QUESTION
How to calculate HP of submersible pump for a particular application and what all factors should be taken into consideration?

Every pump manufacturer provides a Head-Capacity curve or a Table that says how much water to expect from what depth for a given HP and efficiency range. Obviously deeper one installs a pump, less water it pumps. Please note that higher the HP one uses does not mean that more water is pumped out. Ideally, a pump should extract only as much water as the yield rate of the aquifer permits, so as to maintain a steady state in the pumping water level.

A submersible pump suitable for a 6” (inch) bore well, will have outer diameter of the main pump body (motor + pump) at about 5 and odd inches to allow necessary margin of gap for lowering and rotation of the pump. The outer diameter of a pump suitable for a 6” bore is already fixed by the pump manufacturers. Usually the diameter of the rising pipe which brings water to the surface for a 10 HP pump is to the order of 2” – 3” depending on discharge.

Also a pump lifts water depending on the speed, diameter and stages of the impellers which again the pump manufacturers decide. For a given discharge capacity, one needs more stages (counted by the number of rings in the pump body) as one goes deeper.

Ideally a bore well should be tested for its optimal yield before selecting a pump. Such test is known as “Step Drawdown Test”. In this test a submersible pump is lowered at a suitable depth and the pump is run for a fixed time (say 1 hr.) in steps.

In each step the pump is made to lift water at very low, low, medium and high discharge rates using a Glove Valve in the delivery pipe to maintain a constant discharge for the particular step. The drawdown (fall in the pumping water level) is measured during and at the end of each step (1 hr.). The discharge drawdown curve gives the optimum discharges of the well.

One can try to do this test while installing a pump as recommended by the local pump dealer.

Lowering the pump at a suitable depth with sufficient submergence is important as the pumping water level usually falls depending upon the discharge rate of the pump and aquifer yielding capacity (permeability).

Ideally a pump should extract water at such a rate so that the water level in the well stabilizes after some times and do not fall further. A pump running dry with lowering water level can burn out easily. To prevent this, you are advised to install a 1.0”-1.25” flexible PVC pipe in the bore well till the pump depth so that you can monitor the pumping water level using an electrical water level recorder at any time. The PVC guide pipe needs to the clamped at the top securely to prevent it from falling into the bore hole.

Finally, do not expect your well to yield more than your neighbor’s well of similar depth. In fact you will join the groundwater extraction club feeding on the same aquifer albeit tapping a few extra fracture zone due to a greater depth.

Contributor: Dr. Mihir Maitra

by
20 July 2012