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The major types of variable displacement pumps are as follows: 1. Centrifugal Pumps 2. Jet Pumps 3. Air-Lift Pumps.
Type # 1. Centrifugal Pumps:
Centrifugal pumps are the most popular types of variable displacement pumps. They are capable of delivering large volumes of water against high as well as low heads with good efficiency. Besides, these pumps are relatively simple and compact. The basic principle of centrifugal pumping can be understood by considering the effect of swinging a bucket of water around in a circle at the end of a rope.
The centrifugal force causes the water to press against the bottom of the bucket rather than spill out of the bucket. If a hole is cut in the bottom, water would discharge through the opening at a velocity which would depend on the centrifugal force. If an airtight cover was put on the bucket top, a partial vacuum would be created inside the bucket as water leaves through the opening in the bottom.
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If a water source is connected to the airtight cover through an intake pipe, the partial vacuum could draw additional water into the bucket as the water is being discharged through the bottom hole.
The bucket and cover of this example correspond to the casing of a centrifugal pump; the discharge hole and the intake pipe correspond to the pump outlet and inlet respectively; the arm that swings the bucket correspond to the energy source and the rope performs the function of a pump impeller.
Suction-lift pumps create negative pressure at the pump intake. The atmospheric pressure at the free surface of water in the well forces the well water into and up the intake pipe.
The maximum suction lift depends on the atmospheric pressure (10.4 m of water head), vapour pressure of water, head loss due to friction and the head requirements of the pump itself. Under field conditions, the average suction-lift capability of a suction-lift centrifugal pump is about 7.5 m.
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Deep-well vertical turbine pump consists of one or more impellers housed in a single- or multi-stage unit called a bowl assembly. Each stage gives a certain amount of lift and sufficient number of stages (or bowl assemblies) are assembled to meet the total head requirement of the system.
Vertical turbine pumps in high capacity wells are highly reliable over long periods of time. The motors of these pumps are not susceptible to failure caused by fluctuations in electric supply. Motor repairs can be carried out easily because of their installation on the ground surface. These pumps, however, cannot be used in wells which are out of alignment. Besides, these pumps require highly skilled personnel for installation and service.
Submersible pumps have the bowl assemblies which are the same as those of vertical turbine pumps. But the motor of submersible pump is submerged and is directly connected to and located just beneath the bowl assembly. Water enters through an intake screen between the motor (at lower level) and the bowl assembly (at higher level), passes through various stages and is discharged directly through the pump column to the surface.
The motor of submersible pump is directly coupled to the impellers and is easily cooled because of complete submergence. Ground surface noise is also eliminated. The pump can be mounted in casings which are not entirely straight.
Pump house is also not necessary. There are, however, electrical problems associated with submerged cables. These pumps cannot tolerate sand pumping and work less efficiently. The motor is less accessible for repairs and cannot tolerate voltage fluctuations.
Type # 2. Jet Pumps:
Jet pump is a combination of a centrifugal pump and a nozzle-venturi arrangement, as shown in Fig. 4.8. The nozzle causes increased velocity and reduced pressure at point A. The lowered pressure at A draws additional water from the intake pipe, and this water is added to the total volume of water flowing beyond A.
The venturi tube helps in recovery of pressure at B with minimum loss of head. Compared to the centrifugal pumps, the jet pumps are inefficient but have some advantageous features too. These are adaptable to small wells down to 5 cm inside diameter. All moving parts of the jet pump are accessible at the ground surface. Their design is simple which results in low equipment and maintenance costs.
Type # 3. Air-Lift Pumps:
Water can also be lifted from inside of a well by releasing compressed air into an air discharge pipe (air line) lowered into the well. Because of the reduced specific gravity, aerated water is lifted to the ground surface. Air-lift pumping is inefficient and requires cumbersome and expensive equipment and is, therefore, rarely used as a permanent pumping system.
