A good sump design must therefore avoid the formation of vortices;
this can be achieved by directing the flow uniformly across the sump width
with an approach velocity of around 0.3 m s^1 and by avoiding abrupt
expansion of the side walls and large stagnation zones in the sump (mini-
mizing large-scale swirling flow). For larger expansion ratios (area at
outlet/area at inlet2) vanes may have to be provided for uniform flow
distribution. The pump intake must be located in the direction of the
approach flow if possible. Multiple intakes from the same sump should be
separated by dividing walls (to minimize interference). The most effective
free-surface vortex-suppressing device is a horizontal floor grating,
installed about 100 to 150 mm below the water level. Other devices such as
floating rafts, grating cages, and curtain walls are also used as vortex sup-
pressors. Provision of a bellmouth entry (Fig. 13.9(a)) at the inlet to the
suction pipe minimizes entrance losses and facilitates smooth axial flow.
Some typical sump layouts of good design (Prosser, 1977) are shown in
Figs 13.9(b)–13.9(e).
For intakes with proper approach flow conditions (without any
vortex-suppression devices) the minimum required submergence could be
written as
h/dabFrd (13.16)
wherehis the depth of submergence to the centre of intake pipe of dia-
meter,d,Frd(V/(gd)1/2,Vbeing the velocity in the intake pipe) is the
Froude number and a0.5–1.5 and b2–2.5 (Knauss, 1987). At low
Froude numbers (0.3, i.e. large-size intakes) a value of aof at least 1 is
recommended.
The minimum sump volume for good flow conditions also depends
on the maximum allowable number of pump starts in a given time. Start-
up of an electric motor generates considerable heat energy in the motor,
and hence the number of starts must be limited.
The minimum sump volume, Vmin, between stop and start of a single
pump unit is given by
VminQPT/4 (13.17)
whereQPis the pumping rate and Tis the time between starts. Equation
(13.17) then suggests that for a frequency of 10 starts per hour (T6 min)
the minimum volume is 1.5 times the pumping rate per minute. When two
or more pumps are used the start levels are normally staggered and equa-
tion (13.17) is applied to the largest pump.
For a multisystem an additional volume of 0.15 times the plan area of
the sump (in m^3 ) should be provided. However, there are certain restraints
in the maximum volume of the sump, e.g. a septicity problem of sewage in
the sewage sump (BS, 1987).
560 PUMPING STATIONS
