(Section 13.6). Archimedes screw pumps are used to lift wastewater with
large floating bodies into treatment works.
(e) Abstraction from boreholes
The installations are normally of the wet well type with pumps located
within the wells. In deep wells special multistage pumps are used, whereas
for shallow wells the pumps may be located at ground level. Well screens
are essential to prevent sand from entering into the system. The deep well
pumps are normally less bulky (around 100–400 mm diameter) to fit into
well diameters of 150–600 mm.
13.5 Sump design
The most important aspect of pumping installation design is the suction
well (sump), with a good intake (inlet to suction pipe) arrangement. In the
case of shallow submergence of the inlet, the formation of a local vortex at
the water surface is often observed. Such a situation may also arise if the
volume of water in the sump is inadequate, in which case the sudden start-
ing of the pump lowers the water surface. Equally sudden steep velocity
gradients, high velocities, sudden changes in the flow direction as water
enters the bellmouth inlet in the sump and several pump units set in a long
narrow sump with inflow from one end also aggravate the formation of
vortices.
The vortex entrains large quantities of air into the pump suction
pipe. This causes a drop in efficiency and structural vibrations, and
increases the corrosion damage of the pump and its accessories.
SUMP DESIGN 559
Fig. 13.8 Pump sump and intake for sewage pumping