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

 

[PUMPDESIGNER]

Kawartha
I gave no one stars yet, was too involved. Will now do so however.

I saved this post entirely.
I appreciated all the comments and I really needed them. I studied the photos and worked up potential models for what could have possibly caused the problem. With your input I am now more confirmed in my opinion and that makes the knowledge more useful to me.

BobPE
I wrote the explanation on the website. I brought up the increase in NPSHa because I wanted people to think about that possibility. But you are correct, that was not an option. Remember who my audience was though. But then again, perhaps I should show a little passion about the failure of the owners to use knowledgeable engineers.

d23
Yes about the water. Surface waters often have high nitrates and phosphorus from fertilizer run-off, much more than you might think. BobPE is correct however a cast iron volute pump should by my experience last 20 years without a problem. Cast iron has no problem handling surface water, but cast iron does not deal well with stupidity.

d23
There seems to be some confusion about the stages.
Stage 1 is the first stage in the flow path. Stage 5 is the last stage in the flow path. Stages 1 & 2 had all the damage. Stages 3, 4, and 5 have no damage, none at all. Richard Neff
Irrigation Craft

 

[BobPE]

pumpdesigner:

I did not realize you wrote that whole thing. yes considering your audience, you were right to bring that to you audiences attention.

I use a factor of safety when designing verticle turbine pumps for NPSHr suppled by the manufacturer. These types of pumps all have high NPSHr curves no matter the manufacturer. You note that the pump manufacturer did not readily advertise NPSHr only solidifies my opinion that the person that designed the pump was not an engineer since they did not know they needed the value. In the USA, that is illegal for that person to have designed that if not a professional engineer. I have sent many problems like this and their illegal designers to court with my reports unfortunately, it is a common problem.

take care

BobPE

 

[ChasBean1]

I think cavitation would more strongly affect softer materials in the impeller versus the carbon steel of the volute. It is the volute and not the impellers that show the main ware. To me, it looks like you have failure points on the volutes of stages 1 and 2 because they are your most susceptible to corrosion at the higher pressures (and yes, perhaps additional cavitation). Looking at the latter stages, which have not yet failed but appear to be near failure, I would guess this to be more of a material selection issue. It appears that a high carbon, low allow steel has been used in an application where corrosion protection is paramount. I would be interested to know the chloride level of the water and the material makeup of the pump casing.

 

[Kawartha]

PUMPDESIGNER,

I gave you a star also. You really got this thread moving, and agree completely with your analysis.

BobPE

As I mentioned in my previous post, this observation is not uncommon with larger submersibles. It is generally the case also that the impeller does not exhibit serious signs of cavitation. It is known that different materials will behave differently in cavitation. There is no question in my mind that a good quality bronze impeller is much more resistant to cavitation than C.I. S.S. and Ni.Al-Bz are far superior to bronze. Duplex SS is somewhat better and I believe that Titanium is used in some high speed impellers because of its superior resistance to cavitation. I think one of the reasons for this is the strong bonds between the molecules, however I can't give you a scientific answer. Another characteristic of the metal which probably affects its resistance to cavitation is the ability bend or flex without shear (this is why C.I. is so poor).

I think that over the years I have seen a list of metals vs resistance to cavitation, but don't remember where to locate it. Maybe someone on this thread will have such info.

 

[PUMPDESIGNER]

I would enjoy much seeing that list of materials relative to cavitation damage.

BobPE
What area of the country you work in? I have a lot of people that I would send to you. I help a lot of people but advise them constantly to hire an attorney and consulting engineer. Because of my position I cannot really help them. I hesitate a little to refer however because to be honest I do not know any engineers that are really qualified. It would take a while for me to find one and then trust him so that I could refer.

ChasBean1
Stages 1 & 2 have the lowest pressure. Water flows from 1 to 2 to 3, etc. with pressure increasing each stage.

ChasBean1
This is my best guess as to why I think brass/bronze would resist cavitation better than iron. The damage is caused by a shock wave emanating from the collapsing bubble (pressures can reach 22,000 psi I read). That shock wave passing over a rigid material like iron would tend to shatter the material. Shock wave traveling over more ductile or resiliant material would not crack or shatter the material.

