Editing Cavitation (section)

==Cavitation damage{{anchor|Cavitation erosion}}==
[[Image:Turbine Francis Worn.JPG|thumb|right|upright=1.05|Cavitation damage to a [[Francis turbine]]]]

Cavitation is usually an undesirable occurrence. In devices such as [[propeller]]s and [[pump]]s, cavitation causes a great deal of noise, damage to components, vibrations, and a loss of efficiency. Noise caused by cavitation can be particularly undesirable in naval vessels where such noise may render them more easily detectable by passive [[sonar]]. Cavitation has also become a concern in the renewable energy sector as it may occur on the blade surface of [[Tidal stream generator|tidal stream turbines]].<ref>{{cite journal |last1=Buckland |first1=H. C. |last2=Baker |first2=T. |last3=Orme |first3=J. A. C. |last4=Masters |first4=I. |doi=10.1177/0957650913477093 |year=2013 |title=Cavitation inception and simulation in blade element momentum theory for modelling tidal stream turbines |journal=Proceedings of the Institution of Mechanical Engineers, Part A: Journal of Power and Energy |volume=227 |issue=4 |pages=479–485 |bibcode=2013PIMEA.227..479B |s2cid=110248049 }}</ref>

When the cavitation bubbles collapse, they force energetic liquid into very small volumes, thereby creating spots of high temperature and emitting shock waves, the latter of which are a source of noise. The noise created by cavitation is a particular problem for military [[submarine]]s, as it increases the chances of being detected by [[passive sonar]].

Although the collapse of a small cavity is a relatively low-energy event, highly localized collapses can erode metals, such as steel, over time.<ref>{{Cite journal| last1=Fujisawa| first1=Nobuyuki| last2=Fujita| first2=Yasuaki| last3=Yanagisawa| first3=Keita| last4=Fujisawa| first4=Kei| last5=Yamagata| first5=Takayuki|date=2018-06-01|title=Simultaneous observation of cavitation collapse and shock wave formation in cavitating jet|journal=Experimental Thermal and Fluid Science| volume=94| pages=159–167| doi=10.1016/j.expthermflusci.2018.02.012| bibcode=2018ETFS...94..159F|issn=0894-1777}}</ref> The pitting caused by the collapse of cavities produces great wear on components and can dramatically shorten a propeller's or pump's lifetime.

After a surface is initially affected by cavitation, it tends to erode at an accelerating pace. The cavitation pits increase the turbulence of the [[fluid flow]] and create crevices that act as nucleation sites for additional cavitation bubbles. The pits also increase the components' surface area and leave behind residual stresses. This makes the surface more prone to [[stress corrosion cracking|stress corrosion]].<ref>{{Cite book|author1=Stachowiak, G.W. |author2=Batchelor, A.W. | title=Engineering tribology |url=https://archive.org/details/engineeringtribo00stac_240 |url-access=limited | year=2001 | isbn=978-0-7506-7304-4 | page=[https://archive.org/details/engineeringtribo00stac_240/page/n549 525]|publisher=Elsevier |bibcode=2005entr.book.....W }}</ref>

===Pumps and propellers===
Major places where cavitation occurs are in pumps, on propellers, or at restrictions in a flowing liquid.

As an impeller's (in a pump) or propeller's (as in the case of a ship or submarine) blades move through a fluid, low-pressure areas are formed as the fluid accelerates around and moves past the blades. The faster the blade moves, the lower the pressure can become around it. As it reaches [[vapor pressure]], the fluid [[vaporization|vaporizes]] and forms small [[liquid bubble|bubble]]s of gas. This is cavitation. When the bubbles collapse later, they typically cause very strong local shock waves in the fluid, which may be audible and may even damage the blades.

Cavitation in pumps may occur in two different forms:

====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.

Common causes of suction cavitation can include clogged filters, pipe blockage on the suction side, poor piping design, pump running too far right on the pump curve, or conditions not meeting [[Net positive suction head|NPSH]] (net positive suction head) requirements.<ref>{{Cite news|url=https://info.triangle-pump.com/blog/pump-cavitation|title=Common Causes of Cavitation in Pumps|last=Kelton|first=Sam|date=May 16, 2017|publisher=Triangle Pump Components|access-date=2018-07-16|archive-date=2018-07-16 |archive-url=https://web.archive.org/web/20180716224100/https://info.triangle-pump.com/blog/pump-cavitation|url-status=dead}}</ref>

