The Anatomy Of A Gravity Flow Water System
This section contains photographs that detail the construction of a gravity flow water system for 32 families. The system cost £130 per family and took 30 people 18 days to build. Before the system can be designed the area must be surveyed (for details on these processes click here).
The system consists of:
- A spring tank;
- A main pipeline, including a pipe bridge;
- A reservoir tank;
- A distribution network that leads to a tap at each house.
For a gravity flow system to work properly the pipes must run full of water with no air locks. Gravity can then be used to move water, over hills and undulations, between the spring and the reservoir tank. This method works for as long as the spring tank is at the highest point in the system and that there is enough height difference, between the spring tank and the reservoir tank, to give a sufficient flow rate once friction losses have been taken into account. The distribution network also uses gravity to move water to the taps through thinner pipes.
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Spring
Tank
Water is taken from a spring, rather than the river, to reduce the
risk of contamination by parasites and other pernicious micro-organisms.
A tank is built at the spring to ensure a continuous head of water
above the start of the mainline. Thus, the mainline should always
run full without any air locks.This tank is cast from conctret and
consists of a sedimentation area and a 380 liter tank.
- Click here to see the photographs showing the spring tank construction.
The Main pipeline
This system has 4 km of pipe. The main
pipeline runs from the spring tank to the reservoir tank. Because
of the large drop between the spring tank and the pipe bridge PVC
pipe (rather than the usual polyethylene) was used to cope with
the large pressures. Due to the topography a break pressure tank
was not included in the design. A galvanised steel pipe bridge spans
a river at the lowest point below the spring tank.
- Click here to see the photographs showing the pipeline being laid and the pipe bridge construction.
- For details of friction loss calculations click here.
Reservoir Tank
The reservoir tank is placed at a point
near the village that is higher than all the houses that are to
be supplied. The tank must be large enough to ensure an uninterrupted
supply during the times of day when there is the peak demand. The
tank is thus sized according to the size of the population and the
flow rate from the spring during the dry season. The tank in this
system is a 13 000 litre tank made from reinforced concrete using
a pre-fabricated mould. The tank has a vaulted brick roof, a method
that is both cheaper and quicker than making a reinforced concrete
roof.
- Click here to see the photographs showing the mould being used to cast the tank wall.
- Click here to see the photographs of the vaulted brick roof being made.
- For details of reservoir tank sizing click here.
The Distribution Network
In this system each family got their own tap that is connected to
the reservoir tank using a polyethylene pipe network.
- Click here to see the photographs of the distribution network being laid.