cavitation defination 5

[bsfc9]

while troubleshooting an engine's oiling system problem on our engine dyno. the phrase "pump cavitation" is thrown around rather loosely. can anyone simply define cavitation as it would apply to spur gear type oil pump. this is a "dry sump" oiling system in that the pump is external of the engine and is fed from a tank of hot(240 degree F), deaerated oil. thanks

 

[imok2]

Cavitation means that cavities( gas pockets) are forming in the liquid that we are pumping. When these cavities form at the suction of the pump several things happen all at once.

We experience a loss in capacity.
The efficiency drops.

PD pumps are often selected to move liquids with a low vapor pressure point, or fluids with a lot of entrained bubbles. This means that NPIP required (NPSH) is difficult to test. The Hydraulic Institute establishes the point at the first indication of any of the following.
Cavitation noise is heard.
A 5% reduction in capacity at constant differential pressure and speed
A 5% reduction in power consumption at constant differential pressure and speed.

 

[zdas04]

The definition I learned in pump school in 1971 was "the formation and subsequent collapse of bubbles in continious-phase liquid, resulting in high velocity liquids impinging upon control surfaces". The collapse and high velocity liquid impinging is what does the dammage - actually ripping metal off a smooth surface and changing the performance of that surface.

David Simpson, PE
MuleShoe Engineering

http://www.muleshoe-eng.com

 

[25362]

Since cavitation is started by a local drop in pressure, a NPSH has been instituted and used all through. It is the difference between the absolute pressure head available at the pump inlet and the head that corresponds to the vapour pressure of the pumped liquid.

Liquid enters the pump by the pushing force created by the reduction in pressure at the pump inlet. Whenever the reduced pressure falls below the vapor pressure of the pumped liquid, vapor-filled cavities appear.

When the vapor bubbles enter zones of higher pressure they collapse (implode) vigorously. This effect manifests itself in noise, vibration, serious damage (pitting) to the surfaces against which the bubbles collapse by the developed enormous local stresses, and a reduction of output by obstructing the flow of the pumped liquid.

With gear pumps there is also the possibility of entrained and/or dissolved air by the oil exposed to mechanical agitation, leaks, etc. Upon coming out of solution when pressures drop, air will expand taking up part of the available volume of the moving pump cavities, with a consequent reduction of capacity and a noisy operation, usually dismissed as "cavitation" and let go at that, since gear pumps are PD pumps and thus self-priming. Vibration and pressure pulsations can follow suit. However, a gear pump should work vibration-free and quietly. There are a series of steps that can be taken to diminish and minimize these effects. The Pump Handbook by Karassik et al. lists them in chapter 3.

 

[PUMPDESIGNER]

There are several issues as follows:
There is no argument whatsoever as to the nature of cavitation, it is the formation and subsequent collapse of cavities in a fluid, cavities which form because the pressure on the fluid dropped below the vapor pressure of that fluid.

Air or other entrained gasses are NOT the cause of cavitation, they do not contribute to cavitation, they do not sound like cavitation, and do not cause damage like cavitation. In fact, entrained gasses can prevent cavitation.

The difference between entrained gasses and cavitation is critical, and very large. Cavitation involves a phase change from a liquid to a gas, and then back into a liquid again, all very quickly.

The numbers in the following statement are close but not dead accurate because I am lazy and do not want to look it up, and because the numbers vary based on conditions.

The change back into a liquid is where the damage occurs. This is because the implosion results in the following on a microscopic scale, which obviously causes the damage.
1 - The gas converts back into a liquid in nano seconds.
2 - Pressures inside the microscopic bubbles can easily exceed 20,000 psi.
3 - Temperatures rise to 100,000 degrees F. and higher.

Entrained air can never produce those effects.

Entrained air sounds like bubbles and gurgling and other soft sounds moving through a pressurized system.

Cavitation produces very hard sounds like gravel or metal shot moving through a system.

Cavitation is easily proven sometimes by simply re-producing the sound, then reducing flow rate (or increase incoming head, or reducing temperature) until the sound quite suddenly disappears. The "suddenness" of the disappearance of the noise corresponds to the head pressure suddenly passing the vapor pressure of the liquid.

