Pump Cavitation

[bwags]

How does air injection in high velocity pumps help reduce cavitation?

 

[25362]

I'm sorry of having used such a non-technical term as ambient temperature (room temperature).

BTW, the VP lowering of water by salts is tabulated in the CRC Handbook of Chemistry (Ed 77) page 5-109, as a function of moles per liter water at 100^oC; the VP of pure water at this temperature would be 760 mm Hg.

A few examples:
VP drop
mol/L mm Hg

CaCl₂ 5 319.5
MgCl₂ 5 377.0
CaBr₂ 5 368.5
AlCl₃ 3 318.0
LiBr 10 438.0 (more than 50% !)

Regretfully I'don't have a source for the VP reduction at other temperatures, but the general trend is clear.

 

[BigInch]

I don't know. First, additives are usually a big disadvantage unless its drag reducer, or for turning kero into A1. Secondly, I never want to be so close to NPSHr where that seemingly minimal contribution would help. I'd fix the suction system.

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[Artisi]

Don't know about mixing anything with kero - out of my field, but agree running so close to NPSHr is either crazy or you're a masochist

 

[25362]

Thanks to all contributors for their explanations.
I found this link on NPSH that may interest readers.
The attached link implies that, in some pumps, too much of a difference between NPSHA and NPSHR in water systems below 150^oF can be detrimental from the viewpoint of cavitation.

http://www.irrigationcraft.com/cavitation_types.htm

 

[Artisi]

The link may say that - sure - but how can "too much" be detrimental?

 

[BigInch]

No link there explains that one.
I wrote to them and asked them to explain.

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[ione]

Not in absolute terms (I’d dare to add obviuosly), but in specific conditions it has been said that it might be advantageous, to reduce the NPSHA to a "cavitating" pump.

Take a look the link below.

http://www.pump-zone.com/pumps/cent...fugal-pump-can-you-provide-too-much-npsh.html

 

[BigInch]

Couldn't make contact. I got an undeliverable error.

ione. That link appears broken. Can you attach the doc?

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[25362]

I'm no expert on the subject but it appears to me that a "simple" example of how a high NPSHA can indeed create cavitation is when a pump is transferring liquid from a tank at ground level to a larger tank (of almost constant liquid level) at a higher elevation with negligible friction drop and no control.

When the liquid level in the suction tank is high, the NPSHA is at its maximum, but since the differential head to overcome becomes smaller, the flowrate increases rising the NPSHR to cavitating conditions.

In this case a lowering of the suction tank level, with a concomitant reduction in NPSHA can suppress cavitation.

Am I wrong ?

 

[ione]

BigInch,

The link sometimes works and sometimes doesn't.

Better to save a copy on my database too (it is quite interesting)

 

[BigInch]

I thought they might have been talking about higher a/r ratios than that. Its still within the margins where a lot of applications experience cavitation anyway, so I don't count it as unusual, or even as unexpected. In fact, I don't even think it should have been written about in the context of "How much NPSHa is too much?" That doesn't have anything to do with the contents of the article, nor anything to do with the true story, which is that cavitation occurs in a range, not at a point. We even have to define NPSHr at a point related to head, because the onset of cavitation itself is otherwise such a non-specific event.

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[ione]

Given that, in absolute terms, the only detrimental effect of too much NPSH is an higher initial cost, the article was anyhow interesting to me.
The article reported an approach quite unusual to fight cavitation in a specific region.
The article noticed that, in a region where cavitation is absolutely not unusual, an increase of NPSHa, if not enough adequate, can produce a detrimental effect by emphasizing damages produced by cavitation erosion.

 

[BigInch]

The "dosage" has to be right for each patient.

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[ione]

BigInch you got the point once again!

 

[BigInch]

32 replies distilled to one line.

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/