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Pump Cavitation

Pump Cavitation

Pump Cavitation

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

RE: Pump Cavitation

Throttling will reduce flowrate which normally tends to increase suction pressure.  If you can increase suction pressure high enough, cavitation can be eliminated.  I feel you really should try to increase suction pressure by some other method, such as increasing the diameter and shortening the length of suction piping if at all possible, or throttling with a discharge valve, but if you can't, you can't.  Anyway, you might be able to accomplish the same throttling using air injection.  Air injection also reduces the pump liquid  flowrate.  However, as I have recently learned, adding pressured air may tend to increase the fluid's effective vapor pressure, so you must inject high pressure air into the discharge, not into the pump suction.   

**********************
"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/

RE: Pump Cavitation

I'm not sure that injecting a little air into the inlet stream of a cavitating pump actually stops cavitation, however it seems to quietens (in most cases) the cavitation noise.

Why?  Not sure about this either, maybe it reduces pump flowrate or maybe the air which expands in the impeller eye softens the cavitation noise or maybe a combination of both - but as stated in most cases it works - so if it works why worry. Of course the downside of this is you need a source of compressed air plus a metering valve and then you may have the added problem of getting rid of the air further down the process.

RE: Pump Cavitation

I think it "works" by raising the suction pressure above the boiling point of the suction fluid.  No magic, just changing thermodynamic conditions.

David

RE: Pump Cavitation

Air injection is something we would never do in a petroleum facility unless it was the only method available to keep the pump running just long enough to get a bigger diameter suction line in there.  I would think that the potential for increasing surge pressures and the reduction of flowrate in the piping you would get with the injected air would greatly overweigh the costs to correct the situation properly.  

In high value product appliations, the lost revenue from reduced flowrate, or the shutdown/hot tap to install the air release valves, would be enough by themselves to convince the managers a proper fix would be a super-high priority item.

**********************
"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/

RE: Pump Cavitation

Air injection into the suction of the pump could be an option to reduce flow rate and so decrease the likelihood of cavitation. Not properly the best solution as BigInch advised, anyhow it could be a possibility also to cushion the effect of vapour bubbles implosion and reduce noise. When dealing with high temperature fluid it wouldn't be opportune to throttle the discharge as this could promote cavitation.

The paper attached is interesting.
 

RE: Pump Cavitation

Another point of concern with air injection is the inherent oxygenation of the water and the associated corrosion of steel pipes.

The more oxygenated your water the faster your pipes will corrode.

Ione: Exactly where is this mine of useful information you have access to? That's got to be at least the fourth useful paper you have posted.


 

Always remember, free advice is worth exactly what you pay for it!   

RE: Pump Cavitation

Two questions I learned that you never ask in Saudi Arabia.  1, "Where's the money going?", and 2, "Where did you get that?"  

Just download, save, download, save , download, save ...

**********************
"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/

RE: Pump Cavitation

This topic was touch on in the recent thread on dissolved gases and cavitation. It seem to me that for most pump people the term "cavitation" comes loaded with a lot of baggage. It means collapse of bubbles with very damaging forces on the surfaces where the bubbles collapse. In many cases that is what happens, but the term cavitation actually refers to the creation of bubbles (or cavities) where none existed before. So it seems not to make sense that introducing air can prevent cavitation when you are introducing bubbles.

Air introduction does not prevent bubbles. It prevents damage from bubbles that collapse and cause damage to surfaces, because of liquid impacting it with such force. A significant amount of air in a bubble will prevent the bubble from collapsing to nothing and will act as a cushion between the liquid and the solid surface. Most cavitation noise (but not all) is caused by liquid hammering a solid surface.

RE: Pump Cavitation

That's pretty much it and as we have found out, the introduction of air may actually increase the likelyhood of cavitation by, at least in some cases, raising the effective vapor pressure of the fluid.

**********************
"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/

RE: Pump Cavitation

Another reason for introducing air into a pump inlet is to regulate capacity with some savings on power consumption over valve throttling (Pump Handbook 2nd Ed.2.239, Igor J. Karassik et al)
Seems the introduction of air into a pump inlet can in some cases be of benefit whereas in others it needs to be avoided at all costs.

RE: Pump Cavitation

Maybe it's just me, but I have always been a bit uneasy about injecting air into a hydrocarbon stream and then pressurizing it.

Regards,

SNORGY.

RE: Pump Cavitation

Well don't do it, keep it for water.

RE: Pump Cavitation

Compositepro,

This was my point (maybe my not perfect command of English has distorted the message): a cushion effect on vapour bubbles impact is what you get with air injection. If you cannot exploit other possibilities to avoid cavitation, this could be a method to mitigate effects of cavitation.


ColonelSanders83,

I've appreciated you've found useful the papers previously posted. No secret, to quote BigInch: ""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." and so it is important to stay updated in this matter.
 

RE: Pump Cavitation


I wonder whether a colligative property, such as the lowering of vapor pressure by the addition of a nonvolatile solute, has been tried out to mitigate cavitation in those cases where corrosion, erosion or other unwanted effects aren't felt.

At 35oC water has a VP = 43.4 Torr that drops to 41.9 Torr upon addition of 1 mol/kg NaCl (common salt).

On cold water the addition of 0.1 mol fraction of sucrose dropped the VP of water from 4.6 to 3.8 Torr.

Artisi, BigInch, and many others.. can you enlighten me on this subject ?

Something that is not always mentioned in these forums is that even without active participation, one can learn a lot from reading the experts' contributions.

 

RE: Pump Cavitation

25362,

I've never heard about the approach you've described to increase NPSHa.
If I wanted to increase NPSHa by lowering the fluid vapour pressure I'd firstly try to cool the fluid. Looking at a chart, which shows the pressure vapour vs temperature (for water) I think (but I am far from an expert) that this method is much more effective.
 

RE: Pump Cavitation

25362, the effects you point out are real but a change in vapor pressure of 1.5 Torr is equivalent to 0.005 ft. of head.

RE: Pump Cavitation


Compositepro, I'd say 1.5 Torr = 0.065 ft of water. Indeed, still low, but I didn't insinuate eliminating cavitation, I just referred to mitigating cavitation.
Ione, at ambient temperatures a VP reduction of 1.5 Torr for water is equivalent to about 2oC cooling.  

RE: Pump Cavitation

25362,

Using Wagner and Pruss correlation I got 2.2 Torr (from 22°C to 20 °C), but I also got 9.3 Torr (from 52 °C to 50 °C). The deltaT is the same, but the effect is quite different.

RE: Pump Cavitation


Correct, that is why I said at ambient temperatures.

RE: Pump Cavitation


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 100oC; 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.

RE: Pump Cavitation

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/

RE: Pump Cavitation

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

RE: Pump Cavitation


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 150oF can be detrimental from the viewpoint of cavitation.

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

RE: Pump Cavitation

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

RE: Pump Cavitation

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/

RE: Pump Cavitation

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/

RE: Pump Cavitation


I'm no expert on the subject but it appears to me that a "simple" example smile 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 ?

RE: Pump Cavitation

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/

RE: Pump Cavitation

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.
 

RE: Pump Cavitation

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/

RE: Pump Cavitation

BigInch you got the point once again!

RE: Pump Cavitation

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/

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