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ME 4880 Experimental Design Lab

Centrifugal Pump Performance Experiment

Instructors:

Dr. Cyders, 294A Stocker, [email protected]. Ghasvari, 249B Stocker, [email protected]

Spring 2014

1


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Part I.• General topics on Pumps• Categories of Pumps• Pump curve• Cavitation•

NSPH


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Pumps

– Basic definitions to describe pumps and pumpingpipe circuits

– Positive displacement pumps and centrifugal

pumps – The ‘Pump Curve’ – Net Positive Suction Head


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Pump analysis: energy equation

• Shaft work delivered by pump is translated into apressure rise across the pump: P 2 > P 1

•

How does h pump vary with Q? – Typically data is gathered from experiments bymanufacturer and is presented in dimensional form(pump curve)

2 2

1 1 2 21 2

2 2 friction pump P V P V z z h h g g g g

1 2

Q


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Definitions in a typical pump system:• Liquid flows from the

suction side to thedischarge side

• Suction head is headavailable just beforepump, h s:

• Discharge head is head atthe exit from pump, hd :

• Pump head, h p:

= head requiredfrom pump• Flow rates affect

terms h fd & h fs

2 2

1 1 2 21 2

2 2 friction pump P V P V z z h h g g g g

s s s fs

P h z h

g

d d d fd

P h z h

g

p d sh h h


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Positive Displacement Pumps• Properties of a PD pump:

– Pumps fluid by varying the dimension of an inner chamber.Volumetric flow rate determined size of chamber + RPM ofpump.

– Nearly independent of back pressure.• Application for metering fluids (example, chemicals into a process,

etc.) – Develops the required head to meet the specified flow rate

• Head limit is due to mechanical limitations (design/metallurgy).Catastrophic failure at limit.

• High pressure applications – Able to handle high viscosity fluids. – Often produces a pulsed flow


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Types of Positive DisplacementPumps

A. Reciprocating piston (steam pumps)B. External gear pump

C. Double-screw pump

D. Sliding vaneE. Three lobe pump

F. Double circumferential piston

G. Flexible tube squeegeeH. Internal gear


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Positive Displacement Pumps


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Centrifugal pumps

• Characteristics – Typically higher flow rates

than PDs. –

Comparatively steadydischarge. – Moderate to low pressure

rise. – Large range of flow rate

operation. – Sensitive to fluid viscosity.


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Efficiency of centrifugal pumps:2 2

1 1 2 21 2

2 2 fr ic tion pump P V P V z z h h g g g g

• From the energyequation, pumpsincrease the pressurehead

•

The power delivered tothe water (water horsepower) is given by

• The power delivered bythe motor to the shaft(breaking horse power)is given by

• Therefore, efficiency is:

P H

g

w P gQH w P Q P

bhp P T

w

BHP

P gQH P T

Note: 1HP = 746W


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Centrifugal pumps – pump curves• Real pumps are never ‘ ideal ’ and theperformance of the pumps are determined

experimentally by the manufacturer andtypically given in terms of graphs or pump

curves.• Typically performance is given by curves of:• Head versus capacity• Power versus capacity• NPSH versus capacity

– As Q increases the head developed by the screendecreases.

– Maximum head is at zero capacity – The maximum capacity of the pump is at the point where

no head is developed.


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Centrifugal pumps – Sample PumpCurve

• 3500 is the RPM• Impeller size 6¼ to 8¾ in. are shown• Maximum efficiency is ~50%.

– Note that pumps can operate at 80-90% eff.• Maximum normal capacity line

– Should not operate in the region to the rightof the line because pump can be unstable.

• Semi-open impeller – Max sphere 1¼” – This pump is designed for slurries /

suspensions and can pass particles up to1¼”. This is why efficiency is relatively low.

• Motor horse power. – Remember to correct for density using

previous equation• Operating line (system curve)

– This is dependent on the system you areputting the pump into. It is a plot from theenergy equation.

– That is, analyze the system to determine thepump head required as a function of flowrate through the pump … This will form thesystem line.

22

2 12 1 2

42

pump m D

P P L Q H z z f h

g D g


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Pump cavitation and NSPH• Cavitation should be avoided

due to erosion damage topump parts and noise.