Because vapor pressure is the key factor, temperature, pressure, and fluid type determine the critical points at which cavitation occurs.

PUMPDESIGNER

 

[25362]

A clarifying note to reinforce what imok2 has said: cavitation has a broader meaning in fluid mechanics and includes the formation of air- or vapour-filled cavities in moving liquids, not only the damaging effect of their implosion on solid boundaries. It broadly includes even bubble formation when water is brought to a boil and the effervescence of carbonated drinks. More specifically it refers to the damage that can be caused by the swift bubble collapse.

I've witnessed the dismembering effect of cavitation on suspended solids in a liquid irradiated with ultrasound waves of the right frequency. Vacuum cavities can be also created by propellers.

If we consider that the forces developed by the collapsing "vacuum cavities", travel at the speed of sound, about 1500 m/s for water, they would cover a distance of 1 mm in just 0.6 microseconds.

 

[PUMPDESIGNER]

There is only one proper definition of Cavitation:
The formation of cavities in the fluid due to a phase change of that fluid due to pressure drop below the vapor pressure of the fluid, and the subsequent collapse of that cavity also due to the reverse phase change.

Phase change being the change of the fluid from a liquid to a gas, and then back to a gas again.

There is an incredibly vast difference between cavities of gas in a fluid by means other than phase change.

Gas bubbles due to other than phase changes are wimps, they have practically no force compared with phase change cavities (bubbles).

Hey, a little informal, but the point is made.

Check the literature, nowhere will you find cavitation defined otherwise that I know of, and certainly not in any known book on hydraulics or pumps.

I know, this is not a complete statement, but accurate enough for the discussion at hand.

PUMPDESIGNER

 

[ubrales]

The definitions of Cavitation in the replies are accurate. Usually cavitation is the formation and subsequent collapse of bubbles formed by negative pressure, around 6 psi or less, absolute. When a perfectly round bubble collapses, a finite amount of force is directed to the center of the bubble which is a point. A point has no dimensions, therefore, when any force is directed to a zero area, the pressure is infinite! In practice, the bubble is never perfectly round and because of this, the real force is around 10,000 psi, not infinite. It is this force that erodes the surface of any solid object when cavitation occurs. Generally, a nucleus (foreign matter, dirt particles) will aggravate the situation.

The problem can be addressed by designing components that do not have wide pressure gradients, by increasing the flow volume thereby reducing velocity, by good filtration to reduce the nuclie, to name some examples.

 

[PUMPDESIGNER]

ubrales,
I have studied cavitation much.
Have never seen any references to nuclei.
Do you have any sources I could read for that?

PUMPDESIGNER

 

[BRIS]

I agree with Pumpdesigner - cavitation is a phase change in the fluid from liquid to gas. The cavities are cavities of the fluid in its gaseous state not cavities of air. I.e the liquid boils. Nuclei (foreign matter) will promote boiling at slightly lower temperatures or put another way the temperature of pure distilled water has to be raised somewhat above 100 degree c before boiling occurs.

The pressure at which cvitation occurs depends on the fluid and its temperature. Water at 20 degree C may start to cavitate at around 6 psi absolute but at 100 degree C cavitation will occure at atmospheric pressure 15 psi. It is basically the boiling point of the fluid at the respective pressure.

Brian

 

[BRIS]

ie nuclei (particles) in the fluid change the boiling point hence the pressure and/or temperature at which the onset of cavitation for that particular fluid occurs. But I have never come across providing filtration to increase the boiling point as a solution to cavitation. It would be a very expensive and impracticable solution - for water I suspect that to have any effect it would need some form of Reverse Osmosis Plant upstream of the point of cavitation !!

Brian

 

[25362]

I've the feeling of being the older of the respondents because I'm basing myself on technical articles on the subject 20 years old and more.

Those articles mentioned two types of cavitation, the normal vapour cavitation mentioned by most or all of pump experts in this forum, and the gas evolving cavitation which generally happens at higher pressures than the liquid's vapour (saturation) pressure. No doubt all efforts should be made to avoid bringing the operating pressure below the vapour pressure of the flowing liquid to avoid normal vapor cavitation.

