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Basic Pump SelectionPresented by:
Larry Konopacz, Manager Training & Education
This presentation is being brought to you by:
Rocky Mountain ASHRAE Chapter
McNevin CompanyAurora, CO
www.mcnevinco.com
and
ITT Bell & GossettThe Little Red Schoolhouse®
Friday, April 15, 2011
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What is a Centrifugal Pump?
Three basic components:
•Volute, casing, body■ or Diffuser
•Impeller■ or impellers
•Driver (motor)
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Centrifugal Acceleration
LowVelocity
At rest Rotating
HighVelocity
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Pump Impeller
Vanes
Direction ofrotation
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Single Suction Double Suction
Typical Impellers
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Full Diameter Impeller
Rotation
Impeller
Blades
Vt
Vr Vs
Vr = Radial VelocityVt = Tangential VelocityVs = Vector Sum Velocity
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Trimmed Impeller
Rotation
Full ImpellerFull Impeller
Vt
Vr Vs
Trimmed ImpellerTrimmed Impeller
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Impeller and Volute
Suction Eye
Cutwater
Arrows represent thedirection of water flow Discharge
Nozzle
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Velocity Through the Pump
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Small Circulators…”Boosters”
Close CoupledSystem Lubricated Circulator
Close CoupledSystem Lubricated Circulator
3 Piece CirculatorOil Lubricated
3 Piece CirculatorOil Lubricated
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Large, Line Mounted Pumps
• Close Coupled
• Special Purpose Motor
• Spacer Coupler (Rigid)
• Special Purpose Motor
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Base Mounted End Suction Pump
• Single Suction Impeller
• Flexible Coupler
• General Purpose Motor
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• Horizontal or Vertical Installation
• Special Purpose Motor
Close Coupled End Suction Pump
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Double Suction Pump
• Vertical Split Case
• Reduced Axial Loads
• General Purpose Motor
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• Horizontal Split Case
• Reduced Axial Loads
• General Purpose Motor
Double Suction Pump
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This Is Not A Pump Curve
Tota
l Hea
d In
Fee
t(F
oot
Lbs
Per
Lb
Wat
er P
um
ped
)
Capacity In US Gallons Per Minute
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Typical Pump Curve – Constant Speed
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Typical Pump Curve – Variable Speed
Control Curve
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Pump Selection - Things to Consider•Pump location
■ Equipment room or plenum?
■ Close to occupied areas?
•Available space■ Installation footprint
■ Maintenance footprint
•Maintenance requirements■ Parts availability
■ Special skills, tools etc.
•Reliability
•Hydraulic requirements■ Flow rate, head, efficiency, horsepower
■ NPSH
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Hydraulic Requirements
•Analyze the system: determine head and flow
•Evaluate individual pump curves for:■Duty point with respect to BEP
■Horsepower requirement
■Efficiency
■NPSHR
•Estimate life cycle costs■ Initial cost
■Annual operating cost
■Use software to speed the process
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Pump Life Cycle Costs
Life Cycle Costs
Operating 10%
Maintenance 25%
Energy 40%
Installation 7%
Downtime 3%
Environmental 5%Pump 10%
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You have many decisions to make:
•“Suitable” pump configuration.
•Pump size within that configuration.
•Impeller diameter.
•Seal type and materials.
•Pressure rating.
•Motor speed, type of enclosure, size.
•Costs
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What’s “Suitable”?
•Base mounted or in-line?
•Close coupled or flexibly coupled?
•Single or double suction impeller?
•Pump installation and maintenance details.
•Packed pumps or internally flushed mechanical seals?
•To the left or right of the best efficiency point (BEP)?
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Pump Selection – Preferred Region
2008 ASHRAE Handbook – HVAC Systems and Equipment, p 43.10
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Pump Selection – Radial Thrust
2008 ASHRAE Handbook – HVAC Systems and Equipment, p 43.9
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•Operation well above BEP■ Lower efficiency■ Excessive bending stress on the shaft■ NPSHR increases■ High fluid velocity may lead to noise, vibration, or
erosion.
•Operation well below BEP■ Lower efficiency■ Large radial forces acting on the shaft■ Incipient cavitation■ A common rule of thumb defines low flow at 25% of best
efficiency flow.
Pump Operation in Relation to BEP
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Pump Selection - Which RPM…?
•The three most common pump speeds in HVAC applications:■2 pole, 3500 rpmlow flow rates, high head, intermittent operation
■4 pole, 1750 rpmmost common, low noise, long bearing and seal life
■6 pole, 1150 rpmhigh flow rates, low head, quiet operation
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Inertial loads imposed on the motor shaft by the 1750 rpm impeller are 16 times greater
than those of a 3500 rpm impeller.
g
WrI
2
2
1750 impeller is twice the diameter, and four times the
weight of 3500 impeller
3500 RPM for Intermittent Operation
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Closed/Open System Definitions
•Closed System■Has only one point of contact with a
compressible gas.