• Cavitation occurs when P < P v somewhere in the pump

• Since pump increases

pressure, to preventcavitation we ensure suctionhead is large enoughcompared to vapourpressure P v

• Net positive suction head

• Often we evaluate NPSHusing energy equation andreference values – don’t

measure P inlet

s v s fs

P P NPSH z h

g


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NSPHrequired• Manufacturers determine

conservatively how muchNPSH is needed to avoidcavitation in the pump

– Systematic experimentaltesting

• NSPHrequired (NPSHR) isplotted on pump chart

– Caution: different axis scaleis common – read carefully

• Plot NPSH vs NSPH required to give safe operatingrange of pump

QQmax


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Part II.• Dimensional analysis• Affinity Laws


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Dimensionless pump performance

• Previous part: everything dimensional – Terminology used in pump systems – Pump performance charts – NPSH and avoiding cavitation (NPSH vs NPSHR)

• This part : – Discuss how centrifugal pumps might be scaled – Best efficiency point – Examples


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Dimensionless Pump Performance

• For geometrically similar pumps we expectsimilar dimensionless performance curves

• Dimensionless groups? – Capacity coefficient – Head coefficient – Power coefficient – Efficiency – NPSH?

• What to use for n (units 1/time): rad/s ( ), rpm, rps

3Q QC nD

2 2 H gH

C n D

3 5

bh

P

P C

n D

H QC C C

2 2 NPSH

g NPSH C

n D


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Dimensional Analysis

• If two pumps are geometrically similar,and

• The independent ’s are similar, i.e.,C Q,A = C Q,B

Re A = Re B A /D A = B /D B

• Then the dependent ’s will be thesameC H,A = C H,B C P,A = C P,B


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Affinity Laws• For two homologous states A and B, we can use

variables to develop ratios (similarity rules, affinitylaws, scaling laws).

• Useful to scale from model to prototype• Useful to understand parameter changes, e.g.,

doubling pump speed.

3

,,

A

B

A

B

A

B BQ AQ D

DQQ

C C


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Dimensional Analysis: ideal situation• If plotted in nondimensional

form, all curves of a family ofgeometrically similar pumpsshould collapse onto one set ofnondimensional pump

performance curves • From this we identify the best

efficiency point BEP• Note: Reynolds number and

roughness can often be

neglected


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Dimensionless Pump Performance•

In reality we never achieve truesimilarity – E.g. manufacturers put different

impeller into same housing – Following figure illustrates a typical

example of 2 pumps that are ‘close’ tosimilar

• Note:• See that at BEP: max = 088 • From which we get

• From which you can calculateQ, H, NPSH, P

* * * *, , ,Q H HS xC C C C


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Part III.• More on Centrifugal Pumps• Pump selection


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Pump selection

• Previous part : – Other types of pumps – Centrifugal and axial ducted – Pump specific speed

• This partNon-dimensional Pi Groups for pumps

– Application to optimize pump speed (BEP) – Scaling between pumps

3QQ

C nD

2 2 H gH

C n D

3 5

bh

P

P C

n D

2 2 NPSH

g NPSH C

n D


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Dynamic Pumps•

Dynamic Pumps include – centrifugal pumps : fluid enters

axially, and is discharged radially. – mixed--flow pumps : fluid enters

axially, and leaves at an anglebetween radially and axially. – axial pumps : fluid enters and

leaves axially.


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Centrifugal Pumps

• Snail--shaped scroll• Most common type of

pump: homes, autos,industry.


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Centrifugal Pumps


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Centrifugal Pumps: Blade Design


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Vector analysis of leading andtrailing edges.


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Blade number affects efficiency and introduces circulatory losses (too

few blades) and passage losses (too many blades)


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Axial Pumps

Open vs. Ducted Axial Pumps


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Open Axial Pumps

Propeller has radial twist totake into account for angularvelocity (= r)

Blades generate thrust likewing generates lift.


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Ducted Axial Pumps

• Tube Axial Fan: Swirldownstream

• Counter-Rotating Axial-Flow Fan: swirl removed.Early torpedo designs

• Vane Axial-Flow Fan: swirlremoved. Stators can beeither pre-swirl or post-swirl.


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Pump Specific Speed

Pump Specific Speed is used to characterize theoperation of a pump at BEP and is useful forpreliminary pump selection.


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Centrifugal pumps-specific speed

Proper

Lazy

'17,182 s s N N

Use Dimensionless ‘specific speed’ to help choose. Dimensionless speed isderived by eliminating diameters in C q and C h at the BEP.

12

34

1/ 2*

*'3 / 4

**

Q s

H

n QC N C gH

12

3/ 4

( / min)

( ) s

Rpm Gal N

H ft


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What we covered:• Characteristics of positive displacement

and centrifugal pumps• Terminology used in pump systems• Head vs flow rate: pump performance

charts• NPSH and avoiding cavitation (NPSH vs

NPSHR)• Examples


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What we covered:• Today we

– Developed dimensionless pumpvariables

–

Extrapolate existing pump curveto different pump speeds,diameters, and densities

– Examples

3QQ

C nD

2 2 H gH C n D

3 5

bh

P

P C

n D

2 2 NPSH

g NPSH C

n D


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What we covered•

Today we: – Examined axial, mixed, radial

ducted and open pump designs –

Used specific speed to determinewhich type is optimal


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Part IV.• Lab procedure• Venturi Measurements• Summary of equations and calculation way• Preparing graphs


