The selection of right type of pump will require a knowledge of systems head and pump characteristics curves . The normal procedure is to first determine the system head curves and the desired discharge. The characteristic curves or tables giving the performance of various models of pumps are used to select the right type of pump.
The selection of right type of pump will require a knowledge of systems head and pump characteristics curves . The normal procedure is to first determine the system head curves and the desired discharge. The characteristic curves or tables giving the 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.
System head curve
The system head curve gives the relationship between the discharge and the total head in pumping system. The contribution of various parameters to the total head such as well drawdown, frictional head and operating head usually increases with discharge.
To determine discharge drawdown relationship for particular well, a pumping test should be done by adopting proper procedure. When the pump is used to lift water for operating a pressurized irrigation system, the operating pressure is to be worked out using standard procedure and should be included in the total head.
Discharge capacity of pumps :
The data on the safe discharge rate of the well ( or other source of water ) and the discharge rate required for the crops to be irrigated with a particular cropping pattern are estimated, if necessary cropping pattern can be adjusted according to safe yield of well in different season. Thus, pumps capacity should be designed considering two factors : 1) Safe yield of well and 2) Water requirement of selected cropping pattern in peak periods. If there is no constraint on the availability of water , the pump capacity may be determined on the basis of the water requirement of the crops to be grown : otherwise pumping plant is to be designed to match safe yield of well. The safe yields of wells can be determined by conducting a pumping test.
If discharge capacity of pump is to be designed from crop water requirement following relationship can be used.
27.78n n An.Yn
Q = -------- ----------
T i= 1 Rn
Where, Q = pump discharge, lps
An = Area under nth crop, ha
Yn = Depth of Irrigation for nth crop, cm
Rn = rotation period for nth crop, days
T = Duration of pumping, n/day
More simple expression can be used for determining discharge capacity for drip irrigation system.
n An.
Q = 2.778 -----------------Edn
i= 1 (LSn x Edn)
Where ,
Q = Pump discharge, lps
An = Area under drip irrigation, ha
LSn& ESn = Lateral and emitter spacing respectively for crop n, m\
Edn = Desired emitter discharge for nth crop, lph.
Power Requirement in pumping
The power requirement in pumping can be calculated after knowing total head under which pump operates and discharge required. Water horse power (WHP) is the theoretical power required for pumping. It is expressed as,
Discharge , lps x total head, m
WHP = -----------------------------------------
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Brake horse power (BHP) is the actual power to be supplied by the engine or electric motor for driving a pump.
WHP
BHP ---------------------------
Pump efficiency
Power input in kw = BHP x 0.746
Pump characteristic curves
Centrifugal pumps have well defined operating properties, which vary with the type of pump, manufacturer and model. Those properties are expressed as characteristics curves. These curves also known as performance curves show the interrelationship between capacity, head, power and efficiency of pump.
A knowledge of pump characteristics enable the selection of pump which is best adapted to a particular set of conditions, thus obtaining a high value of efficiency at a low operating cost. For this purpose, it is usual to plot the head, the power input and efficiency as ordinate against the capacity as abscissa at a constant pump speed.
Head capacity curve
The H-Q curve shows how much water the pump will deliver at a given head. As the discharge increases, the head decreases.
Efficiency capacity curve
The efficiency capacity curve shows relationship between the efficiency and the capacity of the pump. The efficiency may be observed to increases from O, when the discharge is O, to a maximum and then decreases. There is generally only one peak efficiency, which is related to a specific capacity. Efficiencies vary with the type of pump manufacturer and model.
Input power discharge curve
The input power is related to as the break horse power to drive the pump. The BHP- Q curve for a centrifugal ump increases as the discharge increases, reaching peak at a somewhat higher rate of discharge than the which produces the maximum efficiency. The nature of curve varies with speed.
Selection of proper centrifugal pump
A centrifugal pump is designed to operate efficiently within specified range of capacity and total head when applied to a different set of conditions, the same pump will not work efficiently, irrespective of amount of power applied to operate it. Therefore, by matching the system head curve for a range of discharges above design discharge rate, with the characteristics of various models of pumps, the right type of pump giving maximum efficiency can be selected. Alternatively, tabular data on pump characteristics can also be used for this purpose.
It is usual to draw system head curve on tracing paper, on the same scale as the pump characteristic curves. The tracing paper is placed on the graph showing the pump characteristics. The characteristics of pumps of various models should be matched with system head curve. This will lead to the selection of the pump, which gives the maximum efficiency and the desired discharge at the estimated total head. The point of intersection of head capacity curve of pump and system head curve provides a point of selection. At this point, the pump will give a discharge of 3150 lhp against to total head of 31 m and efficiency of 65 per cent. The power input is 12 hp. By careful selection from wide range of products , it is possible to select a pump , which will give desired discharge at the estimated total head and provide an efficiency of 75-80 per cent or more
For pressurised irrigation system, usually a high head low discharge ump is desired. If such type of pump is not available in market, then required head can be developed by using multistage pump or connecting pumps in series. In this case, the number of stages will be equal to the total head divided by the head developed by a single stage pump.
Important points to be considered while selection proper size of pump :
1. If pumping unit is to be selected for a site with fluctuating well water level, a pump with steeper head capacity curve should be selected so that a minimum variation in discharge with change in operating heads will be resulted. A pump with flatter head- capacity curve in such a condition results into large variation in the discharge with smaller change in head.
2. Multi-stage pumps should be used when the required head can not be developed by a single stage. It is particularly important in case of pressurised irrigation systems, where usually a high head low discharge pumps are required. The number of stages are equal to the total head divided by the head developed by a single stage pump. The selection of right type of multi-stage pump require development of characteristics curves for two or more pumps operating in series and matching system head curve with multi-stage pump curve
3. Pumps installed in parallel should be used where low head high discharge pumps are required as in case of pumping water from scarp well. The combined discharge of two pumps operating in parallel is equal to the sum of the individual discharges for s specific head.
