1. Pump Characteristic Curves & Selection of centrifugal pumps
M. U. Kale
Assistant Professor
Deptt. of Irrigation and Drainage
Engineering, Dr. PDKV, Akola

2. Pump Characteristic Curves

3. Operating properties of pump are expressed as characteristic curves
Also called as performance curves
Shows inter-relationship between capacity, head, power and efficiency of pump
Helps – high value of efficiency at low operating cost
Head, power input and efficiency plotted as ordinates against capacity as abscissa, at constant pump speed
Net positive suction head, when shown, is also plotted as ordinate

4. Head-Capacity curve
Shows how much water the pump will deliver at a given head
As the discharge increases, the head decreases
The curve will dip downward to the right
When a pump is operated against a closed valve, the head generated is referred to as the shut off head
The efficiency of the pump at this point is zero as there is no discharge, but the pump requires energy to drive

5. Efficiency-Capacity curve
Shows the relationship between the efficiency and the capacity of a pump.
Efficiency increases from 0, when the discharge is 0 to a maximum, and then decrease
Generally only one peak efficiency, which is related to a specific capacity
Efficiency vary with the type of pump, manufacturer and model

6. Input Power-Discharge curve
Input power – brake horse power
Called as bhp-Q curve
BHP increases as the discharge increases
peak at somewhat higher rate of discharge than that which produces the maximum efficiency
Nature of curve varies with the speed of the pump

7. Net Positive Suction Head-Discharge curve
Called as NPSHR-Q curve
NPSHR is the amount of energy required to move the water into the eye of the impeller
It is a function of pump speed, impeller shape, liquid properties, and discharge rate
If the energy is not sufficient to move the water into the eye of the impeller, the liquid will vaporize and cavitation will occur.
NPSH available must be more than net NPSH required (NPSHR)

8. Selection of centrifugal pumps

9. Pump selection, what is this?
pump selection – on the suggestions of distributor
one time high investment
once –wrong decision – assured failure of system
before going to purchase pump system, determine the pump system actually required to meet the demands with respect to water availability
The selection of the right type of pump will require a knowledge of use of the system head and pump characteristic curves

10. Determine the discharge capacity of pump
Based on – a) safe yield of well,
b) maximum water requirement of the crops per day in peak periods
c) If there is no constraint on the availability of water like river, canal etc, the pump capacity may be determined on the basis of the water requirement of the crops to be grown

11. Discharge capacity of pump based on crop requirements
The rate of pumping depends on
a) area under different crops
b) water requirement of the crops
c) the rotation period (irrigation interval)
d) duration the pump to be operated

12. Calculated as follows

15. Therefore, capacity of pump required for meeting the maximum water requirement of the crops is l/s
To allow the conveyance losses of water between the pump and the field, the pump capacity may be increased by 20%
Q = X 1.20 = l/s

16. Procedure for selection of centrifugal pumps

17. Match the system head curve with pump characteristic curve
Select the pump giving maximum efficiency for designed discharge and head
If pump characteristics are given as tabular data, the pump which requires the minimum power to operate at the design total head and capacity is to be selected

18. Matching Pump Characteristics and System Head Curves

19. Draw the system head curve on the trace paper
place the tracing paper on graph showing the pump characteristics
The point of intersection of head capacity curve of the pump and system head curve – point of selection
Select the pump which gives maximum efficiency and the desired discharge at the estimated total head

20. Thank You