Electrical VFD (Variable Frequency Drive) Application in a Pumping Plant Which Presently Spills Excess Pumpage

A.    Pump Selection Criteria

     1.     The pump to automate with a VFD (in a location with multiple pumps supplying the same

          pipeline) is the smallest which will meet both of the following criteria:

          a.     (Flow Rate of the VFD pump

                + Sum of the flow rates of all the smaller pumps)



                               must be greater than or equal to



                                         (The flow rate of the next bigger pump)



                     ie, (QVFD + [sum of all Qsmaller pumps])>= QNext bigger pump



          b.    No larger pump flow can exceed the combined flow of all pumps which are smaller than it

                (including the VFD at full speed)



     2.    There is generally little or no energy savings associated with converting to VFD control for more

          than one pump at an installation.



B.    Estimation of a annual KW-HR savings for a VFD installation.



     1.     Estimating the maximum potential savings.

          An estimate of savings requires an estimate of the historical amounts of spilled water.             If, for

          example, the spilled water is 5% of the total pumped water, then the maximum KW-Hr savings

          can be:

                               Max. KW-Hr savings = .05 x Annual KW-Hr



          If pumping amounts vary significantly from year to year, an average of 3 years of data should be

          used.



          The savings may be somewhat less than this (if the VFD operation puts the pump into a less

          efficient operating range) or somewhat more than this (if the new controlled water level is lower

          than the previous spill level).    An examination of      pump efficiencies may show the greatest

          savings possible can be obtained by simply improving the efficiencies of existing pumps.



          a.     The following information is necessary in most cases:

                - Monthly power bills or pump test data providing KW-Hr per Acre-Foot (AF) pumped for

                  each pump

                - Monthly hours of operation per pump

                - Monthly water deliveries (as opposed to pumped amounts)



                             Irrigation Training and Research Center (ITRC) – www.itrc.org



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----------------------- Page 2-----------------------

     Estimating  the Payback Electrical VFD Application - Farm Energy Assistance Loan Program - CEC  Nov. 1994

                   http://www.itrc.org/reports/vfd/vfdpayback.htm       ITRC Report  94-002



          - Pump test data, providing Acre-Feet (AF) pumped per hour of operation for each pump



     b.    Compute AF pumped per month for each pump



                 AF

           AF = hour   x Hours of operation



     c.    From district delivery records, determine the total AF delivered to users (plus seepage and

          conveyance losses) supplied by the pumping station, by month



     d.    Sum the monthly totals



     e.    For each water year, find the % spilled



                              AF Pumped - AF Delivered

               % Spilled =                                    x 100

                                       AF Pumped



     f.    Compute the total KW-Hr savings possible



                                                      % Spilled

               Annual KW-Hr savings possible =            100      x  (Annual KW-Hr consumed)



2.    Estimating KW-Hr which would have been consumed if one of the pumps had been converted to

     VFD.



      This second step should serve as a check on the first step, in which the "possible" savings were

     computed.    By doing this computation, the effect of the overall pump efficiency of the selected

     VFD-controlled pump is accounted for.



      Again, use historical data to make these "what-if" computations.



     a.    Estimate the AF which will be pumped by the VFD unit



                 (Hours) x GPM

           AF =

                       5428



               where

                         Hours = The total hours per year that water is delivered from the pump station

                              (the VFD will operate continuously)

                         GPM = 67% of the maximum GPM of the pump with the VFD controller (the

                              actual percentage can be determined with a detailed analysis, but it is

                              probably not warranted.     The 67% provides a weighted average for a typical

                              condition, accounting for the KW-Hr consumed at various flow rates)



     b.    Estimate the annual KW-Hrs which would be used by the VFD



                            GPM x TDH x 0.0188 x Hours

           Kw-HrsVFD =               Efficiency/100



               where

                         TDH = The total dynamic head of the pump, in feet.



                         Efficiency = The total efficiency of the pump (generally in the range of 40 - 70),

                              which depends upon:



                       Irrigation Training and Research Center (ITRC) – www.itrc.org



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

Estimating  the Payback Electrical VFD Application - Farm Energy Assistance Loan Program - CEC  Nov. 1994

               http://www.itrc.org/reports/vfd/vfdpayback.htm         ITRC Report  94-002



                               Panel Efficiency (Panel) -about 97

                               Motor Efficiency (Motor) - depends upon motor size and model;

                                     typically somewhere between 85 - 93

                               Impeller Effiency (Impeller) - the efficiency of the impeller and bowls.

                                     The Impeller Efficiency to use will occur at a flow rate of about

                                     67% of the maximum flow rate

                               Losses (Losses) - a measure of the losses which occur due to bearings;

                                     typically about 98 on a short lift.



                                               Panel x Motor x Impeller x Losses

                               Efficiency =                     106



c.    Estimate the annual KW-Hrs used by the other pumps at the station.



     1.     Estimate the AF delivered by the other pumps

                     AFother = (Total AF delivered to users plus conv. losses) - AFVFD



     2.    Compute the average pump efficiency          (Effother) for the other (non-VFD) pumps.

             The information from individual pumps will come from a pump test.                Ideally, the

             average efficiency should be determined by taking a weighted average after

             considering the KW and the Hours of each pump, as anticipated after the VFD is

             installed.  In practice, a simple average may be sufficient because the pumps with the

             lowest KW will be cycled on and off more often than the larger pumps, so they will

             have more hours of operation than the larger KW pumps.



     3.    Make the final KW-Hr computation for the other pumps



                               TDH x 102 x AFother

              KW-Hrother =              Effother



d.    Find the total annual KW-Hrs to be used by all pumps



      Total KW-Hr = KW-Hrother  + Kw-HrsVFD



e.    Compute the total KW-Hr savings possible