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Lab Objectives

• Understand operation of a dc motor• Analyze fluid flow using

– Centrifugal pump – Venturi flow meter

• Evaluate pump performance as a function ofimpeller (shaft) speed

– Develop pump performance curves – Assess efficiencies


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Lab Set-up

Motor

E I

T

Pump

Water Tank

Venturi P( )

ValvePaddle meter

Dynamometer

Pin

Pout


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D.C motor

Figure 1. dc motor (howstuffworks.com)

• Armature or rotor• Commutator• Brushes• Axle• Field magnet• DC power supply

http://auto.howstuffworks.com/enlarge-image.htm?terms=motor&page=0


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Centrifugal pump operation

• Rotating impeller delivers energy to fluid

•

Governing equations or Affinity Laws relate pump speed to: – Flow rate, Q – Pump head, H p – Fluid power, P


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0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.000 0.002 0.004 0.006 0.008 0.010 0.0120

2

4

6

8

10

12

14

16

18

20

22

24

0.000 0.002 0.004 0.006 0.008 0.010 0.0120

2

4

6

8

10

12

14

16

18

20

22

24

pump head 1709 rpm

Flow Rate (m 3 /s)

Head(m)

0

200

400

600

800

1000

1200

1400

fluid power 1709 rpm

fluidpower(W)

operating point

pump efficiency 1709 rpm

pumpefficiency,

system load - head


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Pump Affinity Laws

• N Q• N 2 H p•

N 3 P

2

13

2

1

2

12

2

1

2

1

2

1

P P

N N

H H

N N

QQ

N N

p

p


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Determination of Pump Head

12

21

22

2 Z Z

g

V V

g

P P H inout p

g

P P H inout

p


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Determination of Flow Rate

• Use Venturi meter to determine Q

• Fluid is incompressible (const. )Q = V fluid Area


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Venturi Meter

• As V , kinetic energy• T = 0• Height = 0• Pv or P


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Calculate Q from Venturi data

22V AC Q d • V 1 = inlet velocity• V 2 = throat velocity• A1 = inlet area• A2 = throat area


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Throat Velocity

2222

1121

22 Z

g P

g V Z

g P

g V

0 Z 2

21

221 BV

A AV V 21 P P P

),,(2 B P f V vAmm

21

..


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Discharge Coefficient

eDd

R

BC 53.6907.0

1

2

D

D B

11 DV R eD

22

1

221 BV

A AV V


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Solve for Q• Use MS EXCEL (or Matlab)• Calculate throat velocity• Calculate discharge coefficient using

Reynold’s number and throat velocity • Calculate throat area• Solve for Q


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Power and Pump Efficiency• Assumptions

– – No change in elevation – No change in pipe diameter – Incompressible fluid – T = 0

• Consider 1 st Law (as a rate eqn.)

0Q

122122122

1 Z Z g V V hhmW Q


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Pump Power Derivation

Pvuh

v P uv P umhhmW 112212

12 P P vmW QV Avm

12 P P QW


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Efficiencies

EI

P P Q EI T

T P P Q

input output

overall

motor

pump

12

12


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Summary of Lab Requirements

• Plots relating H p , P , and pump to Q• Plot relating P to pump • Regression analyses• Uncertainty of overall (requires unc. of Q )• Compare H p , P , Q for two N ’s

–

For fully open valve position – WRT affinity laws


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905 rpm 1099 rpm 1303 rpm 1508 rpm

Flow Rate (m 3/s)

P

owerDelevered to Fluid (W)

1709 rpm


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pump efficiency

Flow Rate (m 3/s)

905 rpm 1099 rpm 1303 rpm 1508 rpm

1709 rpm


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Pump Efficiency

pump power delivered to fluid (W)

905 rpm 1099 rpm 1303 rpm 1508 rpm 1709 rpm


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Start-up Procedure1. Fill pvc tube with water (3/4 full)2. Bleed pump3. Switch breaker to “on” 4. Push main start button

5. Make sure variac is turned counterclockwise6. Make sure throttle valve is fully open7. Turn lever to “pump”

8. Push “reset” button 9. Push “start” button 10. Adjust variac to desired rpm using tach.


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Pump lab raw data

Shaftspeed(rpm)

DCvoltage(volts)

DCcurrent(amps)

InletPressure(in Hg)

OutletPressure(kPa)

Venturi DP(kPa)

Dyna(lbs)


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Shut-down Procedure

1. Fully open throttle valve2. Turn variac fully counterclockwise

3. Push pump stop button4. Turn pump lever to “off” 5. Push main stop button

6. Switch breaker to “off”

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