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.

 

[gunnarhole]

Thread,

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

 

[Kawartha]

BobPE,

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.

 

[25362]

To bobPE first comment, cavitation is indeed designed for special applications, not for pumps.

 

[gunnarhole]

Gentlemen,

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

 

[25362]

Although there are single-stage, low-temperature, low-energy pumps (generally with drivers smaller than 10 HP) that can withstand low or no-flow conditions w/o much harmful heating, in general, low-flow problems are worse for high-energy units, for pumps handling hot or abrasives-laden fluids, for pumps designed to work at high efficiencies at their BEP, and for pumps for low NPSH. The Pump Handbook by Karassik and co-authors is quite explicit about these factors and the methods of protection.

 

[aberta]

Gunnarhole:

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.

 

[25362]

Aberta: NPSH required (NPSH[sub]r[/sub]) is represented by a single curve rising with flow rates. NPSH available (NPSH[sub]a[/sub]) could be represented by a series of droping curves (almost parallel to the pump's characteristic curve).

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 NPSH[sub]a[/sub] curves with that for NPSH[sub]r[/sub] 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 NPSH[sub]a[/sub] meets the NPSH[sub]r[/sub] line. When the condensate flow rate into the hotwell increases beyond the present pump withdrawal rate, level rises, with it the NPSH[sub]a[/sub], 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 H[sub]2[/sub]S-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.

 

[mlm2]

CD4MCU is the superior material. the solution is to eliminate the cavitation! there are basically 4 types (or sources) of cavitation. a cooling tower could have any or all of them. 1)classic cavitation npsha lower than npshr 2) discharge recirculation - the pump is being operated far to the left of the BEP 3)air ingestion - the most common in cooling tower applications - the pump is picking up the aerated water. the fix is to baffle the sump 4)vane passage syndrom - this one needs a maximum diameter impeller and is identified by the damage being done to the casing only (not the impeller) just past the cutwater.
cavitation will kill bearings and seals besides making impeller look like swiss cheese.