Electrosubmersible units, which are in widespread use nowadays,
eliminate the need for suction pipes, and the problems of cavitation and
cooling are avoided.
13.2.6 Priming of the pump
Prior to the starting-up of a centrifugal-type pump, both the pump and its
suction pipe must be full of water. If the pump is located below the water
level in the sump this condition will always exist; otherwise the air in the
pump and its suction pipe must be expelled and replaced by water, i.e.
‘primed’. A foot valve (non-return type) at the suction inlet end, together
with an air vent cock in the volute casing of the pump, assist the priming
process. Self-priming pump units which do not need any external priming
are also commercially available.
13.3 Design of pumping mains
13.3.1 System characteristic, duty point, and operational range
The efficient design of a pumping station largely depends on the piping
system used to convey the fluid. Friction and other losses in the system,
which are a function of discharge, have to be overcome by the pump, the
performance of which is interrelated with the external pipe (system,) char-
acteristic.
The pump characteristic at a given speed, N, is a function of dis-
charge and is written as
HmAQ^2 BQC (13.14)
whereA,BandCare the coefficients which can be evaluated from its
Q(H) curve.
The system curve is written as
HmHshmhf (13.15)
wherehmandhfare the minor and major system losses respectively; both
these losses can be expressed as KQ^2 ,Kbeing the appropriate loss coeffi-
cient. The solution of equations (13.14) and (13.15) (either analytical or
graphical – Fig. 13.3) gives the duty point of the installation at which the
pump delivers the required discharge. It is important that the duty point
coincides with the peak efficiency of the pump for its economical opera-
554 PUMPING STATIONS
