Pump Cavitation - Suction Side vs. Pressure Side

This sounds like homework. Do you know the cause of cavitation?

The source of cavitation is the intake fluid flow is less, basically, the level in the reservoir ( where the pump is actually sucking water from) will be decreased progressively, to find out the minium amount of fluid needed to prevent cavitation...

Cavitation has been correctly described by several others as cavities formed due to pressures below the vapor pressure of the fluid being moved. In hydrofoil watercraft, the pressure on the low pressure side of the foils can be low enough to cause cavitation, and the implosion of the cavities can be forceful enough to cause pitting of the foil surface.

Your description is a little hard to follow, but if you are saying that a higher level in the reservoir will reduce cavitation, you are generally correct, because it increases suction head pressure. There are however, many causes for cavitation beyond fluid level.

For amusement, see "blue light and cavitation".

" I am looking into observing bubbles that are formed due to cavitation. Would it be better to view from the suction side of the pump or the pressure side? "

Bubbles are not the symptoms of cavitation. Pl. read the cavitation information in detail. It is available anywhere and everywhere on internet. Then this question will be automatically resolved or it will disappear like steam disappears in the air. Wish you all the best.

On the pump discharge side, the cavitation bubbles have collapsed and are no longer visible.

http://www.youtube.com/watch?v=3TTBs3o_09I&feature=related

Are you confusing cavitation with vortex formation?

Level at input side is less will form vortex. Pressure at inlet is less (ie restriction in inlet) will cause cavitation if the pressure drops below the vapour pressure of the liquid.

To check vortex formation, you may best look at the surface of the reservoir. If you have some accumulator/ header on the downstream side you will be able to collect the air bubbles on a top side tapping point.

As someone mentioned, you need a window first then either a high speed camera or a simple strobe (with the main lighting completely out).....one where you can adjust the speed of the strobe to match the pump's speed.

I find the word bubble to not say it properly, its a vacuum void.....that quickly collapses, expands, collapses expands several times (each time smaller than the time before) before it actually stops/finishes....all at a really high speed....we should find/invent a better name that says it better.

You need to mark the end of each vane with a different colour, as you may find one vane is "worse" or "better" than the other(s)....its good to be able to pick the best one....

The noise for example, made by propeller's cavitation is huge, ships and submarines are often located and identified just from this noise alone....even today.....though the noise levels have dropped dramatically since WW2.

A semantic quibble, but they are bubbles in the same sense that steam in a kettle bubbles. The vapor in a cavitation bubble has enough energy in the low pressure pump suction zone to expand and become visible to an eye that happens to be looking in the right spot. As the bubble passes through the impeller and is pressurized, its collapse will cause erosion on the blade of the impeller.

The reason I would like to see another name is simple, a true bubble has a gas/liquid at a higher pressure than the surrounding liquid/gas.

Something wrong with the word "cavity?"

If I had a nickel for every time I've been called "mis-informed", Un-informed, or just plan "full of it", . . . well maybe not rich, but I would be a little less broke! The sad part is sometimes folks like you are right!

The photo attached to your posting appears to be a propeller turning backwards in water in an open tank? Fluid flow in the open is much more complex than in an enclosed pipe! With the propeller in an open channel, the energy imparted from the propeller is not restrained and is free to eddy in many directions. Some of these eddies will have high and low pressure zones, and if thew zones are low enough, bubbles will appear.

I do agree that cavitation has little to do with incipant boiling in a tea kettle, but bubble formation in a tea pot is a slow motion picture of bubbles forming from "nothing" but a change of energy state.

Cavitation is very dynamic and turbulant event, but I have to disagree that a vacuum "bubble" can form in a high pressure zone. Cavitation forms in fluid pressure disturbances, when a portion of the fluid is at a lower pressure than the vapor pressure of the liquid. What makes cavitation so noisy and destructive is how rapidly these bubbles form, collapse, and reform as they travel through a pump.

This is one of the very few times I have to disagree with you, even though you are normally a very agreeable fellow!

In cavitation these are bubbles, that come and go very quickly in a very turbulant environment. Even though the backside of a boat propeller is normally a high pressure zone, there are very turbulant eddies in the zone that under the right conditions will allow bubbles to form within the small "low pressure" parts of the liquid.

Andy,

The cavitation of the ship's propeller is in the hi pressure zone, so I guess we agree on that point. However, the location of the bubbles downstream from the propeller tips is very similar to a phenomena called wake turbulence down stream from wing tips. Within any highly turbulent zone there are high and low pressure points.

With cavitation, the low pressure points are low enough to allow vapor to form bubbles. What make cavitation particularly damaging is the rapid formation, collapse, and reformation of these bubbles.

