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ME 4880 Experimental Design Lab
Centrifugal Pump Performance Experiment
Instructors:
Dr. Cyders, 294A Stocker, cyderst@ohio.edu
Dr. Ghasvari, 249B Stocker, ghasvari@ohio.edu
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
• 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)
2 2
1 1 2 21 2
2 2friction pump
P V P Vz z h h
g g g g
1 2
Q
Definitions in a typical pump system:
• Liquid flows from the suction side to the discharge side
• Suction head is head available just before pump, hs:
• Discharge head is head at the exit from pump, hd:
• Pump head, hp:
= head required from pump • Flow rates affect terms hfd & hfs
2 2
1 1 2 21 2
2 2friction pump
P V P Vz z h h
g g g g
ss s fs
Ph z h
g
dd d fd
Ph z h
g
p d sh h h
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. Reciprocating piston (steam pumps)
B. External gear pump
C. Double-screw pump
D. Sliding vane
E. Three lobe pump
F. Double circumferential piston
G. Flexible tube squeegee
H. 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: 2 2
1 1 2 21 2
2 2friction pump
P V P Vz z h h
g g g g • 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:
PH
g
wP gQHwP Q P
bhpP T
w
BHP
P gQH
P T
Note: 1HP = 746W
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%.
– Note that pumps can operate at 80-90% eff.
• Maximum normal capacity line – Should not operate in the region to the right
of 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 to 1¼”. 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 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.
2
22 1
2 1 2
42
pump mD
P P L QH z z f h
g D g
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
s vs fs
P PNPSH z h
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
Q Qmax
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?
– Capacity coefficient
– Head coefficient
– Power coefficient
– Efficiency
– NPSH?
• What to use for n (units 1/time): rad/s (), rpm, rps
3Q
QC
nD
2 2H
gHC
n D
3 5
bh
P
PC
n D
H QC C
C
2 2NPSH
g NPSHC
n D
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).
• Useful to scale from model to prototype • Useful to understand parameter changes, e.g.,
doubling pump speed.
3
,,
A
B
A
B
A
BBQAQ
D
D
Q
QCC
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
• From which you can calculate Q, H, NPSH, P
* * * *, , ,Q H HS xC C C C
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
3Q
QC
nD
2 2H
gHC
n D3 5
bh
P
PC
n D
2 2NPSH
g NPSHC
n D
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
Propeller has radial twist to take into account for angular velocity (=r)
Blades generate thrust like wing generates lift.
Ducted Axial Pumps
• Tube Axial Fan: Swirl
downstream
• Counter-Rotating Axial-
Flow Fan: swirl removed. Early torpedo designs
• Vane Axial-Flow Fan: swirl removed. Stators can be either pre-swirl or post-swirl.
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
Proper Lazy
'17,182s sN N
Use Dimensionless ‘specific speed’ to help choose. Dimensionless speed is derived by eliminating diameters in Cq and Ch at the BEP.
12
34
1/ 2*
*'
3/ 4*
*
Q
s
H
n QCN
C gH
12
3/ 4
( / min)
( )s
Rpm GalN
H ft
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
3Q
QC
nD
2 2H
gHC
n D
3 5
bh
P
PC
n D
2 2NPSH
g NPSHC
n D
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
Motor
E I
T
Pump
Water Tank
Venturi
P( )
ValvePaddle meter
Dynamometer
Pin
Pout
D.C motor
Figure 1. dc motor (howstuffworks.com)
•Armature or rotor
•Commutator
•Brushes
•Axle
•Field magnet
•DC power supply
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
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 0.012
0
2
4
6
8
10
12
14
16
18
20
22
24
0.000 0.002 0.004 0.006 0.008 0.010 0.012
0
2
4
6
8
10
12
14
16
18
20
22
24
pump head 1709 rpm
Flow Rate (m3/s)
Head
(m
)
0
200
400
600
800
1000
1200
1400
fluid power 1709 rpm
flu
id p
ow
er
(W)
operating point
pump efficiency 1709 rpm
pu
mp
eff
icie
ncy,
system load - head
Pump Affinity Laws
• N Q
• N2 Hp
• N3 P
2
1
3
2
1
2
12
2
1
2
1
2
1
P
P
N
N
H
H
N
N
Q
Q
N
N
p
p
Determination of Pump Head
12
21
22
2ZZ
g
VV
g
PPH inout
p
g
PPH inout
p
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
22VACQ d
• V1 = inlet velocity
• V2 = throat velocity
• A1 = inlet area
• A2 = throat area
Throat Velocity
22
22
11
21
22Z
g
P
g
VZ
g
P
g
V
0Z2
21
221 BV
A
AVV 21 PPP
),,(2 BPfV
vAmm21
..
Discharge Coefficient
eDd
R
BC 53.6907.0
1
2
D
DB
11DVReD
22
1
221 BV
A
AVV
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
–
– No change in elevation
– No change in pipe diameter
– Incompressible fluid
– T = 0
• Consider 1st Law (as a rate eqn.)
0Q
12
21
2212
2
1ZZgVVhhmWQ
Pump Power Derivation
Pvuh
vPuvPumhhmW 112212
12 PPvmW
QVAvm
12 PPQW
Efficiencies
EI
PPQ
EI
T
T
PPQ
input
output
overall
motor
pump
12
12
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
Flow Rate (m3/s)
Pu
mp
He
ad
(m
) 905 rpm 1099 rpm 1303 rpm 1508 rpm
1709 rpm
905 rpm 1099 rpm 1303 rpm 1508 rpm
Flow Rate (m3/s)
Po
we
r D
ele
ve
red
to
Flu
id (
W)
1709 rpm
pu
mp
eff
icie
ncy
Flow Rate (m3/s)
905 rpm 1099 rpm 1303 rpm 1508 rpm
1709 rpm
Pu
mp
Eff
icie
ncy
pump power delivered to fluid (W)
905 rpm 1099 rpm 1303 rpm 1508 rpm
1709 rpm
Start-up Procedure 1. Fill pvc tube with water (3/4 full)
2. Bleed pump
3. Switch breaker to “on”
4. Push main start button
5. Make sure variac is turned counterclockwise
6. Make sure throttle valve is fully open
7. Turn lever to “pump”
8. Push “reset” button
9. Push “start” button
10. 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. Fully open throttle valve
2. Turn variac fully counterclockwise
3. Push pump stop button
4. Turn pump lever to “off”
5. Push main stop button
6. Switch breaker to “off”
Recommended