ChasBean1
Whatever damaged stages 1 & 2 was gone by stage 3.
There are not many things that could be there and then gone.
Stage 5 was perfect. Unfortunately I did not chop out stages 3 & 4 so I cannot say they were perfect like stage 5. PUMPDESIGNER

 

[Togel]

Not an expert by any means, but... I think cavitation by all means is the culprit. The pressure experted by the early stage pumps reduces the suction head on the latter pumps, thus reducing the likelyhood of exceeding vapour pressure. I think that wear probably did not occur on stage two until stage one pump's output pressure dropped below a certain level due to the reduction in the impeller surface area. If this system was to continue, I would predict that all five stages would wear through the stages until pump performance dropped below a certain point and suction could not be achieved.

I repaired many a pump back when I serviced oil and gas wells and wearing at the outside of the impeller was common for us. We dealt with high pressures, corrosive chemicals and high flow rates. Chemical wear was always more severe where flow across an impeller is lesser, i.e inner impeller and wear plate/ring.

Like I said, not even close to an expert, but, I would go after your hydraulic system as it is engineered before I worried about how much I paid for the pump in this case. Cavitation that bad will kill all metals right when you can't afford it.

 

[quark]

Great inputs by all and I back BobPE in particular. Infact this thread created much interest in me that I wanted to give it a good thinking before I comment.After you guy's momentum within one day, I kept on thinking.

I have some ideas and appreciate any reply.

If erosion is the problem the thinning of the casing should be uniform. But I see more thinning near the puncture.

As the puncture appeared at specific intervals of the casing periphery, I am thinking the bubbles were generated at the suction side and were thrown towards the casing. (it is nice to imagine bubbles are equishared by the vanes). There is a possibility of serial blasts after first bubble collapses. So how they travel towards second stage and only to second stage if I am wrong?

One good logic by Bob about the pump operating near BEP at later stages. It convinced me to a great extent.

 

[PUMPDESIGNER]

Togel
I agree with the engineering before worrying about cost.
Two things however brought on that comment:

1. Many other pumps would have worked properly in that application because they have much lower NPSHr values. The pump used dominates the market and is used often because it is the least expensive.

2. I have seen the installation, structure, piping, controls, etc. Although I have much criticism of the design and installation, there is really nothing much more they could have done or even should have done as far as pump submergence. There is no reason to increase submergence to 30 feet at a cost of 10 thousand dollars for a deeper wet well when there are many pumps that cost just a little more and would work properly with 10 feet of submergence.

Interesting thing here. The pumps that have 60% less NPSHr values also have higher performance in terms of efficiency and pressure and flow. There is no advantage to having high NPSHr values which is why many pump manufacturers work hard on getting NPSHr values down as low as possible. PUMPDESIGNER

 

[BobPE]

quark:

each stage adds pressure and the cavitation is overcome by the additional pressure in latter stages in this case. I think if the flow were more to the right of bep then cavitation would progress towards the upper stages. I still think the entire pump and all the stages are operating out of their cure as a whole and individually. Vetricle turbines are difficult to diagnose because of their position in the water column. Usually the first sign of problems is the motor suffers fom vibrational loading if the column will transmit it. I think in this case, the holes in the casing reduced the head and flow and caused the owner to take a look....

BobPE

 

[PUMPDESIGNER]

BobPE
You could be correct about operating to the right of BEP.
I was not monitoring the system before or after pump replacement.
What is it in the photos that leads you to that suspicion. PUMPDESIGNER

 

[ChasBean1]

Pumpdesigner - I Had brain flatulation (5 is the highest pressure)... Looking at the pics I thought corrosion looked pretty high in stage 5 also (like a pitting type corrosion, not just a general oxide layer). It could be the cavitation in the early stages causing little pits in the iron, causing an accelerated attack on the metal in those areas. Or it could in part be sand/particulate as stated by Automatic2 way back when. Exellent topic. -CB

 

[BobPE]

Pumpdesigner:

I hail from Philadelphia....Plenty of bad pump designs and lots of work here!!! If you ever need help by all means let me know....

BobPE

 

[PUMPDESIGNER]

quark
You may be correct about the chemical causing a more even corrosion pattern. The "serial blast" you mention got me thinking of cases of very severe cavitation, is that what you were thinking? I have seen and read about cavitation progressing from small independent bubbles into what they call "super cavitation" where large cavities form, and also another phenomenae called "cloud cavitation".

