2. Centrifugal Pump

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Centrifugal pump


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Classication of pumps

Pumps

Rotodynami

cpumps

Low

head

Axialpump

(propeller)

Medium

head

Centrifugalpump

Mixed owtype

High

head

Centrifugal pump

Radial

type

Positie

displacement pumps

Reciprocating

pupms

Piston pump

Plungerpump

!iaphragm

pump

Rotarypumps

"ear pump

#crew ump

$ane pump


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Centrifugal Pumps (Roto-

dynamic pumps)A centrifugal pump imparts elocity energy to the

uid with the help of impeller% which is conerted topressure energy upon exiting the pump casing&

Centrifugal pump conerts mechanical energy froma motor to energy of a moing uid& A portion of theenergy goes into 'inetic energy of the uid motion%and some into potential energy% represented y uidpressure (Hydraulic head) or y lifting the uid%against graity% to a higher altitude&

Common uses include water% sewage% petroleumand petrochemical pumping&


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Positive Displacement pumps

Positie !isplacement (P!) pumps displace a 'nownuantity of liuid with each reolution of the pumpingelements& *his is done y trapping liuid etween thepumping elements and a stationary casing& Pumpingelement designs include gears% loes% rotary pistons%

anes% screws and hoses&Positie !isplacement Pump Applications+ Chemical,processing

Liuid !eliery

Marine -iotechnology

Pharmaceutical

.ood

!airy

-eerage Processing


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Centrifugal Vs

Positive displacement PumpCentrifugal pump Positive displacement (PD) pump/& $arying ow depending onpressure or head&

/& More or less constant owregardless of pressure&

0& .low decreases as the iscosity

goes up&

0& .low increases due to the

thic'ening of the product&1& Changes in pressure hae adramatic e2ecton a Centrifugal pump&

1& Changes in pressure hae littlee2ect on aP!&

3& 4P#Hr in a Centrifugal aries with

owwhich is determined y pressure&

3& 4P#Hr in a P! aries with ow

which isdetermined y speed&

5& Centrifugals are more e6cient inhigh ow conditions&

5& P! pumps are ery well suited forlow ow Conditions&

7& Centrifugal is ery ine6cient ateen

modest iscosity&

7& P! is ery e6cient with highiscosity applications&


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Main components of

centrifugal pump#uction pipe with a foot,ale and a strainer%

!eliery Pipe%

;mpeller%Casing+

(a) $olute Casing%

() $ortex Casing%(c) Casing with guide lades&


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Impeller*he rotating part of a centrifugal pump is called


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Casing

*he casing is an air tight passage surrounding theimpeller and is designed in such a way that the 'ineticenergy of the water discharged at the outlet of theimpeller is conerted into pressure energy efore thewater leaes casing and enters the deliery pipe&

(a)Volute Casing , ;t is of spiral type in which area ofow increases gradually&

*he increase in area of ow decreases the elocity ofow&

*he decrease in elocity increases the pressure of thewater owing through the casing&

;n case of olute casing% the e6ciency of the pumpincreases slightly as a large amount of energy is lost

due to the formation of eddies in this type of casing&


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()Vorte! Casing ;f a circular chamer is introducedetween the casing and the impeller %the casing is

'nown as $ortex Casing&

-y introducing the circular chamer% the loss of energydue to the formation of eddies is reduced to aconsiderale extent&

*hus the e6ciency of the pump is more than thee6ciency when only olute casing is proided&

(c) Casing "it# $uide %lades

*he impeller is surrounded y a series of guide lades

mounted on a ring which is 'nown as di2user&Also the area of the guide anes increases% thus

reducing the elocity of ow through guide anes andconseuently increasing the pressure of water&

*he water from the guide anes then passes through


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&or'ing of centrifugal pump*he liuid enters the suction no@@le and then into eye (centre) of

an impeller&

hen the impeller rotates% it spins the liuid sitting in the caitiesetween the anes outward and proides centrifugal acceleration&

