CAVITATION-RECIRCULATION
CAVITATION-RECIRCULATION
(OP)
In which order would you place the following materials for best performance against impeller cavitation. The fluid being pumped is water from cooling towers and has some chloride treatment but chlorides do not come into the equation. The materials we are considering are:
CD4MCU
AL-Ni-Bronze
HARDCHROME 11-13% cast iron.
We have had very good success with AL-Ni-Bronze in the past. The latest pump was mistakenly supplied with cast iron which was a disaster. The supplier had promised hard chrome. Is hard chrome better than the other two materials.
Regards.
CD4MCU
AL-Ni-Bronze
HARDCHROME 11-13% cast iron.
We have had very good success with AL-Ni-Bronze in the past. The latest pump was mistakenly supplied with cast iron which was a disaster. The supplier had promised hard chrome. Is hard chrome better than the other two materials.
Regards.





RE: CAVITATION-RECIRCULATION
BobPE
RE: CAVITATION-RECIRCULATION
FYI
The phenomenon of recirculation was not understood about 30 years back. Even now it is very difficut to separate cavitation from recurculation as the end result is the same. These pumps lift water from a large sump and this is normal practice. Theoretically these pumps were designed to opearate well away from the cavitation point.
RE: CAVITATION-RECIRCULATION
RE: CAVITATION-RECIRCULATION
Please don't take this wrong. I'm just on the outside looking in with very limited data. I do have a couple things you may want to consider.
First:
If the AL-Ni-Bronze has a good proven history you should stay with it.
Second:
From your original post the reason for changing the material is due to cavation. If you are selecting specialty impeller materials then you are doing some expensive pump repairs. While spending money would it be better to invest a little more and make a pump design that further limits or totally eliminates cavation? That would address the actual problem rather than a band-aid repair.
Just a thought. Good luck!
RE: CAVITATION-RECIRCULATION
d23 has fairly well stated my thoughts except for the question of corrosion and real existence of corrosion. If the original design was intended to avoid cavitation, have operating conditions changed to result in the cavitation problems now being experienced?
Is it possible that corrosion may be contributing to the deterioration of the impellers? I make no pretenses about being a corrosion expert, but from my experience, it always seems that if chlorides are in any way present, corrosion of some sort is involved. Can you be certain that corrosion is not enhancing the cavitation damage that you are seeing?
What about trying non-metallic materials?
If cavitation is really the problem, does a new pump or impeller design make sense economically and operationally? Don't be surprised if tolerating some cavitation damage makes more economical sense than replacing the pumps with a cavitation-free design. Also, what about modifying piping or adding inducers to the pumps?
RE: CAVITATION-RECIRCULATION
As mentioned by most of the above posts, you have a situation which is an engineers dilemma. You have to evaluate the costs of dealing with a cavitation problem vs. correcting the problem. You should thoroughly analyse the cost of your options in this case. Perhaps the answer is to continue to replace impellers, casings, etc, and obviously only you can determine the conclusion to this question. The point is that someone should do a full evaluation of the alternatives. I always think that this is what engineering is all about. We look for the most economical solution based on all of the known factors.
If the solution to a complete evaluation is to replace pump parts on a regular basis, that can also be considered to be an engineered solution.
I don't have any direct experience with high chrome iron in cavitation applications but my expectation would be that it would not provide good results. My thinking is that the high chrome irons are very hard, but not flexible and they do not have high tensile strength. This characteristic (tough) is essential in cavitation applications. If the metal is not able to flex without shear, small pieces of metal will flake off as vapour bubbles implode. It is the tough metals (not hard or brittle), which I always hear as being the best materials for cavitation situations.
Sorry, I'm not a metallurgist and don't have all the right terms to describe metal characteristics.
Ni-Al Bronze and Duplex SS are excellent in cavitation applications. However the best material is probably Titanium. I suspect that as another post above suggested, that the basic resistance to chemical attach is also important in resisting cavitation damage.
RE: CAVITATION-RECIRCULATION
Thank you for your input.
RE: CAVITATION-RECIRCULATION
RE: CAVITATION-RECIRCULATION
RE: CAVITATION-RECIRCULATION
Would appreciate your comments regarding use of high chrome CI impellers. Are you using this material in cavitating pumps? How do you compare life with Ni-Al-Bz or Duplex SS.?
