Improving Pump Performance

Charles M. Burt, Professor of Irrigation

BioResource and Agricultural Engr. Dept.; Chairman, Irrigation Training and Research Center

(ITRC), California Polytechnic State Univ., San Luis Obispo, CA 93407 cburt@calpoly.edu

Franklin Gaudi, Senior Irrigation Engineer, ITRC

Daniel J. Howes, Senior Irrigation Engineer, ITRC



Abstract.       Options can be specified to minimize power consumption by vertical pumps – both

when new and over the life of the pump.  Options discussed include bowl coatings, proper well

development, improved suction screens, using closed impeller designs, increasing column size,

using new bearings, providing proper bearing lubrication, impeller balancing, and polishing

impellers.  The proper TDH and flow rate must be specified, and the advantages of VFD controls

are covered.



Keywords.  pump, efficiency, power, VFD, irrigation



Introduction

On the surface, the basics of good pump performance are relatively simple.                                     They are:

1.    Select a high quality pump.

2.    Select a pump that operates at a high efficiency at your desired flow rate and pressure.



However, in practice, pump efficiencies are not as simple to achieve as it might appear.  In

December of 2003, ITRC published the report “California Agricultural Electrical Energy

Requirements” (Burt et al, 2003) for the Public Interest Energy Research Program of the

California Energy Commission that included the following two figures, demonstrating that

average pump efficiencies are not as uniform they should be throughout California.



               100

                                                                                                      y = 6.7051Ln(x) + 30.209

                                                                                                            2

                                                                                                           R  = 0.2774

                90



                80



                70



                60

            )

            %

            (

               

             E  50

             P

             P

            O

                40



                30



                20



                10



                 0



                   0          200         400         600         800         1000        1200        1400        1600        1800



                                                                 Input Kilowatts (kW)



    Figure 1.         Pumping plant efficiency as a function of motor input kW for each pump tested –

                      irrigation districts.  Data collected by Cal Poly ITRC.  Average efficiency is about

                      64%.



Page 1                                 Improving Pump Performance by Burt et al.                                ©2008 – ITRC

                 Irrigation Training and Research Center (ITRC) – Cal Poly, San Luis Obispo, CA 93407


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             100



              90



              80



              70



              60

           )

           %

           (

             

           E  50

           P

           P

           O

              40



              30



              20



              10



               0



                 0            20           40           60            80          100           120          140          160



                                                              Input kiloWatts (kW)



    Figure 2.        On-farm pumping plant efficiency as a function of motor input kW for each pump

                     tested.    Data collected by CIT.            Average efficiency is about 48%.



So, if the basics of pump performance are so simple, why are overall pumping plant efficiencies

so low?       The answer includes a blend of the following factors:

• Energy prices have historically not been high enough (relative to overall farming costs) for

     farmers to pay more attention to obtaining higher efficiencies.

• Irrigation pump dealers appear to believe that agricultural customers will price-shop and

     therefore they will only be able to sell bare-bones equipment to farmers.

• Both farmers and pump dealers are often unaware of pump options that could be specified to

     improve or maintain high pump efficiencies.

• Some major pump companies have in recent years moved their foundries overseas and some

     of the previous “standard” options that were important for high efficiencies have been

     eliminated.

• There has not yet been widespread usage of variable speed drive controllers, which can be

     very helpful in (a) increasing well life, (b) reducing water hammer, and (c) perhaps most

     importantly for this paper, allowing the pump to operate without producing more pressure or

     flow than is needed on any particular day.



In agriculture, we typically use four general types of pumps:

1.   Vertical line-shaft turbines in wells

2.   Submersible motors for pumps in wells (usually called “submersible pumps” because the

     package often includes an impeller/bowl assembly that is custom-made for submersible

     motors).

3.   Above-ground horizontal “booster” pumps – typically either end suction or split case.

4.   Propeller pumps for low lift, often high volume applications.



Furthermore, there are two ways to power most pumps:

1.   Electric motors (required for submersible pumps, obviously)

2.   Engines



                                                                                                                                         2


----------------------- Page 3-----------------------

This paper focuses on one combination:  Vertical line-shaft turbine pumps with electric

motors.  The authors address two important issues:

1.  What options are important to include in a new pump purchase?

2.  What options will help keep power consumption (per acre-foot pumped) low for 5 to

    10 years after the initial purchase?



