lec10 cavitation

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C vit tiavitation

Dr. Ha d Kandil

Concept of CavitationCavitation means different things to different people:

Some say when a pump makes a rattling or knocking sound alongw v ra ons, s cav a ng.

Some call it slippage as the pump discharge pressure slips and.

When cavitating, the pump not only fails to serve its basicur ose of um in the li uid but also ma ex erience internal

damage, leakage from the seal and casing, bearing failure, etc.

In summary, cavitation is an abnormal condition that can result in loss of production, equipment damage and worst of all,

personnel injury.

’ ,correctly identify the type and cause of the cavitation andeliminate it. A ood understandin of the conce t is the ke totroubleshooting any cavitation related pumping problem.



In the context of centrifugal pumps, the term cavitation mp es a ynam c process o orma on o u es ns e e liquid, their growth and subsequent collapse as the liquid flows throu h the um .

Generally, the bubbles that form inside the liquid are of twotypes: Vapor bubbles or Gas bubbles.

Vapor bubbles are formed due to the vaporization of a processliquid that is being pumped. The cavitation condition induced by

as Vaporous Cavitation.

in the liquid that is being pumped (generally air but may be anygas in the system). The cavitation condition induced by theorma on an co apse o gas u es s common y re erre o

as Gaseous Cavitation.

where the local static pressure is less than the vapor pressureof the liquid (vaporous cavitation) or saturation pressure of thegas (gaseous cavitation).

Vaporous cavitation s e mos common orm o cav a on oun n process p an s.

Generally it occurs due to insufficiency of the available NPSH orinternal recirculation henomenon.

It generally manifests itself in the form of reduced pumpperformance, excessive noise and vibrations and wear of pump

.relatively minor amount of pitting after years of service to

catastrophic failure in a relatively short period of time.

Gaseous cavitation

pump along with liquid. A centrifugal pump can handle air in therange of 0.5 % by volume. If the amount of air is increased to

, e pump s ar s cav a ng. It seldom causes damage to the impeller or casing. The main

effect of aseous cavitation is loss of ca acit . Let us first understand the mechanism of cavitation, i.e. how

cavitation occurs. Unless otherwise specified, the term.



The phenomenon of cavitation is a stepwise process as shown in Figure 2.

Step One, Formation of bubbles inside the liquid being pumped.

e u es orm ns e e qu w en va or ses .e. asechange from liquid to vapor. But how does vaporization of the liquid occur durin a um in o eration?

Vaporization of any liquid inside a closed container can occur if

either pressure on the liquid surface decreases such that itbecomes equal to or less than the liquid vapor pressure at theoperating temperature, or the temperature of the liquid rises,

than the operating pressure at the liquid surface.

For exam le, if water at room tem erature (about 77 °F) is ke t in aclosed container and the system pressure is reduced to its vaporpressure (about 0.52 psia), the water quickly changes to a vapor.

us e n a c ose con a ner, vapor za on o e qu can occurin centrifugal pumps when the local static pressure reduces belowthat o the va or ressure o the li uid at the um in tem erature.



How does pressure reduction occur in a pump syste ?

e re uc on n oca s a c ressure a any o n ns e epump can occur under two conditions:

. greater than that considered during design. As a result, the

pressure available at pump suction is not sufficiently highenough to overcome the design pressure drop inside the pump.

2. The actual pressure drop inside the pump is greater than that.

The mechanism of pressure reduction in the external and



Pressure reduction in the external suction system of the pump

level. The vessel can be pressurized or can be at atmosphericressure or under vacuum.

The pressure just before the pump, hms, is given by:

H = h – h + h +h – V 2 2

= hs – hls – Vs2/2g

s

,

the static pressure.

ms

pressure, Pv, at some point inside the pump the static pressure

In pumping terminology, the head difference term corresponding2 -atm ms vap

or NPSH.

,

the pump may be reduced to a value lower than that of the liquid

.

Pressure reduction in the internal suction system of the pump

e pressure o e u a e suc on ange s ur erreduced inside the internal suction system of the pump.



Flow path of fluid inside the pump

The internal suction system is comprised of the pump’s suction. .



In Figure 7, it can be seen that the passage from thesuction ange point 2 to t e impe er suction zone(point 3) and to the impeller eye (point 4) acts like aven ur .e. ere s gra ua re uc on n e cross-

sectional area.

