DIAGNOSE THIS ... 11

[PUMPDESIGNER]

What do you think caused this 5 stage turbine damage?
See the photos at:

http://www.fisstate.org/test51.htm

This is not a test, just a desire to have second opinion. Richard Neff
Irrigation Craft

 

[automatic2]

Sand

 

[PUMPDESIGNER]

I do not like sand as the culprit because that would affect all stages of the pump.

By the way everyone, I did not post my theory first because I did not want to affect anyone's thinking. If someone has a theory I never thought of then my opinion stated first might prevent them from posting. Richard Neff
Irrigation Craft

 

[BobPE]

Right Pumpdesigner, it is not sand. "iron worms?" LOL

I would have to lean towards pump runout to the right of bep, causing suction cavitation in the lower stages (that is if the numbering system for the stages goes from top stage to lowest stage) which is overcome as head is added in the upper stages of the pump.

You have to love a picture like that. I think the real problem is lack of engineering or no engineer responsible for the design.

BobPE

 

[electricpete]

Corrosion, erosion or some combination. I'm sure that much is already obvious. I don't know why that pattern.

 

[rcrum]

PumpDesigner,
My guess would be a combination of corrosion and erosion. It would be interesting to see what the impellers look like, since they probably are bronze. Also, the wear ring area (suction side) of the impeller and bowl should give some clues, especially if the pump has a stainless steel wear ring. A lack of wear or erosion on a stainless wear ring would tend to point the finger at a chemical rather than mechanical problem. Sand erosion should leave some indication of wear in the wear ring area, although the area being eroded is typical of the presence of sand.
Five years of operation may not be that bad for this type of pump, it appears to be a small pump (under 8" bowl Dia.) and is probably running at 3500 RPM, which will result in high velocities and relatively rapid wear.
Good Luck,
rcrum

 

[Kawartha]

PUMPDESIGNER,

I've seen this fairly often in submersible "deep well" pumps. It often occurs in pumps with larger bowls, for example 10" and larger. People think that submersible pumps will not cavitate because the impeller is always submersed, but this is certainly not the case. It is not unusual especially when operating at 3600 rpm, to have NPSHr of more than 50' with high flow submersibles in the larger sizes.

However, your holding something back. You didn't show the condition of the 1st and 2nd stage impellers. I'm assuming they are not as badly "iron wormed", or should it be "bronze wormed"? Are there signs of cavitation and if there is, on what side of the impeller vane (high or low pressure)?

I would say this is the result of insufficient NPSHa. I'd be very surprised if this is a 1750 rpm pump.

 

[d23]

PUMPDESIGNER:

Good site.

Common since or cents:

Corrosion, erosion, electrolysis or abrasives will affect the whole pump with one exception. If there are shims between the impellers on the shaft it would be possible that the wrong shim material was used. This would allow “downthrust” to occur at this point. In your pictures I did not see any heat marks to support this. The fact of a 5 year run also takes manufacturing errors out too. Only one solution left:

Heavy duty termites!

 

[CB2]

Can we all assume that each of the 5 stages are the same? That is, the same size impellers/trim.

Internal picture shows the discharge end of the bowl, looks like discharge cavitation to me. Can you determine if stages 1 & 2 impellers were properly set (wear rig gap to bowl)? A pic of the #1 & #2 impeller would sure be of interest.

 

[d23]

PUMPDESIGNER:

I'm not real up on irrigation systems. I could be wrong, but I doubt this is corrosion due to only being the top two stages. With irrigation pumps being feed from a lake do you very often have corrosive water (low ph) to pump? I would have guessed it to be close to neutral in most cases.

One other question is do you have any way to see if the holes in the pump caused any casing damage due to fluid cutting from your holy pump? The replacement pump may be pumping some sand, dirt etc due to holes in the casing.

BOBPI & Kawartha ALL:

I think we need an extermenator to check the well. I still think it's termites!!!

 

[BobPE]

I would think the impellers to be in relatively good shape. The cavitation is occuring on the discharge side, but is it really discharge induced??? I would think vapor cavities are formed somewhere in the suction side of the impeller and are collapsing on the discharge side as shown in the pics.

There is definately a problem because these pumps should last for 20 or more years, even at 3600 rpm...

Abbrasives or corrosion would affect the whole pump uniformly, not just center on one or two areas, so thats not it because the rest of the pump is in good shape....

What else do you know about it pumpdesigner, or are you working with what we know too?

BobPE

 

[d23]

BOBPE:

Before PUMPDESIGNER can answer..... I would have to believe that the impeller of the fourth stage as a min has some damage at least on the tip and the impeller of the fifth stage should have a lot of damage. The reason I say this is because pressure is causing the bubbles to collapse. Am I on the right track with this?

Thanks

 

[CB2]

PUMPDESIGNER

Come back Richard, you have us all worked up over this one!

