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, velocity 2 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 hp = Q x h eff X 3,960 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 Head (feet or psi) Operating Point Pump curve System curve Dynamic Losses Static Pressure Flow
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 High Dynamic Head System Curve Normal Low Dynamic Head
Operation at Low End of Pump Curve High Dynamic Head System Curve Normal Low Dynamic Head
Operation at High End of Pump Curve High Dynamic Head System Curve Normal 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 250 200 1,800 RPM curve 1,125 gpm 2,250 gpm throttling valve 70 hp 90 hp Head (feet) 150 1,200 RPM curve VFD: 36 hp System curve 100 50 System curve: throttled 0 0 500 1,000 1,500 2,000 2,500 Flow (gpm)
Pump Selection Seems Easy 70 20 Looks great for peak design conditions 60 40 50 60 Head (feet) 50 40 74 82 80 81 82 74 System curve 60 30 585 rpm 0 0 500 1000 1500 2000 2500 3000 3500 Flow (gpm)
Not So Fast.. 70 Pumps are often unstable and inefficient at average/ minimum conditions 60 20 40 50 60 74 82 Efficiency is just 40% at 500 gpm Head (feet) 50 40 80 81 82 74 System curve 30 60 0 585 rpm 460 rpm 500 rpm 0 500 1000 1500 2000 2500 3000 3500 Flow (gpm)
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 dripproof, 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 % full load efficiency 100 90 80 70 60 50 40 30 20 10 0 75-100 Hp 30-60 Hp 15-25 Hp 0 20 40 60 80 100 120 % full load
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 1800 30 Average Power (kw) 1600 1400 1200 1000 800 600 400 200 0 January February March April May June July August September October November December Month Average Power Consumption Current Condition Pumps - 877 kw After PS 1 Upgrade - 780 kw 25 20 15 10 5 - Average Flow (mgd) After PS 1 Upgrade Current Condition Pumps Projected Average Daily Flow
Energy Before and After Upgrade Current Condition Pumps 76 kw After PS 1 Upgrade 39 kw 16 kw 91 kw 614 kw 190 kw 611 kw 123 kw Average Hydraulic Power Loss Average Hyd Power Average Motor Loss Average Hydraulic Power Loss Average Hyd Power Average Motor Loss AFD Loss Savings Compared to Current Pumps