Minimizing Initial Power Bills with a New Well Pump

Note that the essence of the words above are “minimizing power bills” rather than “maximizing

efficiency”.  It is always important to select an efficient pump, but putting an emphasis only on

“maximizing efficiency” ignores several important concepts:

• Electric power bills can often be reduced if a farmer can avoid pumping during some hours

    of the day or week.  Utilities offer special “time of use” electric rates for pumping during off-

    peak electrical usage hours only.

• A pump may be producing a pressure and flow rate with a very high efficiency, but if there is

    excess pressure that is being dissipated through pressure regulators, the “power utilization

    efficiency” (PUE – a new term by the authors) is much lower than the “pumping plant

    efficiency”.

• The design pressure requirement may be greater than is necessary.        For example, the column

    pipe diameter may be too small.

• Power can be minimized if the well is properly designed to minimize drawdown in the well.



Selecting an efficient pump

• It’s not a question of whether or not the “pump is efficient”.  Rather, it’s a question of

    whether the pump operates efficiently at the specified pressure and flow rate.  In other

    words, someone who understands hydraulics, well drawdown, and irrigation system pressure

    and flow requirements needs to get together with the pump supplier and provide the correct

    flow and pressure specifications.

• Use line shafts with enclosed oil-lubricated bearings rather than product (water) lubricated

    shaft bearings.  If you are not allowed to use standard oil lubrication, instead select 10 weight

    food grade oil.  The motor must provide the power to overcome the mechanical bearing

    friction, which is typically in the neighborhood of 1-2 HP per 100 feet of shaft with drip feed

    oil lubrication. This HP requirement can double with standard rubber water lubricated

    bearings – usually not at first but with time due to abrasion with sand.  If there is no sand in

    the water, product lube can be fine.

• Coat the interior of pump bowls with Scotchkote 134 (SK134) fusion bonded epoxy per the

    manufacturer’s specifications.   It is approved for potable water, and will typically provide an

    improvement in efficiency of 2% minimum, with 4-5% reported in some cases.  Costs vary

    from about $500 - $650/stage for 10” and 14” bowls, respectively.



                                                                                                     3


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   Figure 3.   SK134 fusion bonded epoxy application.



• Specify a C-10/C-20/C-30 polished finish on all impeller passages and removal of burrs.

    Some of the low-end agricultural market suppliers do not have the equipment necessary to do

    this.  This should increase efficiency by 1-3%.



   Figure 4.   The thickness on the bottom of the vane is correct; the thickness must be reduced

               on the upper portion of the vane.



• Specify a sufficiently deep pump setting so that there will be at least 10-30 feet of water

    (while pumping) above the inlet to the pump bowls.  One must take into account variations in

    well water levels from Spring to Fall, and between years.   Some well pumps need even more

    submergence to avoid cavitation.

• Do NOT use semi-open impellers.  Instead, use enclosed impellers.  The performance of

    semi-open impellers is highly dependent upon proper adjustment of the lineshaft nut on the

    top of the motor, and incorrect “rules of thumb” for adjustment of the height are usually used.

• Obtain from the manufacturer the proper setting of the lineshaft for that particular installation

    – considering the lineshaft material and diameter, the bowls, the shaft length, and the

    pressure (total dynamic head).  Make sure the installer uses that information.



Proper initial specifications that help maintain a high efficiency

• Specify that impellers be dynamically balanced to ISO 1940, Grade 6.3.  The cost is about

    $100/stage for a 10” pump and $200/stage for a 16” pump.     This minimizes the possibility of

    imbalance in the bowl assembly – and subsequent damage from vibrations.



                                                                                                   4


----------------------- Page 5-----------------------

• If you specify drip-oil lubrication of the shaft, make sure that the oil drips the way it should.

    This means you must specify a non-standard oil pot assembly.     The design depicted below

    will maintain a fairly constant drip rate (a minimum of 6-7 drips/minute are needed) and

    provides a large reservoir – with the constant drip rate, the pot will empty out more quickly

    than standard pots with reduced drip rates over time.  Another important feature can be a low

    wattage heater coil, covered with insulation, attached to the oil pipe above the adjustment

    valve.



