Basic Pump Selection2 · This presentation is being brought to you by: Rocky Mountain ASHRAE Chapter McNevin Company Aurora, CO and ITT Bell & Gossett The Little Red Schoolhouse®

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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

4

Pump Impeller

Vanes

Direction ofrotation

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Single Suction Double Suction

Typical Impellers

6

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)


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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


Close Coupled End Suction Pump

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Double Suction Pump

• Vertical Split Case

• Reduced Axial Loads


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• Horizontal Split Case

• Reduced Axial Loads


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

22

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


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


System Curve

50 100 150 200 250

10

20

30

40

50 OperatingPoint

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Tota

l Hea

d In

Fee

t


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


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)


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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No intersection Point Poor Pump Selection

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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


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

Documents

Basic Pump Selection2 · This presentation is being brought to you by: Rocky Mountain ASHRAE Chapter McNevin Company Aurora, CO and ITT Bell & Gossett The Little Red Schoolhouse®