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Design Considerations For Use of Variable Frequency Drives

Glen M. Anderson, PEganderson@swsv.com

(408) 246-4848

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Today’s Presentation

About Us

Pumping System BasicsMunicipal applications

Constant Speed Pumping

VFD Basics

Reduced Speed PumpingAffinity Laws

Equipment Requirements/Limitations

Design Considerations/Real World Examples

Questions?

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Schaaf & Wheeler Profile

21 Person Civil Engineering Firm

Established in 1985

Four Northern California OfficesSanta Clara

San Francisco

Salinas

Santa Rosa

Focus Exclusively on Water Resources ProjectsHydrology and Hydraulics

Flood Control

Wastewater Systems

Potable Water Systems

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Performed Condition Assessment on more than 100 pump stationsStorm and Sanitary

Reliability

Controls Strategy

Rehabilitation/Replacement Design of ~75 StationsFlow rates from 100 gpm to 300 CFS

Startup Support and Functionality AssistancePump performance Testing

Control Set Points and Logic

Troubleshooting

About Me

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Pumping Systems

Pumps

Discharge Piping and Forcemain

Two Types of Head in a Pumping systemStatic Lift = Elevation difference between discharge WSEL and pumping WSEL

Friction Losses = Dynamic losses associated with pumping water through piping. Friction loss varies with:

• Flow Rate

• Pipe Material (Roughness)

• Pipe Diameter

• Pipe Length

• Type and Quantity of Fittings

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Pumping Systems

Pump Curve

BEP

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Pumping Systems

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Pumping Systems

Pump Operates where pump curve and system curve intersect

Good pump selection will place operating point at or near the Best Efficiency Point

OPERATING POINT

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Constant Speed Pumping

Pumps Operate at Full Speed between specified on and off points.Pump capacity limited based on published pump curve

Choose pump that operates within ± 10% of its Best Efficiency Point

Example: Water Booster Pumps Pumping to a Hydropneumatic tankPeak Flow Rate = 375 gpm

Minimum System Pressure = 45 psi

Maximum System Pressure = 65 psi

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Constant Speed Pumping

Note that system curve is flat because minimum pressure governs the pump operation rather than a required flow rate.

0

20

40

60

80

100

0 100 200 300 400 500

Flow (gpm)

Pres

sure

(psi

)

Pump StopPump StartPump Curve

BEP

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Constant Speed Pumping

BenefitsPredictable Operation

• Operate between set points (pressure, wet well level, storage tank range)

• Maintain minimum pipe velocities

• Maintain operation around Best Efficiency Point

Great for systems dominated by static lift

Operationally Easy

Reduced Cost

DrawbacksLimited pump capacities

Multiple pumps required

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What is a VFD?

Varible Frequency Drive (VFD): An adjustable-speed drive used in electro-mechanical drive systems to control AC motor speed and torque by varying motor input frequency and voltage

Uses Pulse Width Modulation (PWM) to convert Utility AC Power to DC power and ultimately into simulated AC power

Allows for pump operation at frequencies other than 60 Hz

AC Power

From Utility

AC PowerSimulated

To Pump

VFD

???

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Why Consider VFDs?

Low Hanging FruitEliminates need for throttling valves to limit flow

Constant water surface elevation requirements

Allows use of 3-phase pumps on 1-phase power

VFDs allow for soft-starting of higher HP motorsReduced in-rush current

Reduced wear on equipment

Operating at reduced frequency allows reduction of pump rotational speed

Reduced pump flow and head

Variety of Pump Ranges*

Reduced Wetwell Size*

Energy Savings*

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Reduced Speed Pumping

⎟⎠⎞

⎜⎝⎛=

1

2

1

212

NNx

DDQQ

Q = Capacity (Flow Rate)

H = Head (Feet of H2 0, Pressure)

BHP = Brake Horsepower

D = Impeller Diameter

N = Pump Speed

2

1

2

1

212 ⎟

⎠⎞

⎜⎝⎛=

NNx

DDHH

3

1

2

1

212 ⎟

⎠⎞

⎜⎝⎛=

NNx

DDBHPBHP

Change In

Speed

Change in Q

Change in H

Change in BHP

1/2 1/2 1/4 1/8

Affinity Laws:

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Reduced Speed Pumping

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Reduced Speed Pumping

Equipment RequirementsVFDs – for each pump

PLC

Level Controller (Ultrasonic, Transducer)

Equipment LimitationsMaximum Turn-down

• Often determined by pump

Maximum distance between VFD and Pump motor

Impacts to neighboring electrical systems

• Resonance

• 6-Pulse, 12-Pulse, 18-Pulse

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Maximum turn-downWhat is the system shut off head (or pressure)?

• For pressure controlled systems minimum speed pump curve should intersect shut-off system curve.

Minumum Pump Speed

• Hydraulic Effeciency

• Varies with pump Type

– Sewage Pumps: 50-60%

– Domestic Water Pumps: As low as 20%

– Consult Pump Manufacturer

Forcemain velocity requirements

• In SS forcemain applications, must maintain cleaning velocities within the pipe (~2-3 FPS)

Reduced Speed Pumping-Design Considerations

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Reduced Speed Pumping -Design Considerations

Maximum Distance Between VFD and PumpWhen the VFD converts the AC power to DC power, voltage increases by

480V 679V

Long Leads can allow voltage reflection that can add the reflected voltage to the outgoing 1358V

• Limit Lead lengths ~150-200’

• Install line filters

• Use Inverter Rated Motors*

2

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Reduced Speed Pumping-Design Considerations

Impacts to Surrounding Power SystemResonance associated with Harmonics from VFDs

• Where resonance is a concern, Utility company may require upgraded equipment

– Prevalent harmonics = N±1, where N is the number of pulses of the VFD:

» 6-Pulse 5th and 7th harmonics

» 12-pulse 11th and 13th harmonics

» 18-pulse 17th and 19th harmonics

Interference with breakers/protection devices

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Examples

Local Water DistrictTwo tanks at opposite ends of system

Both Tanks have the same WSEL

Wants to move water between tanks depending on demands/usage

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Examples

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Examples

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Examples

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Examples

Community Water SystemBoost water from storage tank into distribution system

System pressure regulated/maintained with hydropneumatic tank

S&W brought on board during construction for troubleshooting

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Examples

Schaaf & WheelerCONSULTING CIVIL ENGINEERSSchaaf & WheelerCONSULTING CIVIL ENGINEERS

Examples

Schaaf & WheelerCONSULTING CIVIL ENGINEERSSchaaf & WheelerCONSULTING CIVIL ENGINEERS

Examples

Schaaf & WheelerCONSULTING CIVIL ENGINEERSSchaaf & WheelerCONSULTING CIVIL ENGINEERS

Examples

Schaaf & WheelerCONSULTING CIVIL ENGINEERSSchaaf & WheelerCONSULTING CIVIL ENGINEERS

Examples

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