Bubbles of vacuum will not form unless there is a low pressure point within the high pressure area (high turbulence as in cavitation), or unless there is a localized point of high energy creating a vapor pressure (boiling example).

In my opinion, this is probably the 1% time where you might be less than 100% correct!

Also, I do confess I did not look at the Wiki link. Not enough hours in the day right now.

99% of your last post is still completely wrong

That contention would seem hard to support. The formation of cavities occurs along the low pressure (forward) side of the propeller.

Ried wrote:

Cavitation is very dynamic and turbulant event, but I have to disagree that a vacuum "bubble" can form in a high pressure zone. Cavitation forms in fluid pressure disturbances, when a portion of the fluid is at a lower pressure than the vapor pressure of the liquid.

This is precisely correct, and is supported by the Wikipedia article. Cavities can be carried downstream into a (generally) higher pressure area, but their formation relies on low local pressures. In the propeller shown, the most dramatic cavitation is on the forward surface significantly inward from the tips, at a point where the blade curvature is very great (and where the pressure is therefore very low). (The camera is angled to show the largely white -- from sheet cavitation -- low pressure side of one blade and the mainly black high pressure side of the next.) These cavities are then carried downstream (as you can see from the spiral) and would be expected to oscillate. Some will collapse, causing localized water heating and reemergence of steam bubbles.

You can see that several of the cavitation trails taper off to invisibility within one turn of the prop. The loops in these trails and the general cloudiness of the water behind the prop suggests the presence of eddies and turbulence, as Ried correctly states.

Ried's observation that the propeller appears to be running "backwards" is also correct. In a conventional "cupped" propeller, the concave side of the cup faces rearward, whereas on this propeller, the reverse is true. You can see "sheet cavitation" occurring, and can see that one of these "sheets" has broken off from the prop surface.

I can find little to fault in Reid's post. Perhaps you can give some examples of the 99% that is incorrect?

As per theory

Cavitation is a phenomenon whereby liquid flowing through a device loses its pressure to below its vapour pressure. This forms gaseous bubbles which again collapses as the liquid regains its normal pressure. The collapsing of the bubbles creates a vacuum, which usually even if not at the metal surface, will be too near it and get attached to the metal edge and damage the device through - metal removal (pitting), vibration etc. The next portion is as the bubbles collapse, the liquid rushes in from all sides and at the center of the bubble as the converge an extremely high pressure is created. This water hammer effect creates severe fatigue damage to the metal.

Thus the initiation of the cavitation bubble is at the low pressure area - usually at the suction of the pump, however the discharge end cavitations at the blade/ impeller tips are not very uncommon.

From the same link (wiki)

The physical process of cavitation inception is similar to boiling. The major difference between the two is the thermodynamic paths that precede the formation of the vapor. Boiling occurs when the local vapor pressure of the liquid rises above its local ambient pressure and sufficient energy is present to cause the phase change to a gas. Cavitation inception occurs when the local pressure falls sufficiently far below the saturated vapor pressure, a value given by the tensile strength of the liquid.

To be frank, the boiling bubbles themselves do have cavitation effect on the container.

There are some links to cavitations

1) With some nice sketches :

Vapor bubbles form in a pump inlet whenever the local absolute pressure of the liquid falls below its vapor pressure. These bubbles collapse rapidly and violently when the local absolute pressure increases due to kinetic forces being imparted by the impeller. Cavitation is the rapid formation and collapse of these vapor bubbles. Collapsing cavitation bubbles cause noise, vibration, and erosion of material from the impeller.

2) -Vortex cavitation article (http://cav2001.library.caltech.edu/358/00/b8_006_nagahara.pdf)

c) Turbines - again fall in pressure due to velocity. Cavitation can occur near the fast moving blades of the turbine where local dynamic head increases due to action of blades which causes static pressure to fall. Cavitation also occurs at the exit of the turbine as the liquid has lost major part of its pressure heads and any increase in dynamic head will lead to fall in static pressure causing Cavitation

d) The bubbles form in a lower pressure area because they cannot form in a high pressure area.

e) When a pump cavitates, vapor bubbles form in the low pressure region directly behind the rotating impeller vanes. These vapor bubbles then move toward the oncoming impeller vane, where they collapse and cause a physical shock to the leading edge of the impeller vane.

There are many other literatures available, but may not be really necessaary. There seem to be just a bit of misunderstanding/ communication gap.

The principle of cavitation is that it has to occur in low pressure area - either suction, or any other where the pressure has gone down due to - expansion, velocity or any other reason. But it will be low pressure region nonetheless. It collapses immediately as the flow naturalises and the pressure comes bact to the saturation level. (not necessarily again on the high pressure region, can be at the suction/ low pressure side itself.

