CAVITATION-RECIRCULATION 8

[aberta]

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.

 

[BobPE]

Why people design systems to cavitate is beyond me, but I have seen many applications that were designed to do this. The best material I have seen was high chrome but that is not to say that is the best out there.

BobPE

 

[aberta]

BobPE.
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.

 

[macmil]

High chromium white cast iron comes in various grades. At 11-13% the chrome content is on the low end of the range. I am familiar with high chrome iron with chrome content in the 20-27% range. However other alloying elements can have significant influence on the overall performance. High chrome is frequently used in slurry pump applications with (deliberately) entrained solids. Since you have had success with Al bronze in the past, why not stay with with that ? If wear is an issue, then consider a polyurethane coating on the bronze which can be renewed without actually wearing out the bronze ? Actually, if there is enough left of your old ones, you could restore them in this manner. High chrome has its place, but is difficult to machine without specialised tooling and equipment.

 

[d23]

aberta

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!

 

[ccfowler]

aberta,

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?

 

[Kawartha]

Aberta,

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.

 

[aberta]

Thanks to everyone for their technical contribution on cavitation/recirculation. We are proceeding with Al-Ni-Bronze impeller, as many of you stated no negatives for its use. In all our lift stations, we use Al-Bronze impellers with excellent success. Otherwise we use Cd4MCu or high-chrome CI. We donot have any corrosion issues except in pipelines carrying coke oven gas.
Thank you for your input.

 

[HomeMadeSin]

I'd have to 2nd ccfowlers comment about non-metallics. I've heard good things about plastic impellers absorbing some of the energy from cavitation better than metals. Just a thought, especially as you shouldn't need hardness for solids.

 

[aberta]

ccfowlers comment re; inducers is well taken and we shall pursue that with the pump supplier. Regarding HomeMadeSin's comment regarding plastic impellers: whilst you are probably quite correct, plastics won't make it even as far as the pump before they are dropped, chipped, flame tested, impact tested and any thing else that one can think of in a steel industry environment. Once I tried a palstic back pressure valve in a 1" piping caustic system. It lasted one shift. The operator used a 12" wrench to test how the plunger pump functioned at various back pressure. I became a very quick learner.. no more plastics anywhere!! Thanks again HomeMadeSins, nice to talk to you.

 

[Kawartha]

Aberta,

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

 

[BobPE]

kawartha:

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

 

[Kawartha]

BobPE,

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,

 

[aberta]

Kwartha,
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.

 

[Scipio]

BobPE,

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.

 

[d23]

Scipio

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.

 

[Scipio]

d23

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.

 

[aberta]

Scipio
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.

 

[BobPE]

kawartha:

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

 

[d23]

BOBPE, Kwartha

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