13.2.4 Pump operationA typical installation of a centrifugal pump, transferring water from a
sump (low level) to a higher elevation, is shown in Fig. 13.2. VsandVdare
the velocities in suction and delivery pipes; hfsandhfdare the head losses
in suction and delivery pipes (friction, valves, bends, etc.); hsandhdare the
suction and delivery heads, the sum of which is the total static lift, Hs.Hm
is the total (manometric) head the pump must develop to lift water
through the static lift.
Referring to Fig. 13.2, the manometric head can be written as
HmHshfshfdV^2 d/2g. (13.2)Equation (13.2) can be rewritten with negligible velocity head as
HmHshfshfd. (13.3)If the impeller losses are considered, the efficiency (manometric effi-
ciency) of the pump is affected. The total head the pump must develop to
overcome the impeller and system losses (in pipelines and pipe fittings) in
lifting the water through a given static lift is given by
HHm/ (13.4)whereis the manometric efficiency of the pump.
The pump characteristics, such as discharge versus total head, effi-
ciency and power for the design speed of rotation are obtained from the
manufacturers. Pumps are sometimes required to operate under varying
speeds (due to varying demands) and their corresponding characteristics
change considerably. Within a reasonable range of operational speeds the
performance may be assessed with the help of hill-diagrams (curves of
equal efficiencies).
PUMPS AND THEIR CLASSIFICATION 551
Table 13.2 Specific speeds for rotodynamic pumps
Type Specific speed
Radial flow (i) slow speed 300–900
(ii) medium speed 900–1500
(iii) high speed 1500–2400
Mixed flow 2400–5000
Axial flow 3400–15 000