ME 4880 Experimental Design Lab
Centrifugal Pump Performance Experiment
Instructors:
Dr. Cyders, 294A Stocker, [email protected]
Dr. Ghasvari, 249B Stocker, [email protected]
Spring 2014
1
Part I.
• General topics on Pumps
• Categories of Pumps
• Pump curve
• Cavitation
• NSPH
Pumps
– Basic definitions to describe pumps and pumping
pipe circuits
– Positive displacement pumps and centrifugal
pumps
– The ‘Pump Curve’
– Net Positive Suction Head
Pump analysis: energy equation
P1 V12
P2 V2 2

 z1 

 z2  h friction  hpump
 g 2g
 g 2g
Q
1
2
• Shaft work delivered by pump is translated into a
pressure rise across the pump: P2 > P1
• How does hpump vary with Q?
– Typically data is gathered from experiments by
manufacturer and is presented in dimensional form
(pump curve)
Definitions in a typical pump system:
• Liquid flows from the
P1 V12
P2 V2 2

 z1 

 z2  h friction  hpump
suction side to the
 g 2g
 g 2g
discharge side
• Suction head is head
P
available just before
hs  zs  s  h fs
g
pump, hs:
• Discharge head is head at
Pd
h

z

h
d
d
the exit from pump, hd:
 g fd
• Pump head, hp:
hp  hd  hs
= head required
from pump
• Flow rates affect
terms hfd & hfs
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 of
pump.
– 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
Types of Positive Displacement
Pumps
A.
B.
C.
D.
E.
F.
G.
H.
Reciprocating piston (steam pumps)
External gear pump
Double-screw pump
Sliding vane
Three lobe pump
Double circumferential piston
Flexible tube squeegee
Internal gear
Positive Displacement Pumps
Centrifugal pumps
• Characteristics
– Typically higher flow rates
than PDs.
– Comparatively steady
discharge.
– Moderate to low pressure
rise.
– Large range of flow rate
operation.
– Sensitive to fluid viscosity.
Efficiency of centrifugal pumps:
• From the energy
equation, pumps
increase the pressure
head
• The power delivered to
the water (water horse
power) is given by
• The power delivered by
the motor to the shaft
(breaking horse power)
is given by
• Therefore, efficiency is:
Note: 1HP = 746W
P1 V12
P2 V2 2

z 

z h
h
 g 2 g 1  g 2 g 2 friction pump
H
P
g
Pw  QP Pw   gQH
Pbhp  T

Pw
 gQH

PBHP
T
Centrifugal pumps – pump curves
• Real pumps are never ‘ideal’ and the
performance of the pumps are determined
experimentally by the manufacturer and
typically 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 screen
decreases.
– Maximum head is at zero capacity
– The maximum capacity of the pump is at the point where
no head is developed.
Centrifugal pumps – Sample Pump
Curve
•
•
•
3500 is the RPM
Impeller size 6¼ to 8¾ in. are shown
Maximum efficiency is ~50%.
–
•
Maximum normal capacity line
–
•
–
H pump
 
4
2
Remember to correct for density using
previous equation
Operating line (system curve)
–
2
P2  P1
 L
 Q

 z2  z1   f  hm 
g
 D
 2 g  D2
Max sphere 1¼”
This pump is designed for slurries /
suspensions and can pass particles up to
1¼”. This is why efficiency is relatively low.
Motor horse power.
–
•
Should not operate in the region to the right
of the line because pump can be unstable.
Semi-open impeller
–
–
•
Note that pumps can operate at 80-90% eff.
This is dependent on the system you are
putting the pump into. It is a plot from the
energy equation.
That is, analyze the system to determine the
pump head required as a function of flow
rate through the pump … This will form the
system line.
Pump cavitation and NSPH
• Cavitation should be avoided
due to erosion damage to
pump parts and noise.
• Cavitation occurs when P <
Pv somewhere in the pump
• Since pump increases
pressure, to prevent
cavitation we ensure suction
head is large enough
compared to vapour
pressure Pv
• Net positive suction head
• Often we evaluate NPSH
using energy equation and
reference values – don’t
measure Pinlet
NPSH  zs 
Ps  Pv
 h fs
g
NSPHrequired
• Manufacturers determine
conservatively how much
NPSH is needed to avoid
cavitation in the pump
– Systematic experimental
testing
• NSPHrequired (NPSHR) is
plotted on pump chart
– Caution: different axis scale
is common – read carefully
• Plot NPSH vs NSPHrequired
to give safe operating
range of pump
Qmax
Q
Part II.
• Dimensional analysis
• Affinity Laws
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
Dimensionless Pump Performance
• For geometrically similar pumps we expect
similar dimensionless performance curves
• Dimensionless groups? Q
CQ 
nD
– Capacity coefficient
gH
CH  2 2
– Head coefficient
n D
– Power coefficient
– Efficiency
g  NPSH 
C

