The basic function of the intake structure is to help in safely withdrawing water from the source over predetermined pool levels and then to discharge this water into the withdrawal conduit (normally called intake conduit), through which it flows up to water treatment plant.
Factors Governing Location of Intake
As far as possible, the site should be near the treatment plant so that the cost of conveying water to the city is less.
The intake must be located in the purer zone of the source to draw best quality water from the source, thereby reducing load on the treatment plant.
The intake must never be located at the downstream or in the vicinity of the point of disposal of wastewater.
The site should be such as to permit greater withdrawal of water, if required at a future date.
The intake must be located at a place from where it can draw water even during the driest period of the year.
The intake site should remain easily accessible during floods and should noy get flooded. Moreover, the flood waters should not be concentrated in the vicinity of the intake.
Types of Intake
- sufficient factor of safety against external forces such as heavy currents, floating materials, submerged bodies, ice pressure, etc.
- should have sufficient self weight so that it does not float by upthrust of water.
Depending on the source of water, the intake works are classified as follows:
A pump is a device which converts mechanical energy into hydraulic energy. It lifts water from a lower to a higher level and delivers it at high pressure. Pumps are employed in water supply projects at various stages for following purposes:
- To lift raw water from wells.
- To deliver treated water to the consumer at desired pressure.
- To supply pressured water for fire hydrants.
- To boost up pressure in water mains.
- To fill elevated overhead water tanks.
- To back-wash filters.
- To pump chemical solutions, needed for water treatment.
Classification of Pumps
Based on principle of operation, pumps may be classified as follows:
- Displacement pumps (reciprocating, rotary)
- Velocity pumps (centrifugal, turbine and jet pumps)
- Buoyancy pumps (air lift pumps)
- Impulse pumps (hydraulic rams)
Capacity of Pumps
Work done by the pump,
where, g= specific weight of water kg/m3, Q= discharge of pump, m3/s; and H= total head against which pump has to work.
H= Hs + Hd + Hf + (losses due to exit, entrance, bends, valves, and so on)
where, Hs=suction head, Hd = delivery head, and Hf = friction loss.
Efficiency of pump (E) = gQH/Brake H.P.
Total brake horse power required = gQH/E
Provide even number of motors say 2,4,... with their total capacity being equal to the total BHP and provide half of the motors required as stand-by.
There are two stages in the transportation of water:
- Conveyance of water from the source to the treatment plant.
- Conveyance of treated water from treatment plant to the distribution system.
In the first stage water is transported by gravity or by pumping or by the combined action of both, depending upon the relative elevations of the treatment plant and the source of supply.
In the second stage water transmission may be either by pumping into an overhead tank and then supplying by gravity or by pumping directly into the water-main for distribution.
Free Flow System
In this system, the surface of water in the conveying section flows freely due to gravity. In such a conduit the hydraulic gradient line coincide with the water surface and is parallel to the bed of the conduit. It is often necessary to construct very long conveying sections, to suit the slope of the existing ground. The sections used for free-flow are: Canals, flumes, grade aqueducts and grade tunnels.
In pressure conduits, which are closed conduits, the water flows under pressure above the atmospheric pressure. The bed or invert of the conduit in pressure flows is thus independant of the grade of the hydraulic gradient line and can, therefore, follow the natural available ground surface thus requiring lesser length of conduit. The pressure aqueducts may be in the form of closed pipes or closed aqueducts and tunnels called pressure aqueducts or pressure tunnels designed for the pressure likely to come on them. Due to their circular shapes, every pressure conduit is generally termed as a pressure pipe. When a pressure pipe drops beneath a valley, stream, or some other depression, it is called a depressed pipe or an inverted siphon.
Depending upon the construction material, the pressure pipes are of following types: Cast iron, steel, R.C.C, hume steel, vitrified clay, asbestos cement, wrought iron, copper, brass and lead, plastic, and glass reinforced plastic pipes.
The design of water supply conduits depends on the resistance to flow, available pressure or head, and allowable velocities of flow. Generally, Hazen-William's formula for pressure conduits and Manning's formula for freeflow conduits are used.
U=0.85 C rH0.63S0.54
where, U= velocity, m/s; rH= hydraulic radius,m; S= slope, C= Hazen-William's coefficient, and n = Manning's coefficient.