Some water wells can be very deep at over 800’ or more while others are shallow at 50’ or even less. Water demands vary from just a few gallons per minute to hundreds of gallons per minute. In this overview we are going to explore the factors affecting the selection of the correct submersible well pump for your water well.
Design Point Calculation.
The design point for a submersible well pump is the combination of how much pressure the pump needs to make and how much flow the pump is expected to provide at that pressure. For example: Joe Waterman needs a pump for his well that will produce 216 PSI (500’ TDH) at a flow rate of 15 Gallons per minute.
The first factor is pressure, referred to in the industry as “Total Dynamic Head.” Total Dynamic Head is related to the vertical distance that the water must be lifted by the pump. For example, if you have a pump capable of producing 100 PSI, that same pump will be able to push water to an elevation 231’ above the pump! The second factor is called “friction loss” and is the slowing effect of friction as water runs through the piping between the pump and the house. Friction loss is calculated based on the size, length and type of pipe used. The last factor is the amount of flow required. If you have a 2 bed/2 bath house with no irrigation 10-12 Gallons per minute may be adequate, but a large estate with extensive landscaping may require 50-100 Gallons per minute.
To get the design point for your well pump, your contractor should review data from the well driller's log to find well and water depth in addition to expected flow from the well. They should also look at any well test reports that have been done on your well and investigate the water demands for housing and landscaping.
Once your well pump contractor has reviewed the pertinent information and established an adequate design point, they will then review technical bulletins called “pump curves” from well pump manufacturers. Pump curves from Franklin Water are pictured below. You can see that for any given pump, the higher the pressure, the lower the water flow. Careful review of these pump curves indicate which pump will be best for your application.
Let’s use an example to illustrate proper pump selection. Let’s say that you have a well that is 400’ deep with a static water level that is at 200’ and the water level in the well drops to 300’ when the well is pumped at 15 Gallons per minute. This water well supplies water to a house that is on a hill that is 50’ above the well and the house needs to have 50 PSI of pressure available at the house (this is equivalent to 115’ of head). The house has minimal landscaping and 15 Gallons per minute is adequate. The pipes supplying the house have friction loss values of 35’ TDH at a flow rate of 15 gallons per minute.
The total dynamic head (TDH) is calculated twice, once for the best case scenario with a high water level in the well and again with the worst case scenario, with a low water table:
Best Case Design Point= 200’+50’+115’+35’=400’ TDH @ ~15 GPM
Worst Case Design Point=400’+50’+115’+35’=600’ TDH @ ~15 GPM
In the pump curve bulletin above from Franklin Pump Systems, you can see that there are several pumps that will provide ~13-17 Gallons Per Minute between 400’ and 600’ THD! The correct pump is the one that delivers the water needed with the pump curve staying in the blue “best efficiency” zone. A bigger, higher horsepower pump is not “better.” You will notice that the 5 HP pump in this application will be running and delivering slightly more water, but running outside the blue efficient zone. Choosing the 5 HP pump would cost more to purchase and waste significant amounts of electricity over the life of the pump! Please check out our post Are You Pouring Money Down Your Well? for more information.
Aren’t Pumps sized by Horsepower?
In the pump industry we typically designate pumps with the flow rate AND horsepower to ensure that we cover the bases. The amount of horsepower required is based on BOTH the water flow & pressure requirements. When specifying the correct pump is is more important to specify the amount of pressure and flow required than the horsepower required for the motor. Some pumps may be more efficient and be able to provide the flow and pressure with less power and thus a lower horsepower motor.
Oakville Pump Service has trained professionals with decades of experience. Our professionals will help you select the correct submersible well pump for your residential, commercial or agricultural well in the Napa and Pope Valley areas. OPS knows pumps and pump manufacturers. Wether your water is deep, sandy, hot, or full of minerals; we can help you choose the pump best suited for your application. Our team will ensure you get the pump that will last the longest, work the most efficiently and be installed in a manner that represents the highest industry standards! Call us today at (707)944-2471