NPSH
– NPSH?
n2 D 2
3
CP 
Pbh 
 n3 D 5
• What to use for n (units 1/time): rad/s (), rpm, rps

CH CQ
C
Dimensional Analysis
• If two pumps are geometrically similar,
and
• The independent ’s are similar, i.e.,
CQ,A = CQ,B
ReA = ReB
A/DA = B/DB
• Then the dependent ’s will be the
same
CH,A = CH,B
CP,A = CP,B
Affinity Laws
• For two homologous states A and B, we can use 
variables to develop ratios (similarity rules, affinity
laws, scaling laws).
CQ , A  CQ , B
Q
 D 
 B  B  B 
QA  A  DA 
3
• Useful to scale from model to prototype
• Useful to understand parameter changes, e.g.,
doubling pump speed.
Dimensional Analysis: ideal situation
• If plotted in nondimensional
form, all curves of a family of
geometrically similar pumps
should collapse onto one set of
nondimensional pump
performance curves
• From this we identify the best
efficiency point BEP
• Note: Reynolds number and
roughness can often be
neglected
Dimensionless Pump Performance
• In reality we never achieve true
similarity
–
–
E.g. manufacturers put different
impeller into same housing
Following figure illustrates a typical
example of 2 pumps that are ‘close’ to
similar
• Note:
• See that at BEP: max = 088
• From which we get
*
CQ* , CH* , CHS
, C* x
• From which you can calculate
Q, H, NPSH, P
Part III.
• More on Centrifugal Pumps
• Pump selection
Pump selection
• Previous part :
– Other types of pumps
– Centrifugal and axial ducted
– Pump specific speed
• This part
Non-dimensional Pi Groups for pumps
– Application to optimize pump speed (BEP)
– Scaling between pumps
CNPSH
g  NPSH 

n2 D 2
CP 
Pbh 
 n3 D 5
CH 
gH
n2 D2
CQ 
Q
nD3
Dynamic Pumps
• Dynamic Pumps include
– centrifugal pumps: fluid enters
axially, and is discharged radially.
– mixed--flow pumps: fluid enters
axially, and leaves at an angle
between radially and axially.
– axial pumps: fluid enters and
leaves axially.
Centrifugal Pumps
• Snail--shaped scroll
• Most common type of
pump: homes, autos,
industry.
Centrifugal Pumps
Centrifugal Pumps: Blade Design
Centrifugal Pumps: Blade Design
Vector analysis of leading and
trailing edges.
Centrifugal Pumps: Blade Design
Blade number affects efficiency and introduces circulatory losses (too
few blades) and passage losses (too many blades)
Axial Pumps
Open vs. Ducted Axial Pumps
Open Axial Pumps
Blades generate thrust like
wing generates lift.
Propeller has radial twist to
take into account for angular
velocity (=r)
Ducted Axial Pumps
• Tube Axial Fan: Swirl
downstream
• Counter-Rotating AxialFlow Fan: swirl removed.
Early torpedo designs
• Vane Axial-Flow Fan: swirl
removed. Stators can be
either pre-swirl or postswirl.
Pump Specific Speed
Pump Specific Speed is used to characterize the
operation of a pump at BEP and is useful for
preliminary pump selection.
Centrifugal pumps-specific speed
Use Dimensionless ‘specific speed’ to help choose. Dimensionless speed is
derived by eliminating diameters in Cq and Ch at the BEP.
Proper
Lazy
N s' 
CQ*
1
2
CH *
3
4

 
 gH 
n Q*
* 3/ 4
1
Ns 
Rpm(Gal / min) 2
 H ( ft ) 
1/ 2
3/ 4
N s  17,182 N s'
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
What we covered:
• Today we
– Developed dimensionless pump
variables
– Extrapolate existing pump curve
to different pump speeds,
diameters, and densities
– Examples
CQ 
Q
nD3
CH 
gH
n2 D2
Pbh 
CP 
CNPSH
 n3 D 5
g  NPSH 