Is every one missing the point ?

The source of cavitation is the intake fluid flow is less, basically, the level in the reservoir ( where the pump is actually sucking water from) will be decreased progressively, to find out the minium amount of fluid needed to prevent cavitation...

This is not cavitation this is vortex as I mentioned in #5.

As per Hydraulic institute, you will need approx 1 ft above the suction tapping point (or tank outlet) per ft/sec of flow at its inlet. So if your inlet is at 3 ft/sec, you need at least 3ft of submergence.

However it is not always possible/feasible, for that the choices are

a) Bell mouthing to reduce the speed

b) Vortex breaker (google for details) - basically either channel shape to gradually taper off the velocity, some times like a bathroom basin drain stopper, some times like a strainer or some other shape. This is used to basically disturb the suction streamline that creates the vortex by increasing the section from which the fluid is drawn.

I thought the same, on first reading of the original post. However, he may be thinking that a higher level in the tank causes higher suction head pressure, reducing the likelihood of cavitation.

The quoted statement is hard to interpret, because it is not a logical sentence. Your interpretation is equally possible. The term ventilation is often used to suggest the drawing in of air.

In outboard motors (for boats), there is a "cavitation plate" above the propeller. This is actually an anti-ventilation plate, and has little effect on cavitation.

And the end result is we get confused and OP never clarifies..

Once upon a time, I did get to look through a 1" sight glass to watch pump cavitation. The expansion tank and water level was 10' above the pump impeller, and yet cavitation still occured! Cavitation is not affected by water elevation but by the vapor pressure/pump suction pressure. As engines started using smaller pumps, smaller radiators, and higher temperatures, pressurized cooling systems were developed to prevent cavitation at the water pump.

If a pump is cavitating, mors epressure is required at the pump suction. The only place to "see" cavitation is at the pump suction, and ideally by looking at the eye of the impellor.

Take a look at this.

http://en.wikipedia.org/wiki/Cavitation

your information is towards the end

Sorry should read middle.

Suction cavitation

Suction cavitation occurs when the pump suction is under a low-pressure/high-vacuum condition where the liquid turns into a vapor at the eye of the pump impeller. This vapor is carried over to the discharge side of the pump, where it no longer sees vacuum and is compressed back into a liquid by the discharge pressure. This imploding action occurs violently and attacks the face of the impeller. An impeller that has been operating under a suction cavitation condition can have large chunks of material removed from its face or very small bits of material removed, causing the impeller to look spongelike. Both cases will cause premature failure of the pump, often due to bearing failure. Suction cavitation is often identified by a sound like gravel or marbles in the pump casing.

In automotive applications, a clogged filter in a hydraulic system (power steering, power brakes) can cause suction cavitation making a noise that rises and falls in synch with engine RPM. It is fairly often a high pitched whine, like set of nylon gears not quite meshing correctly.

Discharge cavitation

Discharge cavitation occurs when the pump discharge pressure is extremely high, normally occurring in a pump that is running at less than 10% of its best efficiency point. The high discharge pressure causes the majority of the fluid to circulate inside the pump instead of being allowed to flow out the discharge. As the liquid flows around the impeller, it must pass through the small clearance between the impeller and the pump housing at extremely high velocity. This velocity causes a vacuum to develop at the housing wall (similar to what occurs in a venturi), which turns the liquid into a vapor. A pump that has been operating under these conditions shows premature wear of the impeller vane tips and the pump housing. In addition, due to the high pressure conditions, premature failure of the pump's mechanical seal and bearings can be expected. Under extreme conditions, this can break the impeller shaft.

Discharge cavitation in joint fluid is thought to cause the popping sound produced by bone joint cracking, for example by deliberately cracking one's knuckles.

Cavitation links

http://webwormcpt.blogspot.com/2008/04/how-pump-cavitation-sound-and-looks.html

http://images.google.co.uk/images?q=cavitation&sourceid=ie7&rls=com.microsoft:en-gb:IE-SearchBox&oe=&redir_esc=&um=1&ie=UTF-8&ei=0tCnS7eDA9q5jAfo-eCQAQ&sa=X&oi=image_result_group&ct=title&resnum=4&ved=0CCEQsAQwAw

http://www.youtube.com/watch?v=mQvbispmUF4

http://www.google.co.uk/search?q=propellor+cavitation&hl=en&rls=com.microsoft:en-gb:IE-SearchBox&tbs=vid:1&tbo=u&ei=IdOnS_vKMcyTjAfG8aidAQ&sa=X&oi=video_result_group&ct=title&resnum=4&ved=0CCQQqwQwAw

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

heres a novel use of cavitation

http://www.in.com/videos/watchvideo-hydrosonic-pump-cavitation-heater-3692841.html

I agree with Andy Germany, we need a new word.

