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Static Suction Head
As a thumb rule, the frictional losses in a centrifugal pumpset level in the well
and the centerline of the pump. Evidently, its value should be less than 6 meters
which is the practical limit of total head in a centrifugal pump.
As a thumb rule the frictional losses in a centrifugal pumpset are about 8% of
the total head. It is also known that for good quality pumps the NPSH
requirement is of the order of 10% of the head at first stage of the pump. Velocity
on suction side is restricted to a maximum of 1.5 metres per second which
gives a velocity head of 0.077 metres. Thus, in the equation for total head the
following values can be substituted.
6 = Hs + Hf + Hv + NPSH
Where Hf = 8% of Hs,
NPSH = 10% of Hs
Hv = 0.077 metes or
H = 5.923 = 5 meters
1.18
Thus, as a thumb rule the static suction head for a centrifugal pump should not
exceed 4.5 to 5 metres. Otherwise cavitation may start and the working of the
pump may become inefficient.
Guidelines for selection of Centrifugal Pumps
The following are the main guidelines for selection of centrifugal pumps:-
[NABARD, 1989]
1.
The Horse power (HP) required for a pump can be calculated from the
following formula :
HP = Q x H
75 x e
Where Q = Discharge in litres per second
H = Total head (including friction losses), in meters
E = Efficiency of the pump, in fraction.