 

[d23]

PUMPDESIGNER, BOBPE All:

I would like to add one other possibility for the fifth (top) stage. There could have been some pieces of metal from the fourth defuser (bowl) that caused some mechanical damage to the top stage (impeller and defuser.) This may have contributed to part of the failure of the fifth impeller and/or defuser allowing erosion to play a part of the failure on the top stage. Is there a good way to tell the difference?

OK Richard, quit laughing and fess up!

 

[Kawartha]

PUMPDESIGNER,

I reviewed the pictures and by chance visited the "Florida Irrigation Society" homesite, to find an explanation of the pump damage in your picture.

Very interesting and enjoyed the trip with you.

 

[BobPE]

KAwartha:

Thanks for your hint, I found the answer on their site too. I am not happy with their answer as to the reason. It was poor engineering and bad pump selection and lack of flow control. Increasing NPSHa is not a viable solution for them to be mentioning at all since the pump was fitting the sump. If the engineer had the opportunity to design the pump and the sump, then he would make a foolish choice in increasing the sump depth to increase the NPSHa since the cost would be prohibitive compared to the correct pump.

Makes you think though that there are a lot of non wqalified people out there wasting peoples money pretending to practice engineering!!!


BobPE

 

[PUMPDESIGNER]

On same page:

http://www.fisstate.org/test51.htm

Put up photos of the impellers and one more volute photo.

Kawartha, I really was not hiding anything, I swear. You remind me of a 19th century poem, "...But Father, he always suspicions me".

BobPE, Kawartha, d23, and the others you are sharp indeed.

Pump is 6", 3450 rpm, and probably was originally a 1750 rpm design adapted to 3450 rpm. NPSHr is about 30 feet in the area that they were attempting to use the pump.

I believe (with the others) that severe suction cavitation was the cause. As the bubbles were imploded in stages 1 & 2 they caused the damage. By stage 3 all bubbles were gone.

The last photo I put up of the volute may be diagnostic. Notice the pattern of damage on that photo. The path of the bubbles can almost be envisioned by the pattern. The bubbles can be pictured being forced along the volute guide vane, then as they slammed against the outside edge of the bowl they imploded and "ate" the iron.

I would like more knowledge.
Stage 1 impeller shows some pitting, but nothing that would indicate a problem like the stage 1 volute indicates.
Stage 5 impeller is smoother and seems to have no problem.

I guessed at the time I took these photos and studied them 1 year ago that brass or bronze impellers may be less vulnerable to cavitation damage than iron, perhaps because brass is more ductile? Does anyone know about this?

BobPE - Send me some of those faces, I ain't got none. Richard Neff
Irrigation Craft

 

[d23]

BOBPE, Kawartha, PUMPDESIGNER, All:

I cheated!!! We manufacture multi-stage pumps. Been there done that!

BOBPE FYI; From experience with this type of pump I would expect the termites ate the impellers some too.

I still do wonder if there is some way to tell the difference between cavitation and broken pieces starting erosoinal wear especially after five years run. With multi-stage pumps it seems that the cavitation starts at impeller "C" (cavition) and doesn't stop until the top stage. The bubbles will collasp at a given pressure. Does this mean that the top stage(s) are damaged due to abrasives (pump parts) and errosion? Any ideas?

BOBPE:

You and Kawartha nailed this!!! I put the purple stars for you. I also gave PUMPDESIGNER a star for making us think!

 

[PUMPDESIGNER]

BobPE, you are exactly correct about the lack of engineers.
The reasons for those pumps is low price combined with a lack of knowledge, but then are not those sisters?

I took the photos about 1 year ago.
The particular brand of pump is one of the lowest price pumps on the market. I have been interested in their pumps. But when I saw their curves and the NPSHr values on the pumps I blanched at the thought. That manufacturer controls a large portion of the market. And worse yet, as Kawartha said, many of their pumps require submergence of greater than 50 feet, and no one knows.

As Kawartha correctly said, submerged is not protection against insufficient NPSHa. But as long as no one is called to account for the problem?

Richard Neff
Irrigation Craft

 

[BobPE]

pumpdesigner:

what was the suction entrance like? I would assume the factory bell was on there with no other structure, this entrance cavitated too and damaged the first impeller but this was not the cause of the other cavitation as you noted. Brass is extremely suseptible to cavitation, but from the pictures, there is no way cavitation was occuring inside the impellers.

I had another post on here some time ago that presented a new theory on cavitation damage. As we know the common term is that cavitation forms bubbles. In truth, cavitation forms cavities of absolutely nothing and a little fluid vapor, similar to the vacuum of space. When these cavities implode, as the limit of the vacuum cavity volume approaches zero, temperatures can theoritacaly approach the temperature of the sun at a minutely small point. This is considered to be the force that causes vaporization of the metal that the cavity collapses on. In short, cavitation will distroy any metal or material.

This was a good exercise, thanks for the challenge....

BobPE