   Figure 5.   New well pump oiler



• Vertical hollow shaft motors require special attention.   Premium efficiency motors should be

    specified on 150 HP or less.  It is important to select the correct brand of motor.  “Premium”

    efficiency motors by brand “X” may have a lower efficiency than standard motors from

    brand “Z.”  See later notes on motors for VFD installations.

• Motor life can be extended greatly in many cases if:

    o   A space heater is provided in the motor housing to prevent condensation.

    o   In areas of heavy fog, the motor is enclosed in some type of shed.

    o   The motor is shaded from direct sunlight.

• A common misconception is that if a motor is oversized, the efficiency of the motor will

    drop.  The figure below illustrates the result of ITRC testing of a variety of motors ranging

    from 20 HP to 100 HP.



                                                                                                   5


----------------------- Page 6-----------------------

                                                                                                 A02 GE 20 1200

                                      1.00



                                                                                                 A01 US Motors 20 1800



                                      0.95                                                       A03 US Motors 20 1800



                                                                                                 A09 US Motors 40 1800

                                   0

                                   0

                                   1  0.90

                                   /                                                             A13 US Motors 40 3600

                                   y

                                   c

                                   n                                                             A15 GE 50 1200

                                   e

                                   i

                                   c  0.85

                                   i

                                   f                                                             A11 GE 50 1800

                                   f

                                   E

                                   

                                   r                                                             A12 US Motors 50 1800

                                   o

                                   t  0.80

                                   o

                                   M                                                             A14 US Motors 75 900



                                                                                                 A05 US Motors 75 1800

                                      0.75



                                                                                                 A10 GE 75 1800



                                      0.70                                                       A06 GE 100 1800



                                         0.1  0.2 0.3  0.4 0.5  0.6 0.7  0.8 0.9  1.0  1.1



                                                    Load Factor (%HP/100)



     Figure 6.          Efficiencies of ITRC-tested motors, across-the-line, at various relative loads.



• Install a flow meter that is robust and that is installed properly.                                      Trying to estimate changes in

      pump efficiencies over time without a flow meter is problematic, to say the least.

• If there is any sand in the water, do not use bronze impellers.  Instead, select Ni-Resist.

      Although this material requires more polishing than bronze and loses 1-2 efficiency points, it

      will last much longer (meaning the efficiency will not drop as much).                                           Additional costs are

      about $500 - $1200 per stage for 10” and 14” pumps, respectively.

• If you want to use suction cone screens, be sure to use screens constructed of non-corrosive

      materials with no restriction of open area.                          The photo below indicates that, as screens fall

      apart, pieces of screen go into the impeller.  Additionally, the flow opening can be drastically

      reduced.       The reduced opening can cause pump cavitation and will always increase the Total

      Dynamic Head (TDH) of the pump – resulting in decreased flow rate and usually lower

      efficiency.



     Figure 7.          Corroded pump cone screen with missing sections.



                                                                                                                                                        6


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Reducing the Total Dynamic Head (pressure) requirement

• Start with a well that has a good screen.  Screens cost money up front.  Holes poked in well

    casing are cheap, but a good screen has numerous initial and long-term advantages that save

    power in the long run.    These advantages include:

    o   They allow for good development of a well (see later section).

    o   They have a large percentage of open area – easily 3-4 times as much as inexpensive slots

        or holes in casing.  This means there is less head loss between the aquifer and the well

        (meaning less drawdown), and the lower velocities also help minimize corrosion and

        chemical blockage.

    o   Good materials do not corrode.     Corrosion blocks the entry of water into the well –

        increasing the TDH and decreasing the yield (flow rate).

• Have the well properly developed when it is initially drilled.  Development is the process of

    cleaning out the soil immediately around the well casing to allow for free flow of water into

    the well (and thereby decreasing drawdown).  Proper drawdown involves a lot more than just

    “overpumping” (the common practice), which just improves the opening of already-clean

    zones.  See a well development specialist to learn about various techniques that are available.

• Use one larger size of column pipe and discharge head.          Most customers don’t know how

    much column friction they are paying for, but it can be substantial (a common number is

    about 1 foot per 100’ of column).  By going up one pipe size, the friction can often be cut in

    half.  Another option is to coat the inside of the column pipe to increase the smoothness.

• Use a smart irrigation system design that does not require extra pressure for flushing filters,

    injecting fertilizers, or special valves.