As liuid leaes the eye of the impeller a low,pressure area is

created causing more liuid to ow toward the inlet& -ecause theimpeller lades are cured% the uid is pushed in a tangential andradial direction y the centrifugal force&

*he energy created y the centrifugal force is 'inetic energy&

*his 'inetic energy of a liuid coming out of an impeller is

harnessed y creating a resistance to the ow& *he ?rst resistanceis created y the pump olute (casing) that catches the liuid andslows it down&

;n the discharge no@@le% the liuid further decelerates and itselocity is conerted to pressure according to -ernoulli=s principle&


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*he centrifugal pump wor's on the principle offorced ortex ow which means that when acertain mass of liuid is rotated y an externaltorue% the rise in pressure head of the rotating

liuid ta'es place&*he rise in pressure head at any point of the

rotating liuid is proportional to the suare oftangential elocity of the liuid at that point&

*hus at the outlet of the impeller% whose radius ismore% the rise in pressure head will e more andthe liuid will e discharged at the outlet with ahigh pressure head& !ue to this high pressure

head% the liuid can e lifted to a high leel&


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ead of pumps*Suction #ead ;t is the ertical height of the

centre line of the centrifugal pump aoe the watersurface in the tan' or pump from which water is to elifted& *his height is also called suction lift&

,* Delivery #ead *he ertical distance etween the

centre line of the pump and the water surface in the tan'to which water is deliered is 'nown as deliery head&

* Static #ead-*he sum of suction head and delieryhead is 'nown as static head& *his is represented y

and is written as+

.* Manometric ead-*he manometric head is de?nedas the head against which a centrifugal pump has to

wor'&

( )sh

( )dh

s

H

s s dh hH = +


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Manometric #ead is the head measured across thepump inlet and outlet anges&

;t expresses the increase in pressure energy per unitweight of liuid handled y the impeller&

*he manometric head includes all losses againstwhich pump has to wor' except the 'inetic head&

*hus manometric head is the di2erence etween thereadings shown y the manometers or gauges plusthe ertical distance etween the pressuretappings for the suction and deliery gauges&

_static head +all lossesd s

mano g

p phH

= + =

gh


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/otal0 $ross or 12ective ead this is actualhead against which the pump has to wor'&

;t is eual to static head plus all the head lossesin ow efore% through and after the impeller&

*hus the di2erence of total head and

manometric head is the di2erence of 'inetichead etween the deliery and suction&

*his di2erence is ery less therefore forpractical purpose the manometric head is

eual to the total head (H)&

2 2_ _

2

d s d s

g g

p p v vhH

= + +

( )manoH


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Velocity triangle for impeller vane

2

V2rV

2wV

2u

1rV 1V

1u1wV

1fV

2fV

B;mpeller ane angle at entrance

B;mpeller ane angle at outlet

BAngle etween the direction of asoluteelocity of entering uid and the peripheralelocity of the impeller at the entrance

BAngle etween the direction of asoluteelocity of leaing uid and the peripheral

elocity of the impeller at the exit point


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&or' done 3y impeller

or' done per 'g per second

*his is 'nown as fundamental e4uation of centrifugal

pump&(i) *he ?rst term represent the increase in 'inetic

energy or dynamic head&

(ii) *he second term represents an increase in static

pressure&(iii) *he third term indicates the change in 'ineticenergy due to retardation of ow relatie to theimpeller&

2 2 2 2 2 2

2 1 2 1 1 2

2 2 2

r r manomano mano

mano

V V u u V V H H H

g g g

= + + = + =

2 22 1V V

2g

2 22 1u u2g

r r1 22 2

V V2g


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*he wor' done per 'g per second y theimpeller on the liuid may also e written as

2 2 1 1

1

2 2

. .work done = Euler head(H )

if liquid enters the impeller radially

= ! and hence " !.