BobPE
Send me your info (pump@shadow.net) and I will forward it to City of Bal Harbor (north side of Miami Beach). They got a doosy going on there. I get adrenalin rush thinking about you in there. I cannot even express how funny that would be, and it would be easy to. I am tempted to quit my job just to do that one myself. Perhaps it is a long shot, but the kid from out of town cares nothing about the local engineers, and I'm sure you have seen that problem too. PUMPDESIGNER

 

[quark]

PD!

Two characteristics of cavitation are 1. Coalescing of small bubbles into big ones and 2. When one bubble explodes, it's shock causes further blast of other bubbles due to high localised pressures. Generally with damage of that magnitude I thought there was no chance of escaping from one stage to the other.

As I don't have much knowledge and experience with submerged pumps, I couldn't understand NPSHA causing cavitation. Is it only due to reduced suction or any other factor?

If it is due to reduced suction, when the pump operates at higher pressure at further stages the operating shifts to the left and the problem of cavitation should reduce. As the 3rd, 4th and 5th stages are in good condition, Bob might have pointed out this.

One more question from my side. When you opened the pump did you see black bubbles on the casing (in wet condition) which if you press oozes out water and have rusty material inside? I observed some black spots in the photo but can't make anything out of it.

 

[zdas04]

There are is an incredible amount of fear and superstition around cavitation. It is the bug-bear that keeps engineers from sleeping well at night. There are some hard pieces of information though:

- Cavitation is the formation and subsequent collapse of vapor bubbles in the flow. Even if the bubbles are at near perfect vacuum (call it 1 psia for computational simplicity), with 50 psia at the entrance to the volutes you are only talking 50 compression ratios when the bubble collapses. Since cavitation is the most isentropic compression I've ever looked at, the isentropic compression equation would show a maximum isolated temperature of about 1000F - which is pretty hot, but since the bubble is pretty small the BTU's available to boil metal aren't enough for heat to be the culprit.
- No one ever solved a cavitation problem with material science. PUMPDESIGNER, if you find the list of materials with a cavitation-resistance factor, I'll bet long odds that magnitude of difference is pretty small among metals that could reasonably be used for a pump impeller. A few years ago I was having a major cavitation problem with a downhole jet pump (it turns out the suction piping was plugged, but it cost a lot to pull the pump so I tried metalurgy first). When these pumps cavitate, it is always in the throat after the mixing chamber (much like your cavitation in the Volute, my theory is that the bubbles form in the low-pressure area, migrate with the flow, and collapse as the pressure increases). I used 6 different metals (four stainless steels, Silcon Carbide, and Tungston Carbide) and every one of them failed within a couple of hours and after 24 hours the damage pattern was identical on each of the pumps. The only solution to cavitation damage is preventing the bubbles from forming - raise the NPSHa above the NPSHr or lower the NPSHr below the NPSHa. The US Navy has done more theoretical work in cavitation than everyone else put together (they didn't like either the noise or the damage on submarine propellors) and their solution was to improve the effeciency of the "air foil" shape of propellors to significantly lower the NPSHr. They did a lot of really interesting metals work on the props as well, but that was primarily to improve the perfomance in sea-water corrosion modalities.
- The key to understanding cavitation is that noise that the submariners were trying to get rid of. When the bubbles collapse, you have choked flow to fill the void (i.e., since the body of the fluid is at more than about twice the pressure inside the bubble, the fluid flowing into the bubble is traveling at MACH 1). The shock wave (sonic boom) is very dense and very high velocity. When it hits something, the force is transfered and since the wave is very small it can't deform the metal it hits so the irresistable force knocks some molecules from the surface of the immovable object. A few billion of these impacts in a small area make a lot of noise and remove measurable mass.

I hope this helps get the boogy man from under our beds.

David

 

[PUMPDESIGNER]

quark
zdas04 was correct, the bubbles implode not explode. It is not possible to understand cavitation unless one first knows exactly what is occurring.

Concerning the last two posts. Cavitation is not a phenomenae that we can easily use intuition on unless we understand that it involves a phase change of water from liquid to vapor, then back again. That phase change introduces dramatic physics that we would not normally suspect are present. The pressures inside the bubbles exceed 22,000 psi, and light is emitted because a plasma is created (all electrons disassociate from the nucleus). These events create a shock wave, which we then hear and which causes the damage.

For confirmation do a study on Google. We are fortunate in that the US and other governments have poured millions of dollars into studying the phenomenae because it has bearing on national defense (quiet propellers and high speed underwater missles), and ship propeller efficiency (money!). PUMPDESIGNER