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 flow velocity. This flow velocity causes a vacuum to develop at the housing wall (similar to what occurs in a [[Venturi effect|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.{{citation needed|date=July 2023|reason=applying the principle of the venturi would require the dynamic pressure to be equal to or greater than the total pressure. Is this the case here?}}

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

====Cavitation solutions====
Since all pumps require well-developed inlet flow to meet their potential, a pump may not perform or be as reliable as expected due to a faulty suction piping layout such as a close-coupled elbow on the inlet flange. When poorly developed flow enters the pump impeller, it strikes the vanes and is unable to follow the impeller passage. The liquid then separates from the vanes causing mechanical problems due to cavitation, vibration and performance problems due to turbulence and poor filling of the impeller. This results in premature seal, bearing and impeller failure, high maintenance costs, high power consumption, and less-than-specified head and/or flow.

To have a well-developed flow pattern, pump manufacturer's manuals recommend about (10 diameters?) of straight pipe run upstream of the pump inlet flange. Unfortunately, piping designers and plant personnel must contend with space and equipment layout constraints and usually cannot comply with this recommendation. Instead, it is common to use an elbow close-coupled to the pump suction which creates a poorly developed flow pattern at the pump suction.<ref>{{cite web| last=Golomb| first=Richard| title=A new tailpipe design for GE frame-type gas turbines to substantially lower pressure losses |url=http://cat.inist.fr/?aModele=afficheN&cpsidt=1454644|publisher=American Society of Mechanical Engineers| access-date=2 August 2012}}</ref>

With a double-suction pump tied to a close-coupled elbow, flow distribution to the impeller is poor and causes reliability and performance shortfalls. The elbow divides the flow unevenly with more channeled to the outside of the elbow. Consequently, one side of the double-suction impeller receives more flow at a higher flow velocity and pressure while the starved side receives a highly turbulent and potentially damaging flow. This degrades overall pump performance (delivered head, flow and power consumption) and causes axial imbalance which shortens seal, bearing and impeller life.<ref>[[Pulp & Paper]] (1992), Daishowa Reduces Pump Maintenance by Installing Fluid Rotating Vanes</ref>
To overcome cavitation:
Increase suction pressure if possible.
Decrease liquid temperature if possible.
Throttle back on the discharge valve to decrease flow-rate.
Vent gases off the pump casing.

===Control valves===

Cavitation can occur in [[control valves]].<ref>[[Emerson Process Management]] (2005), Control valve handbook, 4th Edition, page 136</ref>  If the actual pressure drop across the valve as defined by the upstream and downstream pressures in the system is greater than the sizing calculations allow, pressure drop flashing or cavitation may occur. The change from a liquid state to a vapor state results from the increase in flow velocity at or just downstream of the greatest flow restriction which is normally the valve port. To maintain a steady flow of liquid through a valve the flow velocity must be greatest at the vena contracta or the point where the cross sectional area is the smallest. This increase in flow velocity is accompanied by a substantial decrease in the fluid pressure which is partially recovered downstream as the area increases and flow velocity decreases. This pressure recovery is never completely to the level of the upstream pressure. If the pressure at the vena contracta drops below the vapor pressure of the fluid bubbles will form in the flow stream.  If the pressure recovers after the valve to a pressure that is once again above the vapor pressure, then the vapor bubbles will collapse and cavitation will occur.

===Spillways===
When water flows over a dam [[spillway]], the irregularities on the spillway surface will cause small areas of flow separation  in a high-speed flow, and, in these regions, the pressure will be lowered. If the flow velocities are high enough the pressure may fall to below the local vapor pressure of the water and vapor bubbles will form. When these are carried downstream into a high pressure region the bubbles collapse giving rise to high pressures and possible cavitation damage.

Experimental investigations show that the damage on [[concrete]] chute and tunnel spillways can start at clear water flow velocities of between {{cvt|12|and|15|m/s|mph|}}, and, up to flow velocities of {{cvt|20|m/s|mph|||}}, it may be possible to protect the surface by streamlining the boundaries, improving the surface finishes or using resistant materials.<ref name="Volkart_Rutschmann_1984">{{cite conference |author1=Vokart, P. |author2=Rutschamnn, P. |title=Rapid Flow in Spillway Chutes with and without Deflectors – A Model-Prototype Comparison |conference=Proc. Intl. Symp. on Scale Effects in Modelling Hydraulic Structures, IAHR, Esslingen, Germany |editor=Kobus, H. |id=paper 4.5 |year=1984}}</ref>

