PUMPS

Machines that transfer energy INTO fluid system.Pumps have a shaft and an impeller. Impeller is therotating element.

Types of pumps

Fluid flows outward(right angle to its axis)

RADIAL FLOW

Fluid flows spirally(along axis)

AXIAL FLOW

MIXED FLOW“centrifugalpump”

“Propeller pump”

EFFICIENCY OF A PUMP

ωγ

ηT

Qh

shafttogivenpowerfluidtogivenpower p==

where T: torque exerted on the shaft

ω = rotation of the shaft in radians/sec

Note: overall pump efficiency is affected by:

• Hydraulic losses in pump ηh

• Mechanical losses in the bearings, seals ηm

• Leakage of fluid: volumetric losses ηv

Overall vmh ηηηη =

)( powershaftorpowerbrake

CAVITATION IN PUMPS

The potential for cavitation is characterized bydifference in (pressure head +velocity head) onsuction side and the liquid vapor pressure head.This is NPSH

γγγγvss p

gVp

NPSH −+=2

2

There is a value of NPSH that must be maintainedto avoid cavitation called as NPSHR(this isexperimentally determined)

There is available NPSH called as NPSHA

Apply energy equation between 1 and 2

z

1

2

Lhg

Vz

pg

Vz

p +++=++22

22

22

21

11

γγγγ

Lssatm hg

Vz

pp +++=++2

002

γγγγ

Latmss hz

pg

Vp −−=+γγγγ 2

2

Hence

To prevent cavitation

NPSHA > NPSHR

How?

• Small z• Small losses (shorter pipes)

Note: There is a critical value of z above whichpump will NOT operate without cavitation

γγγγv

Latm

Ap

hzp

NPSH −−−= 1

PUMP PERFORMANCE CHARACTERISTICS

Efficiency

η

head

Brake power

h,

BEP: Best efficien

The point whereas close as pos

Water power

head

Q

Efficiency

h, η

s

Design flowrate

BEP

Q

cy point

pump is to be operated should beible to the BEP.

SIMILARITY LAWS FOR PUMPS

Performance prediction from a scaled test modelto prototype

1. 3nDQ α

2. 22Dnh α

3. 53DnP α

4. 5/1

1

2

2

111

≈−−

DD

ηηηη

Q

hBEP

n1

n0

n2

PUMP SELECTION

Efficiency

η

head

h,

Notice: at operatinmaximum

As system curve cbetween BEP and

Design flowrate

BEP Operating point

Systemh

Q

g point efficiency is NOT

hanges so will difference operating point

SPECIFIC SPEED OF PUMPS

and w, Q, and h are the values at BEP

Velocity ofimpeller/√(2gh)

Suction Specific Speed

4/3R

sdNPSH

QS

ωω=

PUMPS IN SERIES AND PARALLEL

SERIES PARALLEL

Increase head Increase Discharge

Pumps in series and parallel should have similarpump characteristics

rpmingpminQftinisNPSHwhere R ωω,,

Q

h

Q

h