Frankenthal, 09.01.2002H 2, Dr. Mollenkopf/GK

Cavitation

1) Definition

The definition of cavitation in pumps includes the occurrence, growth andimplosion of vapor bubbles at the entrance of the impeller eye respectively at theinlet edge of the impeller vanes, in case of a pressure drop at this part of thepump below the vapor pressure of the pumped fluid at the given temperature.

This local drop in the static pressure inside the impeller is caused by thegeneration of head due to the transfer of mechanical energy to the pumpedliquid.

The pressure distribution along the blades for instance at the design point of theimpeller is shown in Fig. 1.

In Fig. 2 is shis going downpump arrange

If the pressurebubbles will dFig. 2c the va

To imagine wlife circle of th

As described vapor pressurwith higher primplosion. Asliquid (microjewith a very hig

Fig. 1

inle

t edg

e

DS

SS

p12

p0

p

xpminown what happens if the pressure po infront of the inlet of the pump, for instance by reducing the waterlevel in the suction sump at thement.

is lowered to poII, so that pmin just equals pD (Fig. 2b) the firstevelop. At a further diminishing of the pressure (to poIII) shown inpor filled bubbles will increase.

hat happens in detail many investigations are made to explain thee vapor bubbles.

the bubble occur if the local static pressure will be lower as thee of the liquid. Now the bubbles are carried by the flow into a regionessure (p > pD). There the bubbles collapse completely like as an observed with high speed movie (106 p.p.s) a microscopic jet oft) shoots out of the collapsed bubble directly to the wall, striking ith velocity.

outlet of vane

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2) Description of the phenominous

If cavitation appears in a pump different effects influence the behaviour of thepump:

- the implosion of the vapor bubble and the impact of the microjet on thematerial create a noise which is very easy to detect. If the cavitation is in afurther estate the sound can imagine that a lot of pebble are pumpedwith the liquid.

- When the bubbles are very numerous and form a bubble cloud then the headof the pump is influenced and also the efficiency drops down.

- In this estate the flow through the pump is macroturbulent and result also inhigher vibrations.

- Under certain presuppositions the impact of the microjet on the surface ofmaterial is strong enough to damage the structure of the material and erosiontake place. (From a combination of measurements and calculations pressureso 105 bar have been ascertained)S o Fig. 3.

fig. 2a fig. 2b fig. 2cFig. 2

Fig. 3

vapourbubble

begin ofcollapse torus microjetf up toee als

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3) Concept of NPSH

To avoid these consequences, a certain margin between the static pressure atthe impeller inlet and the vapour pressure of the fluid is required. Since thepressure directly at the impeller inlet is difficult to ascertain, the total pressure, i.e.the energy content of the fluid, determined at a specified location upstream of thepump (2 x DN) is chosen as a more easily measured reference value instead.The difference between this total pressure and the vapour pressure is calledNPSH (Net Positive Suction Head).

( ) ( )[ ] g/p/2cppg/ppNPSH D21amb1v1totalabs ++==The NPSH value of a pump or pumping system is therefore a neutral numberwhich only takes on a meaning when there is agreement about which property ofthe pump it is to characterise.

In practice several criteria are applied:

Incipient cavitation NPSHi Head drop by a specific percentage 0 %

NPSH0 (= start of the head drop)1 % NPSH12 % NPSH23 % NPSH3X % NPSHx

Breakdown of pumping, fully cavitating NPSHfull Efficiency drop by a certain value NPSH Erosion of a certain quantity of material per unit of time NPSHm Exceeding a certain noise level NPSHPhon Operation at a certain vibration level NPSHVibr

All pump manufacturers generally use the criterion of a 3 % reduction in thedeveloped head due to cavitation (NPSH3) as the value reported in salesdocumentation.

Different pumps can therefore be compared on a common basis. Whether thecriterion is used for pump selection and how it is applied depend on the intendeduse of the pump.

To run a pump in a plant at this intended conditions it is necessary that the givenNPSH-value in the plant (NPSHavailable) is equal or higher than the NPSH-valuerequired by the pump under the running conditions

NPSHavailable > NPSHrequired

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The following recommendations provide a starting point for choosing a criterion:

Fig. 4 shows the relationship of the criteria in terms of the drop in developedhead due to cavitation as a function of flow rate for a volute casing pump.

The operating point with optimum flow entering the impeller (zero incidenceangle), at or at least near the pumps best efficiency point, is characterised by aminimum in the criterion for incipient cavitation (NPSHi).