.work done=

w we

w

w mano

mano

V u V u

g

thus

V u H

g

=

=

=


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155ICI16CI1S 75 8

C16/RI59$8: P9MP* Manometric e;ciency -*he ratio of the

actual measured head or gross lift to the headimparted y the impeller to the uid is 'nownas manometric e6ciency& Mathematically% it iswritten as

.*his is also 'nown as ydraulic e;ciency&

2

2 2 2 2

2

. .

dfstatic

mano mano

mano

w w mano mano

vH

H Hg

u V u V H Hg g

H

+ += = =

+


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,* Mec#anical e;ciency


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* Volumetric e;ciency


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3& 7verall e;ciency is the ratio of the powersupplied y the pump to the power deliered tothe pump shaft&

fluid or water power output

power input to pump shaft

%reak power of dri&in' unit#power lost in couplin'

overall

mano

mech v mano

QgH

=

=

=


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Minimum starting speed

hen pump is started% there will not e anyow of water until the pressure rise in theimpeller is large enough to oercome the grossor manometric head&

Centrifugal or pressure head caused ycentrifugal force on rotating water whenimpeller is rotating% ut there is no ow

.low will commence only if

2 2

2 1

2

u u

g

=

2 2

2 1

2 2 2

2 1 2 2

2

.or

! 2

mano

wmano

u uHg

N d d u V

g g


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Pump c#aracteristics


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Multistage centrifugal pump

Centrifugal pump consists of two or moreimpellers% the pump is called a multistagecentrifugal pump&

*here are two types of arrangements&/& Multistage centrifugal pump for high heads or

impellers in series&

0&Multistage centrifugal pump for high discharge

or impellers in parallel&


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Impellers in series, .or deeloping a high head anumer of impellers are mounted in series or on thesame shaft as shown in ?gure elow&

*he water from suction pipe enters the /st impellers atinlet and discharged at outlet with increased pressure&

*he water then from /st impeller ta'en to inlet of the

0nd impeller with the help of connecting pipe& #o atoutlet of 0nd impeller pressure of water will e more&

*otal head deelopedB n G

here n B no& of impellers in series&manoH


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Impellers in parallel


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Cavitation.low area at the eye of the impeller is usually smaller than either

the ow area of pump suction line or ow area of impeller aneso when water enters a pump% its elocity increases causing a

reduction in pressure within the pumping unit&

;f this pressure falls too low% some of the water will aporise%forming ules entrained in the liuid&

*hese ules collapse iolently as they moe to areas of higherpressure creating the noise and iration from the pump&

*he pressure head aailale at the pump inlet should exceed the4P#H reuired to aoid caitation&

*homa=s caitation factor is de?ned as

( ) ( )a v s fs

mano mano

p ph h

g NPSH

H H

= =


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6et positive suction #ead4et positie suction head is the term that is used to descrie the

asolute pressure of a uid at the inlet to a pump minus the apourpressure of the liuid&

*he resultant alue is 'nown as the 4et Positie #uction Headaailale& *he term is normally shortened to the acronym 4P#Ha%the


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SP1CI5IC SP11D

#peci?c speed is a term used to descrie thegeometry (shape) of a pump impeller&

#peed of an imaginary pump geometricallysimilar in eery respect to the actual pump and

capale of deliering unit uantity against a unithead& ;t is denoted y 4#+,

4# B 4 (E)/I0I(H)1I3

here+4,pump speed in r&p&m

E,discharge in m1Isec

H,head per stage in meter

Pump #peed #peci?c speed(in r&p&m)

Radial ow #lowMediumHigh

/J,1J1J,155J,KJ

Mixed ow KJ,/7J

Axial ow /JJ,35J


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Similarity condition.or complete similarity etween the model and

prototypeIactual centrifugal pump the followingconditions should e satis?ed

( ) ( )*+ *+1. mano manom p

N Q N Q

H H

=

2. mano mano

m p

H H

DN DN

=

.

m p

Q Q

D N D N

=

, , .

m p

P P

D N D N

=


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2. Centrifugal Pump