A few years ago, I coined the expression “Cosford’s Law.” There was a gap in understanding from neophyte hydraulic specialists that hydraulic actuators moved because flow happened, rather than because force was applied. The truth, of course, is that no mechanical motion occurs without some force being applied.
Cosford’s Law states that “pressure makes it go, and flow is the rate in which you can create force.” I created this not because of narcissism, but to bridge the gap in understanding created by the misuse of the false expression, “flow makes it go.”
I’ve come to another gap in understanding, but this time related to cavitation. Cavitation is even more misunderstood than is the pressure vs flow juxtaposition. I need to be honest with you; cavitation is absolutely, perfectly harmless. Cavitation, that is, little bubbles created from vacuum, are entirely harmless in their genesis. Some understanding of cavitation helps prove my claim that cavitation is innocent.
A cavitation bubble pops into existence spontaneously when its liquid home can no longer keep the gas in saturation. All liquid has dissolved gases (typically just air), and the amount of dissolved gas depends partially on the pressure applied to the liquid. The vast majority of the time, liquids are exposed only to atmospheric pressure. But as pressure increases, more gas can be held in saturation, and as pressure decreases, less gas can be held in saturation. The absolute pressure is less important than the change, because as pressure drops, and the gas releases, it does so in the form of bubbles in the liquid.
Cavitation is the creation of air bubbles; that’s it. Cavitation is no more harmful than the bubbles blown by your 4-year-old daughter. If you took the outlet hose of a cavitating pump and let it flow freely into a bucket, absolutely no harm is done to the pump. It’s when the bubbles reach the discharge side of the pump under high pressure that problems occur as a result of cavitation.
When a gas bubble reaches the pressure side of the pump, it implodes as a super heated jet, and if the bubble was on the surface of metal, it damages that surface. It doesn’t matter if the bubbles were created spontaneously through cavitation, or from aeration, which is when air is drawn into to the inlet plumbing due to vacuum; either condition can cause the bubbles that do damage.
My problem is this; what are the little super-heated implosion jets called? It is this effect, not the spontaneously created bubbles, that cause damage. When you hear that tell-tale sound of ball-bearings rattling loose in your pump, our first response shouldn’t be “it’s cavitating,” but rather, “it’s super-heated implosion jetting.” This description is more accurate, but lacks the quality of a radio ad jingle.
Does anyone know the name of this effect? If not, I may have to conjure up a name for it, just as I have for two of my other creations. One of these is Cosford’s Law, of course. And the other? The Exhintake Cam. Go ahead and google it; yep, I created that.
tom mallard says
It’s called cavitation erosion, generally, https://www.corrosionpedia.com/definition/244/cavitation-erosion
Ken in SoCal says
Cavitation voids are not air bubbles and are more damaging than entrained air bubbles. The cavitation “bubble” is a vacuum pocket created when the line pressure drops below the partial pressure of water at the temperature of the water – Same idea as boiling water at room temperature, if you pull a vacuum on it.. When the line pressure is increased in the pump, the vacuum pocket slams closed and can result in surface damage. Cavitation has been shown to create hydroxide radicals, which is a highly oxidative species. In contract, as an air bubbles passes through a pump, it does not slam close because it was not a pocket of vacuum. Instead, it just compresses. Thus, it does not damage pump surfaces as cavitation does.
William Grissom says
I understand cavitation to be the generation of bubbles of vapor, not air coming out of solution. The vapor is generated when the pressure falls below the saturation pressure (at the current temperature). Certainly air will also come out of solution as pressure falls, but cavitation can occur in water which has no dissolved air. A cavitation bubble violently returns to a liquid as the pressure increases, indeed some Physicists predict that event is a “singularity” that could in some theoretical approximations release enough energy to cause nuclear events, or at least generate visible light. That may be the mechanism which causes damage to metal surfaces. However, just having the large density differences between gas and liquid could also cause damage as a fast-moving metal surface interacts, so air bubbles may be bad as well. I think this is still a research area. I don’t think air bubbles quickly re-dissolve in the water and can linger a long time once they come out of solution. Re-dissolving certainly isn’t a violent event as a cavitation bubble collapsing is.
Linas Repecka says
The term used for collapsing bubbles which cause damage is, indeed, ‘cavitation’. It refers to both the creation and collapse of bubbles, which is why you have not heard of any other term for the collapse part. Your argument is like saying no one dies from falling, they die from hitting the ground. Also, they are not ‘air’ bubbles. Air in the bubbles cushions the collapse and makes the process non destructive. Cavitation is the creation of a cavity in a fluid due to turbulence and fluid inertia.
Steven Johns says
“Cavitation is the creation of air bubbles; that’s it.” Air bubbles? While you are correct in stating that it is the collapse of the bubbles against the surfaces that cause damage, saying that cavitation is the creation of air bubbles ranks right up there with those that say Nuc-u-lar…….
David Shaddock says
Wait–little bubbles created from vacuum? I thought they came from dissolved gas that comes out of saturation when the pressure drops in the liquid…
But I think you might consider ‘imruption’ for the implosion of superheated jet. It’s kind of the reverse of an eruption.
Nice article–I love clear explanations (that I can then borrow to edify other engineers!).
Tom Gartlan says
https://www.youtube.com/watch?v=T5eNhEDlGOE
A different perspective.
Tom Gartlan says
Scan to about 32:00 and 40:00 of this video.
Kevin Nash says
How about “fluid ping” for a name?
Francis X Gentile says
“Re-Pressurized bubble implosion super-heated jet” ? Jet seems to imply it is going in one direction as in a shaped charge, but the bubble is going in all directions? so it would be “Re-Pressurized bubble implosion super-heating”?
you had me for a moment with the exhintake thing, since I have 2 patents on feeding fuel into engines from the exhaust side, but I see the idea is to use an exhaust cam as an intake cam as a cheap custom intake cam.
Francis X Gentile says
Could it be that the low pressure side of the pump and the high pressure side of the pump is enough on the same side of a proppellor wing section that the full cycle of reduced pressure bubble creation and “Re-Pressurized bubble implosion super-heating” could occur during the flow.. across the cambered lifting side of the blade {wing]. Creation might occur around 25 percent chord and erosion further towards the trailing edge? Or upstream bubbles could be pressurized by the leading edge and erosion could occur there? As an non expert I head about cavitation on giant ship and submarine proppellors so that might be the origin of bubble creation and implosion being created in the same area an being labled with the one word associated with the first step, which can occur elsewhere prior.
Josh Cosford says
Wow, so much feedback. I’d like to see some reference to the claim that cavitation is a vacuum pocket, rather than air (saying it’s vapor is redundant … air is vapor). Let’s continue the conversation on the forum:
http://forums.fluidpowerworld.com/forum/main-forum/troubleshooting/34-super-heated-implosion-jets