VARIABLE SPEED PUMPING SYSTEMS

Rick FuquaBeckwith and KuffelCheney, Washington

Objective

Discuss the differences between fixed speed pump operation and variable speedpump operation.

• Overview of system curves

VS pump performance

• Interaction with systems

• Reduced maintenance cost

• System reliability

• Energy savings

Changing Speed

The operating point of a centrifugal pump is always determined by the interaction of:

The pump head-capacity curveSystem resistance curve

In general:

Increase in speed results in an increase in pump flow and headDecrease in speed results in a decrease in pump flow and head

Common misconception

Variable speed operation allows you to control head and flow at the sametime.

Western Dry Kiln Association 1 May, 2005

Hydraulic System

Frictional Heada.k.a Dynamic Head Loss

2—H

f = fx d 2V

g2

gPipe Intim Ocenpontet

Fticum

Dynamic Head Curve

Western Dry Kiln Association 2 May, 2005

Dynamic Head Curve

System CurveStatic Plus Frictional Head Effects

Operating PointIntersection of Pump Curve and System Curve

Western Dry Kiln Association 3 May, 2005

Static Head...not always "static"

Static HeadAdding Back Pressure

Control Valve OperationChanging System Curve with Valve Position

Flow

Western Dry Kiln Association 4 May, 2005

Variable Speed Performance

Affinity Laws

Affinity laws are used to estimate pump performance with changes in operating speedor impeller trim.

Diameter *eedChange Change

Di

Q2 D2 % N2

12 2Hi (Di HI NI

P1 DI 33H

P2D2 P2

Pump PerformanceFactored for Speed

2D

ID

20 dD OD 3D I DID 12.D

1121:12 1.12 N2

I OD

3D

CO

do

2D

Western Dry Kiln Association 5 May, 2005

Variable Speed Operation

Reduced Speed Implications

Western Dry Kiln Association 6 May, 2005

Reduced Speed Implications

High static head applications: constant pressure

Some applications have a very high static head in relation to the dynamic head curve.

• Pumping against high elevations• Pumping into pressurized vessels• Pumping systems with multiple demands off a header

Reliability

Reliability Index

• F r -RPM fader RI = Fr X Fd X Fq

• Fd - Impeller ciameter factor• F, - Best Eft deny Poirt Factor

Mil- %. fl&canit

Western Dry Kiln Association May, 2005

Reliability Index

Reliability

• Reduced pump and system wear due to reduced fluid velocity and system pressure• Protection against process upsets (dry run, dead head and run-out)• Reduced system damage from hydraulic hammer effect during start up and shut

down• Heat, noise and vibration reduction due to right sizing and demand control

Estimating Energy Savings

Information required to estimate energy savingsSystem curve data

rated operating point and static head, ortwo points on system curve

• Time operating at a given rating (hours)• Pump performance curve at known speed• Knowledge of current control method• Cost per energy unit (typically $/kWh)

Optional data

Valve loss coefficients for estimating savings at maximum rated condition

Energy Saving Calculation

Biding Mel Spe elMI

VS witVineile

Speed HP221 22 36 00

2 18.4 28 98 10 53 12.4 20 88 2.7

Mktg limeHeed Speed

kw HAxed Sp eed

$Maid&

Sp eel Mk

VtliabkSp eel

$1 2190 3 5 942 $ 1,438 3 5942 $ 1,4382 4380 3 0 061 $ 1,202 1 71 54 $ 686

3 2190 2 0 258 $ 810 441 1 $ 176

Iota

$ 3.450

$ 2.300

Hilda-era e: $ 1,150

Western Dry Kiln Association 8

May, 2005