about it.
I will attempt to find more photos, or the impeller. People think I'm nuts to save that stuff and they throw it away when I'm not looking, or hide it somewhere.
BobPE - I agree with all your statements about cavitation, especially the ones intended to get a rise out of me.
I am still stuck with scalleke in not seeing how cavitation could possibly produce this. Overall impeller is smooth, although some cavitation damage can be seen at the discharge side at the bottom of one of the vanes. However my not seeing it does not convince even me.
BobPE, How can cavitation continue as impeller performance drops: flow rate drops dramatically, cavitation drops dramatically. If impeller performance against system pressure drops then suction energy required drops also.
PUMPDESIGNER
RE: DIAGNOSE THIS
scalleke - I've thought about that recirculation also. I I posted way back up there in my original explanation that as the eye shroud disappeared (fron chemical) then recirculation had to become huge, which could then add to the disintegration of the front shroud while nothing happens to the back shroud. Now you have the front shroud being eaten on exterior by recirculation cavitation.
PUMPDESIGNER
RE: DIAGNOSE THIS
Pressure on the front of the impeller is more balanced and can be controlled partially by contriolling the gap between the impeller front side and the casing. The front side of the impeller thus needs not to be so strong.
Can you point us to the pump design on the web? A cross section would tell us a lot?
Scalleke
RE: DIAGNOSE THIS
I wonder how many times the chem tank ran dry and gave you an air line to the intake of the pump?
Just for grins did you sent the impeller to a metallurgist for analysis? The pump manufacture may be willing or capable of doing this for you.
Just a thought. Like your new site, it's simple like me.
D23
RE: DIAGNOSE THIS
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
RE: DIAGNOSE THIS
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
RE: DIAGNOSE THIS
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
RE: DIAGNOSE THIS
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
RE: DIAGNOSE THIS
I thought my little pump world had flat pump curves. Boy was I wrong!
D23
RE: DIAGNOSE THIS
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
RE: DIAGNOSE THIS
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
RE: DIAGNOSE THIS
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") and the sound of pumping rocks is mostly sonic booms.
David
RE: DIAGNOSE THIS
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
RE: DIAGNOSE THIS
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
RE: DIAGNOSE THIS
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
RE: DIAGNOSE THIS
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
RE: DIAGNOSE THIS
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.
RE: DIAGNOSE THIS
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
RE: DIAGNOSE THIS
The link to the article is: http://abb.com/global/abbzh/abbzh251.nsf!OpenDatabase&db=/GLOBAL/SEITP/seitp161.nsf&v=17ECA&v=1052&e=us&l=us&url=/GLOBAL/seitp/seitp161.nsf/viewUNID/C1CD4E660D29D77685256CB7004BC905!OpenDocument
Select the bearing currents paper. If you have trouble with it, email me and I will send it to you.
Mike Bensema
www.dutchmenservices.com
RE: DIAGNOSE THIS
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.
RE: DIAGNOSE 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
RE: DIAGNOSE THIS
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
RE: DIAGNOSE THIS
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