energy efficiency considerations in pumps and pump stations

Energy Efficiency Considerations in Pumps and Pump Stations

Jeff Foray, P.E. (JeffForay@KennedyJenks.com) WSU Extension Energy Program 14 March 2014

How Do You Save Energy?

►Don’t design it wrong ►Operate smart

►Fix stuff

Thanks for your time, any questions e-mail me at JeffForay@KennedyJenks.com

Energy in a Pumping System

Pumps and Other Fluid Movers

► Centrifugal pumps are the most common End suction Split case Turbines Submersibles

► Positive displacement pumps

► Other devices Fans Compressors

Some Basic Stuff

► Flow: 1 mgd = 694 gpm ► Head: 1 psi = 2.31 feet ► Head Loss: liquid friction,

velocity2

► Power: 1 hp = .75 kW ► Money ($.07/kwhr) 1 mgd at 100 feet (22 hp) for a

year is about $10,000 2 mgd in same pipe is about

$18,000 a year 1 hp for a year about $500

What is Efficiency?

► Good divided by the total

► Energy Losses in Pumps: Mechanical

(friction in bearings, etc.)

Volumetric (recirculation)

Hydraulic (liquid friction)

Calculating Pump Horsepower

Q x h eff X 3,960

hp = Q = flow in gpm h = head (pressure) in feet (1 psi = 2.31 feet of head) eff = efficiency

• Water horsepower: ignore efficiency • Brake horsepower: pump efficiency only (size the

motor) • Wire-to-water horsepower: pump x motor

efficiency (size the electrical service)

Typical Pump Efficiencies

► Pump only (brake, no motor) Non-clog centrifugal, 25 hp: 65% Submersible wastewater, 34 hp: 75% Vertical turbine, water, 30 hp: 81% End Suction, water, 30 hp: 75%

► Bigger is better: add about 5 points at 200 hp

► Slower is better: add a couple points below 1200 rpm

Pump Curve: Superbasics

Flow

Head (feet or psi) Dynamic

Losses

Static Pressure

Pump curve

System curve

Operating Point

Pump Curve: Less Basic

► Nerdy but crucial ► Best Efficiency

Point (BEP) ► Typically drops

off about 20% from BEP

► How pump is applied determines how efficient it will be

Minimum/Maximum/Normal

Pump and System Curves System Curve Normal

High Static Head

Low Static Head

Dynamic Losses May Vary System Curve Normal High Dynamic

Head

Low Dynamic Head

Operation at Low End of Pump Curve System Curve Normal High Dynamic

Head

Low Dynamic Head

Operation at High End of Pump Curve

System Curve Normal High Dynamic

Head

Low Dynamic Head

Variable Speed Pumping

► Variable Speed Drives (VFD’s) Vary motor

speed from about 50% to 100%

Drive is about 98% efficient

2 to 4 times cost of starter

► Why? Moving water slower reduces friction

Why VFD’s? Throttling Stinks

Head (feet)

1,800 RPM curve

System curve

Flow (gpm)

1,200 RPM curve

0 500 1,000 1,500 2,000 2,500

250

200

150

100

50

0

90 hp 70 hp

throttling valve

VFD: 36 hp

2,250 gpm 1,125 gpm

System curve: throttled

Pump Selection – Seems Easy H

ead

(feet

)

System curve

Flow (gpm)

70

60

50

40

30

0 0 500 1000 1500 2000 2500 3000 3500

20 40 50 60 74

82 80

81 82

74 60

585 rpm

Looks great for peak design conditions

Not So Fast….. H

ead

(feet

)

System curve

Flow (gpm)

70

60

50

40

30

0 0 500 1000 1500 2000 2500 3000 3500

20 40 50 60 74

82 80

81 82

74 60

585 rpm

Efficiency is just 40% at 500 gpm

500 rpm 460 rpm

Pumps are often unstable and inefficient at average/ minimum conditions

Follow Standards

► Hydraulic Institute Pump Intake

Design (9.8) Allowable

Operating Region (9.6.3)

► Read “Pumping Station Design” (Sanks)

Electric Motors

► Most applications: NEMA standard Enclosure type: TEFC, open drip-

proof, weather protected 1992 Energy Act

► Submersibles use special motors

► Inverter Duty: improved cooling, insulation

Electric Motor Efficiency

► Standard efficiency motors: 90%

► Premium efficiency: 10% more expensive, 5% more efficient

► Efficiency is constant to about 50% load

0102030405060708090

100

0 20 40 60 80 100 120

75 - 100Hp30 - 60Hp15 -25Hp

% full load

% full load efficiency

Improving Pump Efficiency

► Don’t Design it Wrong Invest in a good predesign Decide how it will operate before messing around with

layouts and equipment selection Pick pumps to operate efficiently operate at conditions

where the pump will actually operate most of the time Consider variable speed drives or smaller pumps to

improve efficiency at low flows

Improving Pumping Efficiency

► Operate Smart Pump as slowly as possible Utilize storage to level out pumping rate Eliminate throttling

► Fix Stuff Test pumps regularly (inc electrical measurements) Visual inspection of interior Modify or replace impeller to match conditions Replace old motors (pre Energy Act, 1992 to 1997)

Example: Big Water Pump Station

Figure 3 - Projected Average Day Power Usage, 2011 Projected Average Day Flow

0

200

400

600

800

1000

1200

1400

1600

1800

January February March April May June July August September October November December

Month

Ave

rage

Pow

er (k

W)

-

5

10

15

20

25

30

Average Power ConsumptionCurrent Condition Pumps - 877 kW After PS 1 Upgrade - 780 kW

Ave

rage

Flo

w (m

gd)

After PS 1 Upgrade Current Condition Pumps Projected Average Daily Flow

Energy Before and After Upgrade

Current Condition Pumps

76 kW

614 kW190 kW

Average Hydraulic Power Loss Average Hyd PowerAverage Motor Loss

After PS 1 Upgrade

91 kW

16 kW39 kW

611 kW

123 kW

Average Hydraulic Power Loss Average Hyd Power Average Motor Loss AFD Loss Savings Compared to Current Pumps