Selection Of Pumpset For Irrigation System In Agriculture

P.P.ShelkeA knowledge of pump characteristics enable the
 selection of pump which is best adapted to a
  The selection of right type of pump will require aparticular set of conditions, thus obtaining a high value
knowledge of systems head and pump characteristicsof efficiency at a low operating cost.  For this
curves .  The normal procedure is to first determinepurpose, it is usual to plot the head, the power input
the system head curves and the desired discharge. and efficiency as ordinate against the capacity as
The characteristic curves or tables giving theabscissa at a constant pump speed.
performance of various models of pumps are used 
to select the right type of pump, which will operate 
efficiently at or near the desired discharge and the 
systems total dynamic head. 
 Head capacity curve
System head curve            The H-Q curve shows how much
The system head curve gives the relationshipwater the pump will deliver at a given head.  As the
between the discharge and the total head in pumpingdischarge increases, the head decreases.
system.  The contribution of various parameters toEfficiency capacity curve
the total head such as well drawdown, frictional head             The efficiency capacity curve
and operating head usually increases with discharge. shows relationship between the efficiency and the
 capacity of the pump.  The efficiency may be
To determine discharge drawdown relationship forobserved to increases from O, when the discharge is
particular well, a pumping test should be done byO, to a maximum and then decreases.  There is
adopting proper procedure.  When the pump is usedgenerally only one peak efficiency, which is related to
to lift water for operating a pressurized irrigationa specific capacity.  Efficiencies vary with the type
system, the operating pressure is to be worked outof pump manufacturer and model.
using standard procedure and should be included in 
the total head.Input power discharge curve
              The  input power is related to
Discharge capacity of pumps :as the break horse power to drive the pump.  The
The data on the safe discharge rate of the well ( orBHP- Q curve for a centrifugal ump increases as the
other source of water )  and the discharge ratedischarge increases, reaching peak at a somewhat
required for the crops to be irrigated with a particularhigher rate of discharge than the which produces the
cropping pattern are estimated, if necessary croppingmaximum efficiency.  The nature of curve varies
pattern can be adjusted according to safe yield ofwith speed.
well in different season.  Thus, pumps capacity 
should be designed considering two factors : 1) SafeSelection of proper centrifugal pump
yield of well and 2) Water requirement of selected            A centrifugal pump is designed to
cropping pattern in peak periods.  If there is nooperate efficiently within specified range of capacity
constraint on the availability of water , the pumpand total head when applied to a different set of
capacity may be determined on the basis of theconditions, the same pump  will not work efficiently,
water requirement of the crops to be grown :irrespective of amount of power applied to operate
otherwise pumping plant is to be designed to matchit.  Therefore, by matching the system head curve
safe yield of well.  The safe yields of  wells can befor a range  of discharges above design discharge
determined by conducting a pumping test.rate, with the characteristics of various models of
 pumps, the right type of pump giving maximum
If discharge capacity of pump is to be designed fromefficiency can be selected.  Alternatively, tabular
crop water requirement following relationship can bedata on pump characteristics can also be used for
used.this purpose.
             It is usual to draw system head
        27.78n     n       An.Yncurve on tracing paper, on the same scale as the
Q = --------      ∑        ----------pump characteristic curves.  The tracing paper is
             T        i= 1    Rnplaced on the graph showing the pump
 characteristics.  The characteristics of pumps of
 various models should be matched with system head
Where, Q      =     pump discharge, lpscurve.  This will lead to the selection of the pump,
An     =     Area under nth crop, hawhich gives the maximum efficiency and the desired
Yn     =     Depth of Irrigation for nth crop,discharge at the estimated total head.  The point of
cmintersection of head capacity curve of pump and
Rn     =     rotation period for nth crop, dayssystem head curve provides a point of selection.  At
T       =     Duration of pumping, n/daythis point, the pump will give  a discharge of 3150 lhp
 against to total head of 31 m and efficiency of 65
More simple expression can be used for determiningper cent.  The power input is    12 hp.  By careful
discharge capacity for drip irrigation system.selection from wide range of products , it is possible
 to select a pump  , which will give desired discharge
                  n        An.at the estimated total head and provide an efficiency
Q = 2.778 ∑ -----------------Ednof 75-80 per cent or more
                   i= 1    (LSn x Edn)For pressurised irrigation system, usually a high head
 low discharge ump is desired.  If such type of pump
Where ,is not available in market, then required head can be
 Q                 =   Pump discharge,developed by using multistage pump or connecting
lpspumps in series.  In this case, the number of stages
  An              =   Area under dripwill be equal to the total head divided by the head
irrigation, hadeveloped by a single stage pump.
  LSn& ESn  =   Lateral and emitter spacing 
respectively for crop n, m\Important points to be considered while selection
  Edn             =   Desired emitterproper size  of pump : 
discharge for nth crop, lph. 
 1. If pumping unit is to be selected for a site with
Power Requirement in pumpingfluctuating well water level, a pump with steeper
The power requirement in pumping can be calculatedhead capacity curve should be selected so that a
after knowing total head  under which pumpminimum variation in discharge with change in
operates and discharge required.  Water horseoperating heads will be  resulted.  A pump with
power (WHP) is the theoretical power required forflatter head- capacity curve in such  a condition
pumping.  It is expressed as,results into large variation in the discharge with
 smaller change in head.
              Discharge , lps x total head, m2. Multi-stage pumps should be used when the
WHP = -----------------------------------------required head can not be developed by a single
 Â  76stage. It is particularly important in case of
 pressurised irrigation systems, where usually a high
Brake horse power (BHP) is the actual power to behead low discharge pumps are required.  The
supplied by the engine or electric motor for driving anumber of stages are equal to the total head divided
pump.by the head developed by a single stage pump.  The
 selection of right type of multi-stage pump require
                     WHPdevelopment of characteristics curves for two or
BHP ---------------------------more pumps operating in series and matching system
               Pump efficiencyhead curve with multi-stage pump curve
 3. Pumps installed in parallel should be used where low
Power input in kw = BHP x 0.746head high discharge pumps are required as in case of
 pumping water from scarp well.  The combined
Pump characteristic curvesdischarge of two pumps operating in parallel is equal
            Centrifugal pumps have wellto the sum of the individual discharges for s specific
defined operating properties, which vary with thehead.
type of pump, manufacturer and model.  Those4. Hydraulic characteristics :
properties are expressed as characteristics curves. Test                 Test Temp.       
These curves also known as performance curvesDuration        Induced stress
show the interrelationship between capacity, head,Acceptance test   200C   1 hr.           6.
power and efficiency of pump.Â