n2 D 2
What we covered
• Today we:
– Examined axial, mixed, radial
ducted and open pump designs
– Used specific speed to determine
which type is optimal
Part IV.
•
•
•
•
Lab procedure
Venturi Measurements
Summary of equations and calculation way
Preparing graphs
Lab Objectives
• Understand operation of a dc motor
• Analyze fluid flow using
– Centrifugal pump
– Venturi flow meter
• Evaluate pump performance as a function of
impeller (shaft) speed
– Develop pump performance curves
– Assess efficiencies
Lab Set-up
Paddle meter
Valve
Venturi
(P)
Dynamometer
E
I
Pout
Pump
Motor
T
Water Tank
Pin
D.C motor
•Armature or rotor
•Commutator
•Brushes
•Axle
•Field magnet
•DC power supply
Figure 1. dc motor (howstuffworks.com)
Centrifugal pump operation
• Rotating impeller delivers energy to fluid
• Governing equations or Affinity Laws relate
pump speed to:
– Flow rate, Q
– Pump head, Hp
– Fluid power, P
24
1400
0.6
22
20
1200
0.5
Head (m)
14
800
12
10
600
operating point
8
400
6
pump head 1709 rpm
200
fluid power 1709 rpm
pump efficiency 1709 rpm
system load - head
4
2
0
0.000
fluid power (W)
1000
16
0.002
0.004
0.006
0.008
3
Flow Rate (m /s)
0.010
0
0.012
pump efficiency, 
18
0.4
0.3
0.2
0.1
0.0
Pump Affinity Laws
• NQ
• N2  Hp
• N3  P
N1 Q1

N 2 Q2
2
H p1
 N1 

 
H p2
 N2 
3
 N1 
P1

 
P2
 N2 
Determination of Pump Head
Pout  Pin V22  V12
Hp 

 Z 2  Z1
g
2g
Pout  Pin
Hp 
g
Determination of Flow Rate
• Use Venturi meter to determine Q
• Fluid is incompressible (const.  )
Q = Vfluid Area
Venturi Meter
•
•
•
•
As V , kinetic energy 
T = 0
 Height = 0
Pv or P 
Calculate Q from Venturi data
Q  Cd A2V2
•
•
•
•
V1 = inlet velocity
V2 = throat velocity
A1 = inlet area
A2 = throat area
Throat Velocity
2
2
V1
P1
V2
P2

 Z1 

 Z2
2g
Z  0
g
g
2g
A2
V1  V2
 V2 B 2
A1
.
.
P  P1  P2

m 1  m 2  A v
V2  f (P, B,  )
Discharge Coefficient
B
Cd  0.907  6.53
ReD
ReD
V1D1


D2
B
D1
A2
2
V1  V2
 V2 B
A1
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
Power and Pump Efficiency
• Assumptions
– Q  0
– No change in elevation
– No change in pipe diameter
– Incompressible fluid
– T = 0
• Consider 1st Law (as a rate eqn.)


1 2


2


Q  W  m h2  h1   V2  V1  g Z 2  Z1 
2


Pump Power Derivation
h  u  Pv
 h2  h1   m
 u2  P2v   u1  P1v 
W  m
 vP2  P1 
W  m

 v  AV  Q
m

W  QP2  P1 
Efficiencies
output QP2  P1 
 pump 

input
T
T
 motor 
EI
QP2  P1 
 overall 
EI
Summary of Lab Requirements
•
•
•
•
•
Plots relating Hp, P, and pump to Q
Plot relating P to pump
Regression analyses
Uncertainty of overall (requires unc. of Q)
Compare Hp, P, Q for two N’s
– For fully open valve position
– WRT affinity laws
Pump Head (m)
905 rpm
1099 rpm
1303 rpm
1508 rpm
1709 rpm
3
Flow Rate (m /s)
Power Delevered to Fluid (W)
905 rpm
1099 rpm
1303 rpm
1508 rpm
1709 rpm
3
Flow Rate (m /s)
pump efficiency
905 rpm
1099 rpm
1303 rpm
1508 rpm
1709 rpm
3
Flow Rate (m /s)
Pump Efficiency
905 rpm
1099 rpm
1303 rpm
1508 rpm
1709 rpm
pump power delivered to fluid (W)
Start-up Procedure
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
Fill pvc tube with water (3/4 full)
Bleed pump
Switch breaker to “on”
Push main start button
Make sure variac is turned counterclockwise
Make sure throttle valve is fully open
Turn lever to “pump”
Push “reset” button
Push “start” button
Adjust variac to desired rpm using tach.
Pump lab raw data
Shaft
speed
(rpm)
DC
voltage
(volts)
DC
current
(amps)
Inlet
Pressure
(in Hg)
Outlet
Pressure
(kPa)
Venturi DP
(kPa)
Dyna
(lbs)
Shut-down Procedure
1.
2.
3.
4.
5.
6.
Fully open throttle valve
Turn variac fully counterclockwise
Push pump stop button
Turn pump lever to “off”
Push main stop button
Switch breaker to “off”