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Water Well Pump Curves And Different Gallon Per Minute Pump Ends

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It occurred to me that I have never mentioned how to read pump curves or talked about why submersible pumps ends are offered in so many different gallons per minute ranges.

Lets first attack the gallons per minute issue with pump ends. We use the term pump ends to separate it from the motor. Pump ends have impellers. Impellers are different from propellers in that they pull water in through the impeller where a propeller pushes air around the blades of a propeller. An impeller has volutes, usually 5 or so. These volutes extend outward and are all attached to the eye of the impeller where the water is introduced to the impeller. The faster the impeller is turning (in rpmís) the more water it will ingest and the more pressure it can build. Submersible pump ends have more than one impeller except in cases where a lot of water is needed at very low pressure. As the first impeller passes water to the next in the impeller stack. Itís pressure is added to the pressure of the second impeller and so on.

Wells are all different depths and so are the water levels in these wells across the country. Mountainous areas tend to have deeper wells and lower water levels. The further a pump has to push the water up vertically, the more pressure (head) is required. To put this into perspective, 1 foot of head = .433 pounds. The reason we use the term head is that it makes it easier to size a pump. For instance; lets say we have a 150′ well that has a 75 foot static water level. (The distance from ground level to top of water in the well) With this well we want to irrigate a small garden. The sprinklers in the irrigation system use 10 gallons per minute and the ideal pressure to make them reach out as far as reasonably possible is 40 psi (pounds per square inch). We know that the well has a 75′ water level which is 75′ of head. We know we need 10 gallons per minute at 40 pounds which can be converted to head by multiplying 40 times 2.3. The math says that 40 psi X 2.3 = 92′ of head. Add the 92′ to the 75′ of head for the water level and we have a total head of 167′. Now we go to our pump curve:

As you can see, the 7 gpm pump end poops out at 10 gpm. Without going up to a 1.5 hp pump motor, this pump will not work well at all. Lets see what the 10 gallon per minute pump will do at 167′ of head.

The ten gallon per minute 1/2 hp pump is doing a little over the 10 gallons per minute at 40 psi or a total of 167′ of head. Maybe now you can see why itís easier to convert pressure to head and add it to the water level to size a pump.

If we had gone on further with other pump curves like the 16, 25, 40, 60 and 75 gpm pump ends, they would have been way too big for our job. And as you can see, there are many horse powers to pick from as well as pump ends. Itís just a matter of matching the motor to the right pump end with the needed number of impellers to get the job done. As the curve goes up in horse power, so does the number if impellers in the pump end. With the Shakti pump, the number of impellers is in the model number. The 1/2 hp ten gallon per minute pumps model number is shown to the left of the curve as QF2-9 (1/2 HP) The nine is the number of impellers.

I hope I made this clear enough so you can size your own pump for the application at hand. If you have questions, please post them below and Iíll do my best to answer them for you.
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