In the impeller, the point of minimum radius (eye) withre erence to pump center ine is re erre to as t e “eye” othe impeller (Figure 8).



How pressure reduction occurs as theu ow n e e um

According to Bernoulli’s principle, when a constant amount of liquidmoves through a path of decreasing cross-section area (as in aventuri), the velocity increases and the static pressure decreases.

.of minimum cross-section, the velocity is at a maximum and thestatic ressure is at a minimum.

The pressure at the suction flange decreases as the liquid flowsfrom the suction flange, through the suction nozzle and into theimpe er eye. T is ecrease in pressure occurs not on y ue to t eventuri effect but also due to the friction in the inlet passage.

the liquid strikes and loads the edges of impeller vanes. The neteffect of all the ressure dro s is the creation of a ver low-pressure area around the impeller eye and at the beginning of thetrailing edge of the impeller vanes.

The pressure reduction profile within the pumps ep c e n gure .



,

n ess ere s no c ange n e o era ng con ons, newbubbles continue to form and old bubbles grow in size.

impeller eye to the impeller exit tip along the vane trailing edge.

Due to im eller rotatin action, the bubbles attain ver hi hvelocity and eventually reach the regions of high pressure withinthe impeller where they start collapsing.

e e cyc e o a u e as een est mate to e n t e or erof 0.003 seconds.



, As the vapor bubbles move along the impeller vanes, the

pressure around the bubbles begins to increase until a point isreached where the pressure on the outside of the bubble is

. The bubble collapses.

bursting).

Hundreds of bubbles colla se at a roximatel the same ointon each impeller vane.

Bubbles collapse non-symmetrically such that the surroundingqu rus es o e vo orm ng a qu m cro e .

The micro jet subsequently ruptures the bubble with such force.

Bubble collapse pressures greater than 1 GPa (145x106 psi) havebeen re orted. The hi hl localized hammerin effect can itthe pump impeller.

.





After the bubble collapses, a shock wave emanates outwardfrom the point of collapse.

"cavitation".

color) is shown in a small video clip available here.

In nutshell, the mechanism of cavitation is all about formation,growth and collapse of bubbles inside the liquid being pumped.

The concept of mechanism can help in identifying the type ofu es an e cause o e r orma on an co apse.

General Symptoms of Cavitation and itsEffects on Pump Performance and Pump Parts

erce e n ca ons o e cav a on ur ng um o era onare more or less loud noises, vibrations and an unsteadily workingum . Fluctuations in flow and dischar e ressure take lace

with a sudden and drastic reduction in head rise and pump

capacity. Depending upon the size and quantum of the bubblesorme an e sever y o e r co apse, e pump aces

problems ranging from a partial loss in capacity and head to totalfailure in um in alon with irre arable dama es to the internalparts.

It requires a lot of experience and thorough investigation ofeffects of cavitation on pump parts to clearly identify the typeand root causes of cavitation.

under.



Reduction in capacity of the pump :

The formation of bubbles causes a volume increase decreasing thespace available for the liquid and thus diminish pumping capacity. For

,increases by approximately 1,700 times.

The une ual and uneven formation and colla se of bubbles causesfluctuations in the flow and the pumping of liquid occurs in spurts.This symptom is common to all types of cavitations.

Decrease in t e ead deve oped :

Bubbles unlike liquid are compressible. The head developed

increase the velocity of the liquid used to fill up the cavities, as thebubbles colla se.

The Hydraulic Standards Institute defines cavitation as conditionof 3 % drop in head developed across the pump.

Thus, the hydraulic effect of a cavitating pump is that the pumpperformance drops off of its expected performance curve, referred

, .

Figure 12 depicts the typical performance curves. The solid

whereas the dotted lines depict the condition of inadequate. . .



It is movement of bubbles with very high velocities from low-

ressure area to a hi h- ressure area and subse uent colla se thatcreates shockwaves producing abnormal sounds and vibrations. It hasbeen estimated that during collapse of bubbles the pressures of the

4

. The sound of cavitation can be described as similar to small hardparticles or gravel rapidly striking or bouncing off the interior partso a pump or va ve. Various terms ike ratt ing, knocking, crack ing areused to describe the abnormal sounds.

motor. To distinguish between the noise due to a bad bearing orcavitation, operate the pump with no flow. The disappearance of noise

.