■Elevation differences may exist, but can’t cause flow.
•Open System■Has several points of contact with a
compressible gas.
■Elevation differences can cause flow.
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Source
Pump
Load
• Boiler• Chiller
Closed System
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Static Pressure and Pump Head
Which system has the greater static pressure?
Which one requires more pump head?
A
BPa Pb
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•Q1 = Known (design) Flow
•Q2 = Final Flow
•h1 = Known (design) Head
•h2 = Final Head
Q
Q
h
h2
1
2
2
1
System Curve Construction
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The System Curve
115165185200215230
102025303540
G.P.M. Ft. HD.
Q
Q
h
h2
1
2
2
1
Q
Q
Q
Q
Q
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2
2
2
2
200
10
30
20010
30
200 33
200
.
0.574456
114.8913 115
“Simple”
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Tota
l Hea
d In
Fee
t
Capacity In US Gallons Per Minute
System Curve
50 100 150 200 250
10
20
30
40
50
What we need
System Curve Construction
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Pump and System Operation
Tota
l Hea
d In
Fee
t
Capacity In US Gallons Per Minute
System Curve
50 100 150 200 250
10
20
30
40
50 OperatingPoint
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Tota
l Hea
d In
Fee
t
Capacity In US Gallons Per Minute
1
2
3
4
Where will the pump operate?
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Static Suction HeadLess Than
Static Discharge Head
Static Suction LiftPlus
Static Discharge Head
TotalStaticHead
TotalStaticHead
StaticSuctionHead
StaticDischarge
Head
StaticSuction
Lift
Open System – Total Static Head
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Condenser(Known head loss)
Pump
BasinWaterLevel
(Constant)
TotalStaticHead10’
hf = 30’ @ 200 GPM
Pipe Friction Loss(Varies with flow)
Cooling Tower System
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Tota
l Hea
d In
Fee
t
Capacity In US Gallons Per Minute
System Curve
50 100 150 200 250
10
20
30
40
50Operating
Point
Constant head loss
Variable head loss
Open System Operation
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Tot
al H
ead
In F
eet
(Foo
t Lb
s P
er L
b W
ate
r P
umpe
d)
Capacity In US Gallons Per Minute
Head Capacity
NPSHR
NPSHRfeet
NPSHR
Net Positive Suction Head Required
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to discharge
VaporPressure
Suction Discharge
Ps
VaporPressure
Suction Discharge
Ps
NPSH Required
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Cavitation = Swiss Cheese Impeller
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Static Suction Head• from surface to centerline• adds to NPSHA
Static Suction Lift• from surface to centerline• subtracts from NPSHA
NPSH Available From the System
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Vent
5’
Liquid type and temperature?
MinimumFlow friction loss = 0.5’
NPSHR = 5’
Increasing NPSHA
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•NPSH Required■ Impeller design, shape,
construction
■Plotted on pump curve
■ Increases with flow
•NPSH Available■PositivesStatic suction head
Lower vapor pressure
Higher system pressure
■NegativesFriction losses
Static suction lift
NPSH Summary
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Head
Head
Flow
Pump Curve
NPSHR Curve
Pump Curve
NPSHR Curve
Larger Pump
Smaller Pump
Pump Selection For Low NPSHR
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Pump Selection – Parallel Pumps
System Head
1/2 system flow*
1/2 system flow*
* Size piping for total flow.
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Pump Selection – Parallel Pumps
No intersection Point Poor Pump Selection
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Pump Selection – Parallel Pumps
Intersection Point Good Pump Selection
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Saving Energy in Hydronic Systems
•Good pump selections.
•Insure the system is hydronically balanced.
•Trim the impeller based on measured values.
•Consider:■using multiple pumps
■primary-secondary pumping
■variable speed pumping
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Tota
l Hea
d In
Fee
t
Capacity In US Gallons Per Minute
15 HP12 HP
10 HP7.5 HP
5 HP
**Some impellers can’t be trimmed
Consider Impeller Trimming**
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Good Pump Selections
•Operate most of the time at high efficiencies
•Prevent heavy bearing loads
•Avoid cavitation
•Keep from running off the end of the curve
•Minimize operating costs, maintenance costs
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Avoid Pumping System Problems
•Make good pump selections
•Proper installation - including all required accessories
•Proper system cleaning and commissioning
•Periodic inspection and routine service
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•Lack of a systems approach during the design process
•Overly conservative or improper pump selection, resulting in poor performance
•Improper installation or operation
•Poor maintenance
•System requirements change over time
Why Improvement Opportunities Exist
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Basic Pump Selection
Any Questions?
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Thanks for Attending!
This presentation was brought to you by:
Rocky Mountain ASHRAE Chapter
McNevin CompanyAurora, CO
www.mcnevinco.com
and
ITT Bell & GossettThe Little Red Schoolhouse®
Friday, April 15, 2011