55--Cavitation in PumpsCavitation in Pumps55 Cavitation in PumpsCavitation in Pumps

CavitationCavitationCavitationCavitation

• If the pressure of the liquid stream is lowered up to vapor pressure evaporation will start forming small vapor bubbles inpressure evaporation will start forming small vapor bubbles in the liquid stream.

• The bubbles constitute cavities as far the liquid stream is• The bubbles constitute cavities as far the liquid stream is concerned thus the name ‘cavitation’

A. formation of vapor bubbles;

B. collapse the vapor bubble;

C. material destruction

• If downstream pressure again rises above the vapor pressure• If downstream pressure again rises above the vapor pressure, the vapor phase becomes unstable and it disappears.

• If the bubble was located on the wall the material of the wall• If the bubble was located on the wall, the material of the wall may be destroyed due to impact of the liquid particles rushing towards the wall.

• The destruction proceeds rapidly if the surface of the wall has already distraction marks or chamfers.

• If the bubble is entirely surrounded by the liquid, no destruction of wall material is caused but a hard noise can be h d d th hi ib theard and the machine may vibrate

PumpsPumps

• The pressure acting on the suction water level may be PA (Atmospheric pressure ).

• If the suction water level is taken as reference, the energy content of the water at this level is givenwater at this level is given by (PA/ρ).

• The total energy content of the water at the suction end of the impeller is less. The energy difference is given by:the impeller is less. The energy difference is given by:

A. The geodetic energy difference  (ges [Nm/kg])

– Where  es: height difference between suction water level and the highest point of the suction edge of the vanes.

B. The energy loss of the flow in the suction pipe up to the suction end (pump suction flange ) Z [Nm/kg]suction end (pump suction flange ) Zs [Nm/kg]

• Considering a. and b., the following total energy exists at the height e at the pump suction endheight es at the pump suction end

[ ] ( )A ZPkNE +⎟⎟⎞

⎜⎜⎛[ ] ( )ss

As ZgekgNmE +−⎟⎟

⎠⎜⎜⎝

=ρ

• The dynamic energy and the loss for generating co from the velocity zero at the suction water level may be counted 

h d dtogether and expressed as:

1. 2

20

2 >λλ whereC Because the generatingloss is included1

2. 22 >λλ where loss is included

• Due to the influence of the thickness of the vane and of the di t ib ti d th ti d f th thpressure distribution around the suction end of the vane the 

actually prevailing velocity at the suction end is greater than co.

• The additional dynamic energy included. ( the additional losses for generating this highest velocity from co) is best expressed related to relative velocity Wexpressed related to relative velocity Wo.

12

. 1

20

1 <λλ whereW As the additional dynamic energyis only a fraction of the dyn.

• Consequently

2 energy of the relative velocity

( )20

20 CWEE λλΔConsequently,

E i t l l λ 0 25 t 0 35 λ 1 1 t 1 3

( )220

20

1min. yEE stats λλ +=Δ=−

• Experimental values: λ1≈ 0.25 to 0.35; λ2≈ 1.1 to 1.3

and  ( )ssA

sstatCWZgePyEE ⎟⎟

⎞⎜⎜⎛

+−+−⎟⎟⎞

⎜⎜⎛

=Δ−=20

2

20

1min λλ( )44 344 2144 344 21

pumpofdesignbygiven

pumpoflocationandpipesuction

ofdesignbygiven

sssstat gy ⎟⎟⎠

⎜⎜⎝

⎟⎟⎠

⎜⎜⎝ 22 21min. ρ

• If evaporation of the flow medium has to be avoided, it must be

ρT

sstatPyEE ≥Δ−=min.

Where PT[N/m2] vapor pressureat Prevailing temperature

• The last condition can be expressed also in the following way:

( ) ( ) yEZgePPPE availsssTAT

s Δ≥=+−−

=−ρρ

• Evaporation will takes place if (Es)avail < ∆y

Avoiding of Avoiding of CavitationCavitation in Pumpsin PumpsAvoiding of Avoiding of CavitationCavitation in Pumpsin Pumps

• The locally lowest static pressure found at the suction end of the impeller isimpeller is:

( ) ⎟⎟⎞

⎜⎜⎛

+−+−⎟⎟⎞

⎜⎜⎛

=20

2

20

1iCWZgePE A

t t λλ

• Where:

( ) ⎟⎟⎠

⎜⎜⎝

++⎟⎟⎠

⎜⎜⎝ 22 21min. ZgeE ssstat λλρ

– PA/ρ is determined by the pressure condition on the surface of the suction tank

– (ges + Zs) is determined by the height position above the surface of the suction tank and the layout of the suction pipe system

– (λ1W20 + λ2C20 ) =Δy is determined by the pump design

• The outer point a of the vaneThe outer point a of the vanesuction edge is exposed to thecavitation most as here u andtherefore W has the highesttherefore Wo has the highestvalue along the vane suctionedge.

• By this consideration, thevelocity Co may be assumednearly the same for all point ofy pthe vane suction edge.

