Jet pumps which are used in irrigation practice consist of a combination of a centrifugal pump and a Jet Mechanism, or an enjector. The centrifugal pump, coupled with the electric motor or engine, is placed at the ground surface and furnishes the driving head and capacity for the jet pump placed in the well bellow the water surface. The main parts of a jet pump are a nozzle and a venture. At the pump delivery, a portion of the high pressure water returns through the pressure pipe to activate the nozzle in the ejector.
The nozzle is shaped so that it smoothly but abruptly reduces the area through which the flow must pass, thus increasing the velocity of flow. This creates a low pressure area around the venture which draws more water from the well. The gradual enlargement in the venture tube to the full diameter of suction pipe reduces the velocity of water with a minimum of turbulence. The vacuum created by the impeller of the pump placed at the ground surface draws the flow through the delivery pipe at the desired pressure. The additional supply of water which is obtained from the well is discharged past the control valve while the volume required for producing the flow is recirculated through the pressure pipe. The control valve is set to maintain the necessary' pressure to produce flow at the existing pumping head.
Fig. 7. Jet pump assembly with electric jet assembly motor driven
The most important factor governing the capacity and efficiency of a jet pump is the selection of the jet to correspond to both the type of the pump and depth of low-water level at the source. Jet pumps are by nature inefficient. The highest efficiency usually obtained is about 35%. The efficiency of the pump is influenced mainly by the nozzle throat ratio. For small jet pumps used in domestic water supply, the spacing between the nozzle and the throat is equal to one nozzle diameter. The length of the throat is about six throat diameters.
Adaptability: Jet pumps are generally used for low capacity deep well pumping where it is necessary to locate the moving parts of the pump and the prime mover at the ground surface. The following are the main advantages of a jet pump:
- (1) Its adaptability to be installed in wells as small as 5 cm inside diameter. (When the well diameter is too small to admit two pipes, a single pipe ejector may be used to connect the piping to the pump head. When this is done, the jet should always be submerged and the well casing extended to the low water level).
- (2) Its high lift which cannot be reached by ordinary centrifugal pumps. The suction lifts usually obtained with low to medium size jet pumps vary from 12 to 18 metres.
- (3) Accessibility of all moving parts which are located at the ground surface.
- (4) Simplicity combined with relatively low cost and easy maintenance.
- (5) Adaptability to being installed with the moving parts offset from the well.
In some locations, where water levels are subject to large seasonal variations, or where severe corrosion or plugging at the nozzle occurs, jet pumps are not very satisfactory.