Thanks
RE: CAVITATION-RECIRCULATION
I have seen most metals in cavitation service. I usually get to see them after the users had enough and decide that they need help to stop the cavitation. It seems that maintenance people default to hi chrom iron as the last resort since it holds up so well to cavitation, its usually the pump that fails with the impellers still in good shape. They reach this high chrome conclusion after trying most other impeller materials that failed. Cavitation is a strange thing, current science holds that the damage mechanism is heat, not errosion or other mechanical means.
I do respectfully disagree with you however, no system need be designed to cavitate, doing so is not in my opinion a good engineering solution.
BobPE
RE: CAVITATION-RECIRCULATION
It may not be a good engineering decision, but unfortunately business and economic trade-offs can take precedence over engineering. Engineers have to recognize this and design the best systems based on project parameters (generally set by others). For example, suppose that you are involved in design of a refinery where ambient temperatures can drop to -40 DegF. Do you specify Charpy impact testing on all equipment, piping, vessels, etc., or do you relax the specification to -20 DegF. for the "Low metal design temperature" and save many 100's of thousands of greenbacks.
Unfortunately, there are many trade-offs in engineering, some more important than others (ex. safety, environmental, reliability, efficiency, monetary aspects, etc.). You could probably add hundreds of other factors which impact on our designs.
Regards,
RE: CAVITATION-RECIRCULATION
We use high chrome (or CD4MCU) for abrasion resistance only. These include dirty water systems where there is some refractory dust from off-gases. They work extremely well. For our industrial water recycle systems, such as at the water treatment facilities, lift pumps, we find that Al-Bronze performs much much better than cast iron impellers. On small pumps, we have had good success with cast iron impellers. These system are lime feeding or high pH applications. We have not had much luck with SS mag drive pumps (silicon carbide bearings) in particular the fractionator feed pumps (hign pressure and temperatures). Also winter star-up is very harsh on sillicon carbide bearings. However the mag-drive pumps work extremely well for brine applications. The pump was FRP encapsulated, graphite bearings and with self priming tank.
Regards.
RE: CAVITATION-RECIRCULATION
Actually I've also heard it mentioned before that heat is the damage mechanism in cavitation, but never found any supporting reasoning behind it. Had something to do with heat being released during the phase change from a vapour back into a liquid being high enough to vaporize adjacent metal grains. At the same time, intuitively I'd think this would indicate the poorest materials would be ones with low melting points, i.e. elastomer-coated impellers and materials like Ni-Al Bronze, which apparently have very good resistance to cavitation-induced damage.
It'd be interesting to see a list of materials sorted in order of cavitation resistance compared to lists of the same materials sorted by melting point and by surface toughness.
RE: CAVITATION-RECIRCULATION
I'm not sure if this is what you are looking for, but heat will change the vapor point of the liquid or change the NPSHr of the pump allowing cavitation.
RE: CAVITATION-RECIRCULATION
Nope, in this instance from what I recall the theory was that the failure mechanism was intense, pinpoint heat actually generated by the vapour implosion, as opposed to excessive heat in the fluid contributing to formation of the vapour in the first place. I couldn't get my mind around it at the time, pretty much dismissed it until I saw BobPE's reference to it.
RE: CAVITATION-RECIRCULATION
I have not heard about pin-point heat realease during implosion, but it makes sense. Instantaneous release of thermal and mechanical energies would make some materials better for cavitation than others. Cast iron can withstand the least mechanical (impact) forces and is not a good conductor of heat.Ductile iron is slighly stronger mechanically and about the same as cast iron thermally. Al-Bronze is a good conductor of heat and is mechanically tough. Ni-Al-Broze is much more tougher with slightly lower thermal conductivity. It seems to make good sense to me but now that I have acquired a little knowledge, I am dangerous.
Regards.