The ratio between Coa and Woa is given by the angle βoa. ToTo avoidavoidoa oa oacavitationscavitations--thethe minimumminimum ofof ΔΔyy-- isis givengiven byby thethe followingfollowingequationequation :

( ) 1tan 1=λβ( )

( ) 2.13.0'3217

2tan

210

21

===

+=

λλβ

λλβ

andifoptoa

optoaCase: Case: αα0a0a=90=9000

•• Case:Case:αα0a0a≠90≠9000 :‐ (βoa)opt‐ values are given by the following diagramg

WC1δ

PrePre--rotationrotation factorfactor

a

ou

a

our u

WuC

11

1 =−=δ

( )21

2

21 11

21tan

λλδ

λλβ

+

⎟⎟⎠

⎞⎜⎜⎝

⎛−+

= roptoa

• The value ofΔy may be determined by assuming λ1 and λ2 if Co

212 λλ +

1 2 oand Wo are known or by assuming the suction number

Suction Number SSuction Number SqqSuction Number SSuction Number Sqq

• It is a dimensionless number in regard to prevailing  lowest static pressure

• The design of the pump, the locally lowest static pressure is influenced by:

a.a. The value of The value of ΔΔy: y: i.e. by the friction coefficients λ1 and λ2actually existing and the relation of co to wo as given by the angle  βoaoa

b.b. The value of the speed n:The value of the speed n: ( if for the same pump the speed n is increased, the locally lowest static pressure is decreased)decreased)

c.c. The volume v: The volume v: ( if for the same values of n and βoa the volume V is increased, the absolute value of Co and Wo increase;volume V is increased, the absolute value of Co and Wo increase; hence the locally lowest static pressure in the eye decrease).

Suction number VnSuction number

43yVnS q Δ

=

• As for same n and V, ,a higher Sq value refers to a lower Δy.

The diagram indicates the a prepre‐‐rotationrotation δδrr==00..88 (slight(slightrrprepre‐‐rotationrotation inin directiondirection ofof impellerimpeller rotation)rotation) avoidsavoidscavitationcavitation..

• If the hub is extended through the eye of the impeller a decrease in free cross section of the eye with an increase ofwith an increase ofdecrease in free cross section of the eye with an increase of with an increase of the meridian velocity for same V is noted.the meridian velocity for same V is noted.

• In order to take account of this increase in velocity due toIn order to take account of this increase in velocity due to existence of a hub, the suction number is defined as

43Δ=q y

kVnS

2

1: ⎟⎟⎠

⎞⎜⎜⎝

⎛−=

Δ

n

ddkWhere

y

⎠⎝ sd

Suction Specific Speed Suction Specific Speed nnqsqs; Net Positive ; Net Positive S i H d NPSHS i H d NPSHSuction Head NPSHSuction Head NPSH

• Suction Specific Speed  Vnp p

Where:‐ Δh= Δy/g , 

i d i

43hVnnqs Δ

=

( )n is measured in rpm

According to previous diagram

( ) ( )

( )

availableavails

NPSHy

NPSHg

E

Δ

=

• As the pump requires out of the available (NPSH)available the 

( )requiredNPSHg

=

amount (NPSH)required the demand for avoiding cavitation is given by

( ) ( )requiredavailable NPSHNPSH ≥

Measures to Avoid Measures to Avoid CavitationCavitation

1.1. Measures at the pump designMeasures at the pump design

Demand: ∆y as small as possible, is                20

20 CW λλΔ

y p ,obtained using

a. Low Shape Number:‐ (Large pump, low speed, multi‐suction, thus 

220

20

1CWy λλ +=Δ

Co and Wo kept small)

b. Favorable βoa (assumes favorable ratio of Co to Wo)

c. Low vane loading by means of increasing the vane length, vanes drawn into the impeller eye (influences λ1).

d. Good Streamlining of impellerapproach i.e. especially avoiding orapproach i.e. especially avoiding orrapid changes of the flow direction.

e.      Smooth Surfaces of walls of pump eye and vanes.

f. Use of entrance guide vanes in order to generate a pre‐rotation in direction of the impeller rotation (δr =0.8).

22 Measures outside the pumpMeasures outside the pump2.2. Measures outside the pumpMeasures outside the pump

Demands:   ges, Zs and PT/ ρ as small as possible, high PA/ρ

aa small small gege : : i t b l t d l t th ti ta.a. small small gegess: : pump is to be located as close to the suction water level as possible

b.b. Small ZSmall Zss: loss of the suction pipe is to be kept small: low flow SS ss p p pvelocity (Cs = 1 to 2 m/s normally); 

– short suction line.

– Avoiding of unnecessary bends;

– Smooth inside surface of pipe 

Leak proof suction line to avoid air intake;– Leak proof suction line to avoid air intake;

– Avoiding of pre‐rotation in suction pipe;

Avoiding Air Pockets

C.C. Small PSmall PTT//ρρ: : Pump is to be located where the liquid has its Pump is to be located where the liquid has its TT//ρρ p qp qlowest temperature.lowest temperature. Circulating pumps of heating installations should be installed in the runback.

dd Ri i PRi i P // U f f di hi h i l dd.d. Rise in PRise in PAA//ρρ: : Use of a feeding pump which is located deeper tan the main pump and runs with low speed. Often, also ejector pumps with no rotating parts are used if height es or temperature of liquid are very high.

33.. OtherOther MeasuresMeasures

• Cavitation can also be avoided by using proper pumpmaterials with high fatigue resistance and high ductility andproviding very smooth surfacesproviding very smooth surfaces

• Preferable pump materials are: Chrome‐steel, stainless steel

• The destruction effect of cavitation may also be eliminated partly of fully by inducing air into the liquid at the suction line.

• The air will fill partly the cavities which consequently do not collapse anymore so suddenly