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1. How to calculate the HP of the water pump

Dear Sir,

The horse-power (H.P.) of a pump can be determined by calculating the work done by a pump in raising the water upto H height in meters

Let the pump raise ‘W’ kg of water to height ‘H’ m

Then work done by pump = W X H Kg m  = WQH mkg/sec

Where W → density of water in kg/m3. Is equal to app. 1.0

Q → water discharge by pump in m3/sec

The water horse power, Pw =  (W × Q × H) /75   and the break horse power  =  ((W × Q × H) /75 X Efficiency }

Power or energy required per second to the pump water is Pw = QH kw  and this determines the horsepower

Horse power of the pump is Pw x 1.341 HP

The above is the general formula.  But for the design of your system, you should know the following in addition, the lift and the total head.  The total head involve, friction loss while the water flow through the pipe, bed loss, loss at valve, junction, at various changes of diameter of pipe etc.  Hence to design a system if you know the all values, then you can design the pump and motor horse power, (Discharge rate, Lift, total head)

The total head involve various frictional loss computations at different stage.  These are useful to understand the system and design as a whole.

But in practical cases, due to exhaustive trial and error statistics, either recorded or practically performed  and assessed, given the discharge, pipe diameter, and total height from sump to tank, an experience mechanic can tell the required horse power of the pump/ motor and pipe diameter. 

PVC pipes and HDP hoses have low frictional loss than the metal pipes. Depending up on your overhead tank size, available volume of water, you can chose the pipe diameter and horse power by varying the time of pumping. If you want lesser time, choose higher horse power which is within the available discharge

For a 5 member family, 500 liter capacity tank,  filling twice a day will suffice the need.  Assuming the tank is at a height of say about 25 feet you may fit 1.5 h.p centrifugal self priming pump to lift water from 5 feet depth sump for a pipe diameter of  one quarter inches.   This might fill your tank of 500 liters in maximum of about 10 to 12 minutes.  However this depends on the quality and make of the impeller and motor. An ISI standard pump might do.

This is for your enlightenment.





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