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down. This is known as Drawdown in the case of wells in alluvial formation. For
wells in hard rock areas part of the water column is dewatered during the 4 to 6
hour operation of the pump. Therefore, in these cases the drawdown can be
taken to be equal to half the dewatered water depth in the well.
For lifting groundwater from shallow wells mostly centrifugal pumps operating
at about 1500 RPM are used. The selection criteria given below is applicable to
the above type of pumps only.
Suction limit of Centrifugal Pump
The total head over which a pump is required to lift water comprises of suction
head, the delivery head, the head loss due to friction in suction and delivery
pipes and the velocity head. However, at the well site only the suction and
delivery heads are measurable and are known. The other parameters have to
be evaluated or furnished in the form of a matrix. The suction or delivery head is
always referred to from the centerline of the pump known as axis. In the case of
centrifugal pumps, the suction head is very important as beyond a certain value
of static suction head the working of the pump may be adversely affected.
Suction Head
(i)
It is known that the atmospheric pressure at sea level is about 10 metres
of water head. For pumps operating above sea level the reduction is about 1.15
metres for every 1,000 metes altitude. It has, however, been observed in
practice that if the total suction lift is greater than 6 to 7 meters, cavitations take
place and the pump stops working. If the suction limit exceeds 9 meters
centrifugal pumps are not suitable for use and in that case either vertical turbine
pumps or submersible pumps should be used.
The practical limit of 6 to 7 meters of total suction head can be attributed to the
various components as below. The maximum practical suction lift can be
computed by the following equation. [NABARD, 1989]
