The bellmouth entry to the pump suction pipe suppresses flow sepa-
ration and ensures uniform flow throughout the intake cross-section. The
turned-down bellmouth (Fig. 13.7(c)) allows a lower sump water level than
a horizontal intake (Fig. 13.7(b)) and is less prone to vortex formation
(Section 13.5).
(c) Stormwater pumping installations
Provision of coarse screens with wet well arrangements is recommended in
this type of pumping installation. Since the pump duty is intermittent,
maintenance is possible during dry periods. Similar arrangements apply to
de-watering and land-drainage installations.
(d) Sewage (untreated) pumping to treatment works
The pump sump size is so designed as to keep the approach flow velocities
sufficient for the solid matter to remain in suspension. Stagnant areas or
corners must be avoided. A dry well with a turned-down bellmouth is the
most suitable type of sump, and a typical section of such an arrangement is
shown in Fig. 13.8.
Screening of the sewage prior to pumping should be avoided if pos-
sible as the screening equipment requires regular maintenance with addi-
tional costs for the disposal of the screened wastes. Normally small
pumping stations (pump sizes up to 200 mm in diameter), preferably with
two pump units, are desirable to cope with the pumping of untreated
sewage. Pumps with appropriate impeller design are available to pass
almost all solids that can enter the pump suction bellmouth (Prosser, 1992)
558 PUMPING STATIONS
Fig. 13.7 Wet and dry well (sump) pump installations. Note that the best
location for the pump in a dry well is below WL