RE: CAVITATION-RECIRCULATION
I didn't mean any disrespect. I come from the world on consulting engineers. In this world we have no choice but to do the correct design first, then for the cheapest cost. We have no choice since or PE attaches the design to our liability insurance. If owner imposed parameters make me have to perform a design that I do not agree with, I have no choice but to decline and walk away. This is a terrible thing and I have been forced to do it several times. I think consultants take a lot of flack for this and it is a big reason why people say our designs are over designed and we are not in touch with finances. I don't claim to have a solution, believe me. But I do feel strongly that no system need cavitate, but I also respect your point of view.
The heat theory for cavitation damage is very strange. It has to do with the limit of the vapor space as it collapses and approaches zero. As the limit is reached, the area of the vapor pocket gets infitisimally small. The energy at the point of collapse is 100's of thousands of psi. This is converted to heat in an equally infentisimally small area which does the damage. The heat is not enough to heat the mass of the impeller, just the small area at the atomic level. The material is then vaporized into the fluid.
take care
BobPE
RE: CAVITATION-RECIRCULATION
I couldn't disagree with either one of you or your views. I like Bob's opinion that if it is right in the beginning life will be easy. I understand Kwartha's statement that purchasing agents or budgets must be addressed. The only positive thing I can say to both of you is that in my very limited corner of the world it seems that things are slowly changing. Within the last couple of years I think engineering decisions at long last are being made by engineers with considerably less influence by purchasing agents or their budgets. Hope this trend continues here.
D23
RE: CAVITATION-RECIRCULATION
This comment may be a liitle off the point but the repeated comments to the thermal nature of cavitation damage made me think of some recent articles I have read regarding single-bubble sonoluminescence. You may want to take a look at http://web.bham.ac.uk/mjb594/sbsl.html or do a search on Google for more information.
Regards,
Gunnar
RE: CAVITATION-RECIRCULATION
No disrespect taken. I don't really think that we are in total disagreement on this issue. I agree that Engineers have the responsibility to advise clients of best or preferred design practice. If an Engineer knows that a design will not perform as necessary (or as required), he or she is obligated professionally (and legally as per your comments) to inform the client. It seems to me that if you inform the client (in writing), and the client advises use of an alternate design (also in writing), that there is no legal liability to the Engineer. I would not be surprised if some local jurisdictions have different opinions, however I would expect this would be the normal legal position.
RE: CAVITATION-RECIRCULATION
RE: CAVITATION-RECIRCULATION
In I. Karassik's book, "Centrifugal Pump Clinic", he describes pumps which operate "in the break" or under "submergence control". These systems were designed and sucessfully relying on cavitation to limit the flowrate delivered by the pumps. He goes on to describe the process conditions necessary to make use of this control strategy.
I have never seen such a system during my career but that doesn't mean that it can't or shouldn't be done.
Regards,
Gunnar
RE: CAVITATION-RECIRCULATION
RE: CAVITATION-RECIRCULATION
Does the book by Karassik you refer to, contain any special pump materials for operating under those conditions. It would be interesting just to know such systems do exist.
At some of our lift stations, the suction gets throttled by debris and mud and the pumps operate below there design flow. We usually back flush them after several years of operation and the pump goes back ti its design flowrate.
It seems similar to what you mentioned.
Regards.
RE: CAVITATION-RECIRCULATION
Flow-rate control by cavitation is indeed used on small condensate pumps with low heads to avoid wearing the vanes by uneven and excessive cavitation. The intersection of the variable NPSHa curves with that for NPSHr i.e., at full cavitation will determine the flow rate.
When flow of condensate into the hotwell is lower than design its level drops, the pump cavitates, and there is a lower flow-rate point where the new pump NPSHa meets the NPSHr line. When the condensate flow rate into the hotwell increases beyond the present pump withdrawal rate, level rises, with it the NPSHa, altering the cavitation status and the pumps delivers more, at a new intersection of available and required NPSH.
No doubt this control system's main argument is simplicity.
Following reports by Sulzer, cavitation resistance of Cu (Cu/Al/Be or Cu/Al/Ni) alloys in H2S-free water increases with Brinell hardness to below 1 mg/h metal loss for BH values of > 350. On this subject you may ask pump manufacturers, among them, Sulzer Brothers Ltd, Wintherthur, Switzerland.
RE: CAVITATION-RECIRCULATION
cavitation will kill bearings and seals besides making impeller look like swiss cheese.