13.6 Screening devices
All major pumping stations (especially sewage and stormwater pumps)
have to be provided with bar screens to prevent large objects from enter-
ing the wet well. They consist of steel bars spaced at 20–40 mm with a
blockage ratio (area of bars/total area) between 0.3 and 0.5. The spacings
could be larger if the pumps are able to pump solids (sewage pumps).
Appropriate mechanical cleaning (raking) devices for the screens may also
be provided. The screens are usually laid at an angle of 60°–90° to the hori-
zontal and a head loss of at least around 150 mm (Section 9.2) must be
incorporated in the hydraulic design of bar screen devices.
The presence of screens increases turbulence (proportional to bar
size) which would cause undesirable flow conditions if the screens were sit-
uated very close to the pump. Any swirling in the flow may be reduced by
a screen or grid, or even by a honeycomb device. Alternatively, splitters
may be used under the bellmouth intake as an antiswirl device.
13.7 Benching
As it is a desirable practice to pump all the solids, their deposition on the
floor or walls of the wet well sump must be minimal. This is achieved by
providing a suitable benching for the (self-cleansing) pump sumps.
The benching should be steep with a minimum slope of 45°; if pos-
sible, 60°–75° is preferable. It should extend right up to the pump intake
where it terminates in a small pit in which the intake bellmouth of the
pump must be installed. It is advisable to locate stop levels–override con-
trols in such a way that the sump level is drawn down to the lowest pos-
sible level at regular intervals (e.g. once a day).
13.8 Surges
It is essential to protect the pipeline and the pumping station against
pressure surges (waterhammer) in the system due to the closure or
opening of valves, or sudden power failure causing pump stoppage. The
undesirable effects of surges in pumping station systems can be controlled
by antiwaterhammer devices (flywheels, air vessels, etc.). For the treat-
ment of waterhammer analysis and surge protection see e.g. Jaeger (1977),
Novak (1983), Sharp and Sharp (1996) and Popescu et al. (2003).
562 PUMPING STATIONS