Gas bubbles, re-dissolve, if at all, at much lower speeds than that at which they are released especially with hydrocarbons. Part of the not re-dissolved bubbles may be recirculated back to suction when operating at low flow rates, in particular when dealing with oversized pumps or pumps working at level control. A centrifugal effect by the rotating impellers may help in adding the circulated gas to the freshly formed gas bubbles if no proper suction venting is supplied.

Since released gas in large quantities may cause reduced capacities, and uneven pump operation, some designers opt to reduce NPSHa by using artificial vapour pressures. These are those pressures that enable a maximum of 3% vol/vol gas release considered a safe limit.

For example, for gas-free clean water at 30^oC the vapour pressure would be 0.0425 bar or 0.4 m water head. However, dissolved air, at saturation, may need an artificial vapour pressure of 0.42 bar (4.2 m water head), ten times more, to stay within those design limits.

Namely, the NPSHa would have to be reduced by 4.2-0.4=3.8m or by some 12.5 ft water head! That could be one reason for making the suction lines of pumps sucking from cooling tower basins, short, vertical and wide.

Pump experts are invited to comment on the above, if only for my own benefit. Thanks.

 

[PUMPDESIGNER]

Bris,
I agree with your statement about nuclei affecting cavitation. What caused my interest is that I loosely define particles as macroscopic, and not as dissolved substances or nearly dissolved substances.

It would indeed require expensive filtering such as RO to affect the boiling point.

25362,
I respect everything I have ever read on this forum by you, at least as far as I remember, I'm not so young any longer either.
It is a fact that NPSHa can be raised by injection of gas.
It is also true that small localized areas can experience an increase in pressure momentarily when gas comes out of solution.

I have a rare article that attempts to understand the effects of dissolved gasses coming out of solution and how that affects cavitation phase changes.
But that article did not get too far, very difficult thing to analyze. I think also that the affect is very small because the formulas that we use to predict cavitation appear sufficiently reliable, and those formulas ignore dissolved gasses to a point.

PUMPDESIGNER

 

[vanstoja]

A rather comprehensive review of cavitation, now 30 years old, is Grein,H., 1974, "Cavitation - an Overview", Sulzer Research Number-1974, pp.87-111 which includes 57 of the very best references on the subject up to that time. Pertinent to the ongoing discussions is the following:
"Concerning the air content in a liquid and its influence on cavitation, F.G.Hammitt published a comprehensive summary of the knowledge gathered by a working group of the IAHR. It regarded as certain that gases present in a liquid in the form of gaseous nuclei exert a determinant influence on the onset of cavitation, and that both the free and dissolved air contents are major factors in cavitation erosion. High gas contents appear to encourage cavitation by causing more bubbles, whereas a high air content (partial pressure of air) inside an imploding cavity reduces the implosion velocity. According to observations by Parkin & Kermen, high air conternts inside the bubbles arise mainly through diffusion during the bubble growth."

 

[PUMPDESIGNER]

I will post up some data and sources I have on hand. It appears that much of this data is very late because the technology required to obtain the information is only available of late. This will take me a few posts over a few days.

From the Swiss Polytechnic Institute (2001)
Swiss National Fund, Project No 2100-057253.99/1
Dr. Mohamed Farhat, Professor Francois Avellan, and Philippe Couty
"Although the cavitation phenomenon has received a great deal of attention, the physical process that governs a single cavity collapse near a solid surface is still not fully understood. The knowledge of such process is obviously necessary to characterize the hydrodynamic load and analyze the erosion mechanisms. It has been shown that the supersonic rebound of the vapor cavity after its collapse is associated with the generation of a strong shock wave that causes material erosion. Furthermore, recent studies have shown a tremendous increase of the temperature in the vapor cavity at the final stage of its collapse leading to sonoluminescence generation. Further investigations are needed to understand the interaction between the dynamics of collapsing cavity and the solid surface as well as the physics behind the sonoluminescence phenomenon and its relationship with the collapse overpressure."

From Another paper by Dr. Mohamed Farhat, Professor Francois Avellan, and Philippe Couty

"These phenomena act as intense pinpoint loads as high as 1GPa (Farhat, 1994) on the material and they are therefore directly involved in the erosion process."