I vote for Vubble 8-)

AB1 out.

Thats the best name as of now!!! I hope it was not too much "Trubble"?

And maybe that's why it's called "cavitation" rather than "bubbletation"!

As a general reply to several people:-

The bubbles (as nobody wants to invent a new name!) form in tiny areas of extreme low pressure, either due to imperfections in the basic design of the impeller or when the impeller is revolving too fast for its shape/design or usually because of a combination of the two. They can even form on fixed parts of the vessel/pump where pressure is low, but this is less often.

After forming, the bubbles close up at a very high speed and spring apart again and close up again and spring apart again, many times before they do not exist further. Similar to "clapping".....or oscillation....

It happens at a fairly high frequency, even though the bubbles are now in the relatively high pressure zone after the propeller or impeller.....they "live" for a relatively long time, usually several seconds, but that all depends on several factors....liquid type, temperature, design of impeller, quality of the design, pressure, vacuum etc etc etc...its not a simple matter.

It probably could even be likened to the sound of a bell, rung once (for each individual Bubble for example), the sound gradually dying away...

The energy to make these bubbles comes from the prime mover of the impeller/propeller, it results in a substantial loss of efficiency and extra fuel or power costs....

The picture I posted came from the Wiki article, (I posted the link previously), the ship is moving as I said, from left to right, the corkscrew of bubbles is formed after the water has passed through the prop.

I do not understand how anyone could even dream that this happens BEFORE the water passes through the propeller (as several other posters appear to believe!!).......that shows a level of misunderstanding of the subject that I don't think any post by me or article from a good source can change.....certainly no real Engineer would ever make such an error of thinking/observation.....

I do not understand how anyone could even dream that this happens BEFORE the water passes through the propeller

This might be a huge part of our discussion! Your primary experience with cavitation is in an "open" system, while I have witnessed cavitation in a "closed" system (pipeline). Cavitation can and does occur before and after a fluid impeller.

I have been fortunate to be able to see cavitation occurring at the inlet to a centrifugal pump. As a valve was throttled (affecting suction pressure) the volume of bubbles formation would vary dramatically. For the particular engine cooling system, the solution was to change from an open make-up tank to a closed, pressurized cooling system. Cavitation went away with increase suction pressure.

A centrifugal pump impeller and the ship's propeller are both "impellers", but operate in very different hydro-dynamic environments. In your case, cavitation will normally be behind the impeller; in my example, the cavitation is in front of the impeller.

Thanks for the discussion. My gray cells needed the workout!

You slightly missed the point, sorry, I thought I said it clearly enough, but here goes again. Please concentrate carefully on what I write here and what I have previously written:-

Please look at this picture, taken using a high speed camera, it "stops" the action. Look at the "spiral" form that the cavitation bubbles are making in this particular picture, some people here sincerely believe that these bubbles were formed BEFORE the water passed through the screw....

THAT WAS THE POINT I WAS MAKING.....

Nobody (least of all me!) would say that cavitation Bubbles CANNOT be formed BEFORE the impeller, in fact two posts ago I even mentioned it that it can happen!!!

In fact, if you look carefully, there are large patches of bubbles being formed near the propeller boss, between the blades, one patch has seemingly risen to a point in the top LH corner, but the picture is very old and I may be wrong on that point as its not as the pix has got top quality.

When reading a post, the human brain can get funny, it only reads what it wants to see.......maybe that has happened here.

May I suggest a careful read of all the posts I have made before jumping to any wrong conclusions......

Thanks for your time, again.

The picture I posted came from the Wiki article, (I posted the link previously), the ship is moving as I said, from left to right, the corkscrew of bubbles is formed after the water has passed through the prop.

Evidently, this is the source of your confusion. The cavities form on the surface of the prop in the low pressure area, not in the generally higher pressure area behind the prop. The corkscrew is only evidence of the cavities having formed, and the helical path of the blade through the water. As you can see, in about half a revolution, the corkscrew trail disappears.

If "the corkscrew of bubbles is formed after the water has passed through the prop," and if that is what you think cavitaiton is, then cavitation would not damage propellers. But it clearly does damage propellers.

To clarify the situation, I've added some color coding and arrows.

The prop turns in the direction indicated by the purple arrow, using the blade closest to the viewer as the reference.

The blue arrow indicates the direction of the ships motion.

The brown arrow shows the corkscrew trail disappearing. This trail is from the blade immediately before the one closest to the observer (i.e., the bottom blade).