So in the example in Fig. 4 the pump runs cavitations-free only in the regionbetween and .

Cavitation bubbles form on the impeller blade surfaces even at relatively highNPSH values, and they must lengthen significantly until the developed headstarts to fall measurably at the NPSH is further reduced. It is also worthy of notethat just a small reduction of the NPSH available below the NPSH3 value maysometimes make pumping impossible. This fact was formerly occasionally usedfor self-regulation of a pump in a system (such as a condensate pump).

4) Criteria for al wable pump operation

For a constanon various probelow the valuWith a furtherby the implosierosion rate fofor higher gradstarting point developed hei.e., when the material erosibreaks down c

no caviation

no operation possible

NPSHfull

0 100 200 300 400

30

25

20

15

10

5

0

NPS

H [m

]

Q [m3/h]

on v

ane

pres

sure

sid

eufon vane suction side

incipientcavitation

increasing noise vibration risk of erosion

NPSH3

example ofNPSHavailable in a plant

Fig. 4lot operating point (Q = const.), Fig. 5 shows the effects of cavitationperties of a pump. When the NPSH available in the system fallse of NPSHi, the noise level LPA of the impeller starts to increase.

reduction, depending on the quality of the material, erosion causedon of the vapour bubbles on the impeller vanes can occur (VAI =r less resistant material such as grey cast iron, VA2 = erosion ratee material such as GSNBZ). It must be kept in mind that the

for material erosion is independent of the onset of the reduction ofad. When the cavitation zone length LBL exceeds a certain value,impeller channels become largely filled with vapour, the noise andon are dramatically reduced. The developed head simultaneouslyompletely.

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All this makes obviously that in normal practice the criterium for 3 %-head drop(NPSH3%) can be taken for sufficient operation but for more sophisticated use ofa pump another criterium shall be necessary.

5) Possibilities to avoid trouble and damages caused by cavitation.

An effective step is shown in Fig. 6 where the impeller diameter as a parameterfor NPSHrequired is shown. For an operation point of Q = 300 m/h and H = 36 mthe diagramm is an example for NPSHreq as function of speed of rotation.

The following table supplements these data for the speeds 960/1450/2900 RPMwith the power input requirements and list prices as a percentage of the completepump unit for 2900 RPM.

Assuming that the NPSH available in the system is 3 m, then use of the pumpwith 2900 RPM would not be possible in this case and the speed n = 1450 RPMwould have to be chosen instead. An increase in NPSH, by 2,5 m (for exampleby increasing the height of the liquid level on the suction side) would bring downthe pump unit price by 30 %, since then 2900 RPM should be possible.

960 RPM 1450 RPM 2900 RPMRel. pump pr e [%] 145 75 35Rel. pump sePower [kW]Nominal diam

Fig. 5

Fig. 6

VAHlBLLpA

NPSH

lBl

H

VA2

Lp

VA1

NPSHvoll NPSH3

Q = constn = const

VA1 z.B. GGVA2 z.B. G-CuAl10Ni

NPSHiic

t price [%] 240 130 100

37.2 35.7 35eter [mm] 150 125 100

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An other method to reduce the NPSH-value of a pump is to use an inducer. Thisbooster infront of a pump lifts the inlet energy a little so that the main impellercan run without cavitation or with a small amount of cavitation which doesntinfluence the head. Normaly the inducer will run with cavitation bubbles but thenegative effects can controlled easily.

Fig. 7 + 8 shows the effect in detail and the result in NPSH3% head drop of thetotal pump.

6) Conclusion

In the present time it is only in special cases possible to avoid totally theappearance of cavitation in the whole range of operation. In most cases of usinga pump we have to life with cavitation, minimum in the operation range of part-and overload. If we know more detailed what happens in respect to cavitation inthe pump we can take measures to avoid severe damages, par example by usingb tter material, lower speed or flow (double inlet pump) or by inserting a booster( ducer).

Dr. GKSBEntwDepa

Fig. 7 Fig. 8

pressure insite a pump without and with an inducer

P P

S S

without inducer with inducer

psucpvapour

psuc without

psuc withpvapour

pressure through the pump

area with vapour bubble

pump performance curve

NPSHreq. without inducer

NPSHreq. with inducer

discharge QQopt

NPS

H of

the

pum

phe

ad H

of t

he p

umpeinerhard Mollenkopf Aktiengesellschafticklung Engineered Pumpsrtment H2

Cavitation6)Conclusion