Similarly, vibration is due to the uneven loading of the impeller as themixture of va or and liquid asses through it, and to the local shockwave that occurs as each bubble collapses.

The excessive vibration caused by cavitation often subsequently’ .

likely failure mode of a cavitating pump,



Damage to pump parts :o Cavitation erosion or pitting

ur ng cav a on, e co a se o e u es occurs a son cspeed ejecting destructive micro jets of extremely high velocityu to 1000 m/s li uid stron enou h to cause extreme erosion

of the pump parts, particularly impellers.

The bubble is trying to collapse from all sides, but if the bubbleis lying against a piece of metal such as the impeller or volute itcannot collapse from that side. So the fluid comes in from the

creating the impression that the metal was hit with a "ball pinhammer". The resulting long-term material damage begins tobecome visible by so called Pits (see Fig 11), which are plasticdeformations of very small dimensions (order of magnitude of

. The damage caused due to action of bubble collapse is commonly

referred as Cavitation erosion or ittin .



Fig 13: Photographic Evidence ofCavitation on Impeller and Diffuser

Cavitation erosion from bubble collapse occurs primarily by

fatigue fracture due to repeated bubble implosions on thecavitating surface, if the implosions have sufficient impact

.

The erosion or pitting effect is quite similar to sand blasting.

observed are the low-pressure sides of the impeller vanes nearthe inlet edge.

The cavitation erosion damages at the impeller are more or lessspread out.

e p ng as a so een o serve on mpe er vanes, uservanes, and impeller tips etc.

,holes in the impeller and damage the vanes to such a degree thatthe impeller becomes completely ineffective.



Fi 14: Cavitation a a e on I ellers

More Pictures



C

V

I

AT

ON

e amage m e er s ows a e s oc waves occurre nearthe outside edge of the impeller, where damage is evident.

highest point. This pressure implodes the vapor bubbles,

changing the water’s state from vapor into liquid. Whencavitation is less severe, the damage can occur further downtowards the eye of the impeller.

erosion on impeller, volute, diffuser etc. can help predict thet e and cause of cavitation.

The extent of cavitation erosion or pitting depends on a numberof factors like presence of foreign materials in the liquid, liquidempera ure, age o equ pmen an ve oc y o e co aps ng

bubble.



ar rom eros on o um ar s, n gger um s, ongerduration of cavitation condition can result in unbalancing (due toun-e ual distribution in bubble formation and colla se of radialand axial thrusts on the impeller. This unbalancing often leads tofollowing mechanical problems:

Bending and deflection of shafts,

Bearing damage and rubs from radial vibration,

rus ear ng amage rom ax a movemen ,

Breaking of impeller check-nuts,

ea aces amage e c.

These mechanical deformations can completely wreck the pump.

replacements can be huge.

Cavitation corrosionFrequently cavitation is combined with corrosion. The implosionof bubbles destroys existing protective layers making the metalsur ace ermanen y ac va e or e c em ca a ac . us, nthis way even in case of slight cavitation it may lead toconsiderable dama e to the materials. The rate of erosion mabe accentuated if the liquid itself has corrosive tendencies such

as water with large amounts of dissolved oxygen to acids. Cavitation – heart attack of the pump

Thus fundamentally, cavitation refers to an abnormal condition

formation and subsequent collapse of vapor filled cavities orbubbles inside the li uid bein um ed. The condition ofcavitation can obstruct the pump, impair performance and flowcapacity, and damage the impeller and other sensitive

“. ,attack of the pump”.



Once the um has been built and installed, there is little that

can be done to reduce cavitation damage.

Operation near the best efficiency point usually minimizes

cavitation. The admission of a small amount of air into the pump suction

. , ,because it is difficult to inject the right amount of air undervar in head and ca acit conditions and, fre uentl , there areobjections to mixing air with the fluid pumped.

If a new impeller is required because of cavitation, the designs ou a e n o accoun e mos recen a vances escr e nthe literature. For example: A change in the impeller material may

The table shows the relative resistance of several metals tocavitation pitting produced by magnetostriction vibration.

The net positive suction head (NPSH) is a statement of the minimum

suction conditions required to prevent cavitation in a pump.