The high pressure area of the upper vertical blade is highlighted in green.

The low pressure area of the blade closest the observer, (the one showing the cavitation source) is highlighted in red. A cavitation sheet has formed on this blade, which is why is appears white in the original picture.

The area highlighted in yellow is a sheet cloud of cavities that have shed from the prop.

I hope this helps you get reoriented. I await your elucidation re the 99% of Ried's post that you claim to be incorrect. I note that 1 out of 100 of your posts are considered good, (not a bad ratio, albeit not as good a ratio as Ried's) so if you come up with a good answer re Ried's "errors of thinking/observation" perhaps your ratio will improve.

Another one who only reads what he wants to see!!

Please read my comments mentioned as though they are being addressed to someone who is of the opinion that the bubbles are formed BEFORE the water passes through the prop!!!! (Because that is to whom I addressed them...!) I was talking about the spiral form of bubbles which can be clearly seen, after the prop has moved on.....they are left BEHIND as the prop moves on.

I did not say they "happened/produced" afterwards, but they are the visible evidence of what is happening (or should I have written evidense?)

Getting stuck on fine detail and not addressing the "BIG" picture is a failure of many on CR4...... You are not alone....

Have a great day.

How odd. Your post originally numbered 26 has disappeared. The current numbers make it appear that Ried is replying to himself, but addressing you. It also makes it appear that I am replying to Ried's post (now number 26), but quoting from your original, in which you claimed that 99% of what he posted was wrong.

Getting stuck on fine detail and not addressing the "BIG" picture is a failure of many on CR4...... You are not alone....

It is a matter of perspective, apparently. Claiming that 99% of a post is wrong seems like more than a fine detail. I think of 99% as being a big part of the whole. But the ability to present different perspectives is a strength of a forum such as this.

I certainly I agree with your statement that I am not alone. You are not alone either, I hope.

Our OP is a student, if I recall correctly. I'm sure we have given him some things to think about.

I have mis placed my scorecard, but I do believe post 26 was always my post!

And I am not hung up on the 99% hyperboli statement from Mr. Germany. Sometimes some CR4 folks exaggerate their statements or poke fun at each other. I presume Andy was trying to yank my chain, and I did not take any offense by his claim of my error. I still don't. You might want to step back, take a deep breathe and have a chuckle about this long winded discussion.

Tiny bubbles. I suggest opening a beer and contemplating the difference between slow bubble formation and its calming effect (in contrast to the destruction of rapid bubbles from cavitation!)

Skol!

Prost!

I think it was 100% wrong, but wanted to give you the benefit of some small point possibly being right!!

I await your elucidation re the 99% of Ried's post that you claim to be incorrect. I note that 1 out of 100 of your posts are considered good, (not a bad ratio, albeit not as good a ratio as Ried's) so if you come up with a good answer re Ried's "errors of thinking/observation" perhaps your ratio will improve.

I do not think such elucidation by Andy is required. I would rather suggest that over the years I have learned more from Andy's posts that he may have learned from mine!

This is one of the very few times I have disagreed with Andy, and based on his most recent post and clarification we might be closer to the same opinion on this subject.

In the meantime, I will go off into the sunset humming that old Don Ho classic,

"Tiny bubbles, In the sand.

Make me feel happy . . . "

(Go ahead Andy, vote this "Off-Topic!")

Good link and post.

Good article... and post.

I think we should call it Grubbles and define them as related to gravity and admit that we don't know our high from the low.

Andy deserves a GA for posting the image in #20. It does have one speculating. And it would be nice to have dynamic movement related images to work on.

I was wondering about the spiral of bubbles. Do they rise to the surface? (the last time on a ship I was more concerned about the company)

Good question.

The bubbles are naturally far lighter than the surrounding water and tend to rise to the surface (though not all will make it there), this makes the wake of a ship look like its boiling!! Some of course break the surface and dissappear immediately and it sometimes looks like steam in very small amounts, depending upon wind, weather and temperature.....when the water vapour produced is released in tiny amounts....

Its all wasted power and every year much work is done by many companies to reduce the effects with better designed propellers and the like. Its definitely big business......

Many years ago a company added air jets to the outside edge of the props, I know they were trying to curb cavitation, but what the final effects were I have no idea....

So now since the consensus is reached we can close the thread ?

BUT: where is OP? he never answered my basic question - it is vortex or cavitation he meant

i agree - My brain is now cavitated

It would definitely be the Discharge Side, I think it would be much simpler and easier to look into the open end of the pump discharge to see the "Bubbles", at least you'd get an eye full of "Bubbles"! And an ear full from everybody close by!