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.
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"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)
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.
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.
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"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)
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.
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"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)
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.
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.
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"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)
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.
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.
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.
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.
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.
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.