The required NPSHR must be determined by test and usually will be.

The available NPSHA at installation must be at least equal to the.available NPSH provides a margin of safety against the onset ofcavitation.

Consistent units must be chosen so that each term represents feet(or meters) of the fluid pumped.

sua y a pos ve va ue o ss s ca e a s a c suc on ea w e anegative value is called static suction lift.

2

vaps

msatm A H H H NPSH 2

slossssvapatm A h H H H NPSH )(



A

vaps

msatm A H g

V H H NPSH

2

= Hatm


vaps

msatm A H V

H H NPSH

2




sV

2

vapmsatm Ag

2A > R

Increase Hss: Place the um under the suction level

sossssvapa m

Decrease hlosss:

1- Place the um ver close to the suction tank (L = min.)

2- Use a larger size of pipe for the suction side (ds>dd)

3-Minimize the minor losses in the suction side:

a- Min. no. or no bends, ..b- If strainers are used they have to have very small p

c- No valves in the suction side

Use a pump of small NPSHR (NPSHR < NPSHA)

The above e uation ma be also written as:

)( atmvmms hh H H

)()(

2

atmvm

s

lsss hh H

V

h H

Where: is the cavitation factor Thoma which is a function ofthe pump specific speed.

hv is the vapor pressure of the liquid at working temperature inabsolute pressure units

hatm is the atmospheric pressure

o avo cav a on, e a ove equa on mus e sa s e



Resistance o aterials to Cavitation a a e

,is not suitable for ordinary use because of its comparatively highcost and the difficulty encountered in machining and grinding.

It is seen that cast iron the most commonl used material forimpellers, has relatively little pitting resistance compared tobronze and stainless steel which are readily cast and finished.

Elastomeric coatings have been found to be highly resistant to

cavitation pitting.

damage than stellite which leads the list of metals in the table.

recently has it appeared 'possible to secure an adequate bondbetween the coating and the metal.

Polyurethane and neoprene which show high resistance tocavitation pitting and may be applied in liquid form, should be

be used.



Inducers It is sometimes difficult or

im ossible to rovide the re uiredNPSH for an otherwise acceptablepump. The required NPSH can be

rov e y a sma oos er um ,called an inducer, placed ahead ofthe first-sta e im eller.

The inducer blades are designed tooperate with low NPSH and toprovide enough head to meet theNPSH requirements of the first-

.

Inducers can operate with varyingde rees of cavitation since thecollapse of the vapor bubbles isdistributed over a relatively large

.

Inducer The hubless inducer ejects cavitation bubbles into a central

vaneless core where the colla se without dama e.

The inducers contribute not more than 5% of the total pumphead. Although the efficiency of the inducer alone is low, thereduction in overall pump efficiency is not significant.

Since this type of inducer causespre-rotation, a careful matchbetween inducer and suction

. A suitable inducer-impeller

combination can operate at about50 o t e NPSH require orthe impeller alone at capacities

not exceedin the normal value.The NPSH requirement increasesrapidly for capacities above

,range should be avoided.



Entrained Air Air or other gases may enter the impeller inlet from several

sources. The immediate effect usually will be a drop in capacityand power. This will be followed by loss of prime if more gas ispresent than the impeller can handle.

suction piping. Stuffing box air leakage may be prevented bylantern rin s su lied with li uid from the um dischar e.

If the pump takes water from a sump with a free surface, avortex may form from the free surface to the impeller inlet.

It is sometimes permissible to inject a small amount of air into thepump suction to reduce the noise ; damage from cavitation caused

.

Change the Operating Conditions!! Reduce the pump speed or partially close the delivery valve such

that the pump will work at a flow rate lower than critical as shown.

30 80

2570Efficiency

20

60

H1

Close Valve

15 H 40

50H-2

system

system

Original

System

1030

H3

eff-1

eff-2

N1

5

20N2<N1

0 0

0 10 20 30 40 50

Q



Variation of NPSH and NPSH withflow through the system.



e wo c ures on e e are o e same area o acentrifugal pump impeller. The one on the left shows a typicalcavitation attern durin flow. Bubbles are formin to the leftand imploding at the impeller’s surface in the upper right. Thepicture on the right shows the actual damage caused bycon nuous mp os on o u es n e same area.

The com lete Stor

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lec10 cavitation