PUMPDESIGNER

 

[PUMPDESIGNER]

D.R. Mills, Auburn University, ~2002 (He may have obtained this from another unstated source)
"The bubbles form at low pressure areas, move to high pressure areas (tip of the impeller blades), and collapse. This collapse causes microjets oriented toward the blade at extremely high pressures (>10,000 atm.). This impact (or cavitation phenomena) causes severe erosion or erosion/corrosion (in a corrosive medium) of the impeller blades. Noise and vibration are detected in the pump."

Institute for Hydraulic Engineering and Water Resources Management
"During the collapse, a stream of liquid through the bubble is formed. These so called Micro Jets with a diameter of 10 - 100 µm reach a velocity up to 200 m/s (LAUTERBORN 1980). Because of the gradient of pressure near the solid walls adjacent, the jets are orientated vertical to the wall surface. If this action is continuous and with high frequency, the material in that zone will be damaged, even high quality steel may be destroyed. Material will be dissolved continuously out of this area. An erosion caused by cavitation takes place which, in the worse case, may lead to the failure of a whole construction."

K.M. Kalumuck and G.L. Chahine
Dynaflo, Inc.
Presented at the Cavitation Conference, 2001, session 4.006
"It is generally accepted that water disassociates under intense cavitation due to the formation and collapse of microscopic bubbles. The maximum pressure may be as high as 1.2 x 104 atm. and the temperature could be about 10,000 K."




PUMPDESIGNER

 

[PUMPDESIGNER]

Data and Source Continued:

Joachim Holzfuss, Matthias Rüggeberg, Andreas Billo August 7, 1998, Physical Review Letters 81, No. 23 (1998)
“The pressure in the shock and in the bubble is shown to have a lower limit of 5500 bars.”
“The reflected shock wave has a duration > 40 ns and is distorted.”
“The duration of the shock pulse can be determined to be 10 ns (FWHM). As this is on the order of the optical pulse length of 7 ns, this value is an upper bound”.
"An inner shock wave launched in the interior of the bubble upon collapse has theoretically been assumed to account for the observed short SBSL light pulse and its spectrum".



Should I continue with posting these references?
Also,
It should be noted that cavitation research sometimes, perhaps often, uses methods to create the cavity that are not the same as pressure drop creating the cavity. They use lasers and sound waves in the laboratory which can produce cavities with different features than normal fluid cavities in the real world, how different I have never been able to ascertain even though I have asked them, I think they do not know how different.


PUMPDESIGNER

 

[zdas04]

PUMPDESIGNER,
Thank you for the impressive literature review. I notice that several of the authors refer to huge pressure increases in force per sq area terms. Is this real? I can see a significant force from the mass being accelerated by the differential pressure. The bubble has a (very small) measurable area, so a force/unit area term is theoretically definable, but aren't we talking about a force being disipated instead of some sort of bulk fluid pressure?

When we talk about tens of thousands of psi, GPa's, and hundreds of atmospheres it calls to my mind relief valves, flange ratings, and MAWP values for casings. None of these things matter in a cavitation scenario. What matters is how much force is impinging upon the metal of the item being cavitated. If a micro ounce (mass) of liquid is accelerated at a rapid enough rate to apply 10 lbf to a 0.001 in^2 surface, then the result is 10,000 psi, but does that mean anything?

Am I thinking wrong here? Maybe I'm just having a problem with the language (I did grow up in the hills of Arkansas, so English is not my first language).

David Simpson, PE
MuleShoe Engineering

http://www.muleshoe-eng.com

 

[25362]

Wouldn't it be of more help to bsfc9 to concentrate on gear pumps as his original query referred to ?

 

[PUMPDESIGNER]

25362,
I agree with you somewhat about the original topic,
bsfc9 is probably not as interested as we are.

zdas04,
I have studied this subject for the past 6 years or so. I accept nothing as fact unless I find multiple reliable sources.

The data is real enough, but what do we do with it?
The scale is small, event duration nanoseconds, so the effects are small.

But contemplating this material has been beneficial to me by giving me an intuitive grasp of the subject including material selection, frequency of occurance, and of course the vast difference between gas bubble collapse by diffusion versus and cavitation bubble implosion by phase change.

PUMPDESIGNER