Variable Frequency Drive Controllers

Advantages to VFD control

Power Savings.    The key power savings advantage to using VFD control is simple – the speed of

the pump will be adjusted so that the pump only provides the pressure or flow that is needed – no

more and no less.   For agricultural well pumps, this has huge implications because:

• Well water levels fluctuate during the year and between years.

• Irrigation systems may not always need a constant flow rate and/or pressure.           For example, a

    drip system is typically divided into blocks that may be of different sizes and at different

    elevations, each requiring a different operating point.



How much savings does this represent?       It is impossible to say without knowing the details of the

aquifer and the irrigation system.  There is an inherent extra 6% or so power requirement for

VFD controllers (inefficiency plus air conditioning), so the savings have to be greater than 6% to

break even.  But “experience” seems to indicate that 10-15% overall savings are commonplace.



Ability to use Time-of-Use (TOU) Rates with Well Pumps.            Every time a standard well pump is

started, it has a very high initial flow rate (due to having a low initial pressure requirement).  The

water level in the well drops quickly, and the water on the outside of the casing takes time to

“catch up” in dropping.  Meanwhile, there are large inward pressures on the casing.  This leads to

premature well failure.



                                                                                                         7


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Many farmers correctly understand that their wells have a life measured in the number of

startups, rather than in number of years.  Therefore, these farmers will not start and stop their

well pumps every day to take advantage of low power rates (TOU rates) – the risk of well failure

in the middle of the summer is too great.



VFDs offer the advantage of being able to slowly start and stop the pumps – so that the well

itself is not subject to violent stresses.  This lengthens the life of wells.    We do not have good

field data on this, but it is clear that this is the case.



Reduction of Water Hammer.  The slow start and stop of well pumps is a dream for minimizing

water hammer problems that typically occur during rapid startup.  Pipes fill up slowly.



Motor specifications for VFDs

Besides the general motor recommendations given earlier, VFD installations should include:

• Proper grounding to eliminate bearing corrosion due to stray currents.            Specify a shaft

    grounding ring installed in the new motor.

• “Inverter duty” premium motors.          These are designed to withstand the peculiar electrical

    stresses associated with simulated AC current.



Special lineshaft bearings for VFD applications



Because of the slow start, water lubricated bearings may spin some time before they become

lubricated.  If the water is very clean and and an open lineshaft is used, specify carbon bearings.



Purchasing a good VFD controller

There are large differences in quality between VFD controllers.          ITRC provides guidelines for

VFD specifications at www.itrc.org.  A good VFD controller will:

1.  Allow one to run electrical conduit more than a few feet between the controller panel and the

    motor.

2.  Provide an excellent Power Factor.

3.  Provide high quality power that helps ensure long motor life.

4.  Have a very high efficiency – 98% or so.

5.  Condition the power properly.       For example, a good VFD controller will not be limited to the

    lowest voltage of the 3 leads of a 3 phase power supply.

6.  Be capable of functioning with variations in voltage in the power supply.



ITRC has encountered two common VFD problems in the agricultural market:

1.  The panel must be properly cooled and kept clean.          Often this requires an air conditioner unit.

2.  The VFD controller should usually be one size larger than the motor.           For example, a 125-HP

    VFD controller is needed for a 100-HP motor.



                                                                                                             8


----------------------- Page 9-----------------------

Conclusion

Proper design and the addition of appropriate options can greatly maximize efficiency and

minimize power bills associate with pump systems.     Additionally, VFD controllers have not yet

caught on in popularity, despite the powerful advantages that they bring when properly selected

and installed. With rising energy prices throughout the country, it is important that farmers

become aware of potential improvements to their systems.



Acknowledgement

The input of a large number of pump dealers and manufacturers is appreciated.     Special thanks

are given to Chris Lula of Layne/Verti-Line of Pentair and Bruce Grant of Floway.     They have,

of course, no responsibility for any errors in this paper. Support for ITRC’s pumping activities is

provided by the California Energy Commission’s PIER program.



References

Burt, C.M., D.J. Howes, and G. Wilson.    2003.  California Agricultural Water Electrical Energy

Requirements.   Irrigation Training and Research Center, California Polytechnic State University,

San Luis Obispo, California.  www.itrc.org.



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