When some air is present in the water the resulting mixture is compressible and this damps the high pressure caused by the bubble collapses.<ref name="Peterka_1953">{{cite conference|author=Peterka, A.J. |title=The Effect of Entrained Air on Cavitation Pitting |book-title=Joint Meeting Paper, IAHR/ASCE, Minneapolis, Minnesota, Aug. 1953 |pages=507–518 |year=1953}}</ref> If the flow velocities near the spillway invert are sufficiently high, aerators (or aeration devices) must be introduced to prevent cavitation. Although these have been installed for some years, the mechanisms of air entrainment at the aerators and the slow movement of the air away from the spillway surface are still challenging.<ref name= "Chanson_1989a">{{cite journal| author=Chanson, H. |title=Study of Air Entrainment and Aeration Devices |journal=Journal of Hydraulic Research |volume=27 |issue=3 |pages=301–319 |issn=0022-1686 |url=http://espace.library.uq.edu.au/view.php?pid=UQ:9385 |year=1989 |doi=10.1080/00221688909499166 |bibcode=1989JHydR..27..301C |author-link=Hubert Chanson}}</ref><ref name="Chanson_1989b">{{cite journal|author=Chanson, H. |title=Flow downstream of an Aerator. Aerator Spacing |journal=Journal of Hydraulic Research |volume=27 |issue=4 |pages=519–536 |issn=0022-1686 |year=1989 |doi=10.1080/00221688909499127 |bibcode=1989JHydR..27..519C |author-link=Hubert Chanson |url=http://espace.library.uq.edu.au/view.php?pid=UQ:9386}}</ref><ref name="Chanson_4">{{cite journal |author=Chanson, H. |title=Aeration and De-aeration at Bottom Aeration Devices on Spillways |journal=Canadian Journal of Civil Engineering |volume=21 |issue=3 |date=June 1994 |pages=404–409 |doi=10.1139/l94-044 |issn=0315-1468 |author-link=Hubert Chanson |url=http://espace.library.uq.edu.au/view.php?pid=UQ:9317}}</ref><ref name="Chanson_1995">{{cite journal |author=Chanson, H. |title=Predicting the Filling of Ventilated Cavities behind Spillway Aerators |journal=Journal of Hydraulic Research |volume=33 |issue=3 |pages=361–372 |issn=0022-1686 |year=1995 |doi=10.1080/00221689509498577 |bibcode=1995JHydR..33..361C |author-link=Hubert Chanson |url=http://espace.library.uq.edu.au/view.php?pid=UQ:9322}}</ref>

The spillway aeration device design is based upon a small deflection of the spillway bed (or sidewall) such as a ramp and offset to deflect the high flow velocity flow away from the spillway surface. In the cavity formed below the nappe, a local subpressure beneath the nappe is produced by which air is sucked into the flow. The complete design includes the deflection device (ramp, offset) and the air supply system.

===Engines===
Some larger [[diesel engine]]s suffer from cavitation due to high compression and undersized [[cylinder (engine)|cylinder]] walls. Vibrations of the cylinder wall induce alternating low and high pressure in the [[water cooling|coolant]] against the cylinder wall. The result is pitting of the cylinder wall, which will eventually let [[cooling fluid]] leak into the cylinder and combustion gases to leak into the coolant.

It is possible to prevent this from happening with the use of chemical additives in the cooling fluid that form a protective layer on the cylinder wall. This layer will be exposed to the same cavitation, but rebuilds itself. Additionally a regulated overpressure in the cooling system (regulated and maintained by the coolant filler cap spring pressure) prevents the forming of cavitation.

From about the 1980s, new designs of smaller [[gasoline]] engines also displayed cavitation phenomena. One answer to the need for smaller and lighter engines was a smaller coolant volume and a correspondingly higher coolant flow velocity. This gave rise to rapid changes in flow velocity and therefore rapid changes of static pressure in areas of high heat transfer. Where resulting vapor bubbles collapsed against a surface, they had the effect of first disrupting protective oxide layers (of cast aluminium materials) and then repeatedly damaging the newly formed surface, preventing the action of some types of corrosion inhibitor (such as silicate based inhibitors). A final problem was the effect that increased material temperature had on the relative electrochemical reactivity of the base metal and its alloying constituents. The result was deep pits that could form and penetrate the engine head in a matter of hours when the engine was running at high load and high speed. These effects could largely be avoided by the use of organic corrosion inhibitors or (preferably) by designing the engine head in such a way as to avoid certain cavitation inducing conditions.