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The selection of a proper pumping set is important to ensure continued satisfactory yields from wells and the factors to be considered are:
(i) Finished inside diameter and total depth of the well.
(ii) Yield from the well, the desired pumping rate and hours of pumping per day.
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(iii) The lowest pumping water level (in the dry season).
(iv) The total head on the pump.
(v) The power required.
(vi) The quality of water, whether corrosive, clear or sandy.
The characteristic curves of a pump show the relationship between pump discharge, total head, and horsepower consumed and pump efficiency at a particular operating speed of the pump, Fig. 13.6 (a). Pump manufacturers also give the characteristics of their pumps at different speeds within a limited range. The discharge-drawdown relationship of a well is known as the well characteristics and is obtained by pumping tests, Fig. 13.6 (b).
An accurate test of a water well in advance, more than pays for itself by the saving that can be made in the selection of the proper pump and in the reduction of power costs. The characteristic curves of the well and the pump, enable the selection of a pump which best suits the well. It is usual to draw the well characteristic curve on a tracing paper in the same scale as that of the pump characteristic curves.
The pump characteristics are matched with the well characteristics and the pump which gives the maximum efficiency at the desired head and discharge is selected. The pump selected will give a discharge of 3,150 lpm a total head of 13 m with an efficiency of 65% and requires 12 hp. Some pump manufacturers provide information of their products as tabular data.
Though precise matching of pump and well characteristics is not possible with tabular data, it is possible to select an efficient pump by checking the data of a large number of pumps. When the well yield exceeds the irrigation requirement, it is economical to utilise in full the available well yield. The excess water may be sold to the neighbouring farmers. Pumps could be of plunger, displacement, deep well turbine, submersible, air lift or jet type.
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The suitability of each type is given in the following:
1. Plunger Pumps:
Hand pumps with the cylinder at the ground surface or the hollow brass cylinder lowered below the ground surface (depending upon the ground water level) can discharge 20-60 lpm. A plunger, which is connected to the pump handle rod moves up and down in the cylinder. It can be hand operated or driven by a 1 or 2 hp (V-belt driven) motor, Fig. 13.7. Pump handles often get broken in constant village use; handles should be kept in spare as also extra bolts, nut and bushings needed at the moving joint of the handle. Villagers should be taught to handle the pumps gently. The pump should be set so that the cylinder is within 6.7-7.6 m of the lowest water table, and can pump of a total head of 45-60 m.
2. Jet Pumps:
Jet pumps are often practicable for pumping rather small flows (40-90 lpm) under low heads (15-45 m) when the water level is beyond 7.6 m from the ground surface. Their capacity reduces as the lift increases. A jet pump consists of a pump and a jet, Fig. 13.8. Water is recirculated from the delivery side of the pump to the bottom of the suction pipe and is injected through a nozzle to impart additional kinetic energy.
This gives additional suction lift by creating a partial vacuum at this point. The advantage of the jet pump over most other types of deep-well pumps is that the pump and motor may be set away from the well. Jet pumps are usually used for residential buildings and hotels. There are two type’s twin type for borewells 15 cm and above, and packer type (duplex) for borewells less than 15 cm.
3. Deep-Well Vertical Turbine Pumps:
Deep-well vertical turbine pumps are most widely used for large tubewells. The bowl-assembly (impellers) is kept below the lowest pumping water level, but the driving unit—electric motor or petrol or diesel engine—is on the ground surface and is connected by a long shaft, Fig. 13.9. Usually deep-well turbine pumps are used for fairly high flows under high heads. This type of pump has the advantages of high efficiency, high head pumping capability and excellent serviceability.
The impellers can be obtained semi- open or fully enclosed. This pump requires sufficiently straight and plumb well for installation and proper operation and is subject to abrasion from sand. The maintenance problem is severe when pumping corrosive water unless pump, column, shaft etc., are made of non-corrosive materials. Lubrication and vertical alignment of shaft is critical. The overall efficiencies of turbine pumps range from 50 to 80%.
4. Submersible Pumps:
Submersible pumps have the motor and the bowl assembly as a unit submerged below the lowest pumping water level. A water proof cable supplies power to the motor. Submersible pumps to fit inside 10, 15, 20 and 25 cm borewells are available in India, Fig. 13.10. They can be used for flow rates from 40-3000 1pm and heads from 15-150 m. They can be installed in crooked wells but repair to motor or pump requires removal from well and is subject to abrasion from sand.
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This type of pump has the advantage that it can be installed when there is little or no floor space to install the unit and in locations that require quiet operation. They can be either water or oil lubricated. Their initial costs are lower than those of vertical turbine pumps. Their repair and maintenance costs, however, are high. The new type of voltage regulated starters have solved the problem of over loading.
5. Air-Lift Pumps:
Air-lift pumps have efficiencies ranging from 20 to 35%; the efficiency greatly depends upon the percentage submergence and is reasonable when the percentage submergence is 50 to 60%, Fig. 13.11. These pumps have the advantage of having no moving parts below the ground level. However, the disadvantage of low efficiency, coupled with the pump’s inability to pump against high head requirements, has limited its use. Air lifts are adapted to crooked wells, to wells discharging large amounts of sand, and to installations where reliability is of more importance than efficiency. Capacities for well pumping range from 90 to 9000 lpm.
6. Centrifugal Pumps:
The most common type of irrigation pump is the volute centrifugal pump, Fig. 13.12.
It has the advantage of low initial and maintenance costs and high efficiency. It has the limitation that the available drawdown in the well is limited to about 6.5 m.
It can be used to a tubewell if a pit is made to house the pump, so that the pumping water level is within the suction lift of the pump, thereby saving in the initial cost of a deep well pump. It is also not very efficient under low heads, Fig. 13.13.
The propeller pump is ideally suited for low head high discharge pumping. For pumping from rivers, canals and large tanks, the propeller pump is often the most efficient.
Information to be Supplied to Pump Manufacturers:
The pump should have semi-open type dynamically balanced bronze impellers, with steel column pipes, stainless steel line shaft (preferably hollow shaft) with screw coupling and class-C steel column pipe assembly of 3 m lengths along with standard accessories such as foot valve, perforated brass strainer, pressure gauge etc.
B. Suitable 1 : 1 ratio right angle gear head for installing on the above pumps so that the drive may be through a diesel engine with an intermittent spicer shaft coupled with universal joint cross.
Note:
These pumps are proposed to be utilised as test pumps in borewells and should be capable of withstanding continuous running at a single stretch of 8 to 10 days.
Detailed specifications and catalogues, etc., should accompany the tenders.