August 3, 2012
Title: How to Select a Submersible Well Pump
Disclaimer: This document is for instruction purposes only and is not intended for an actual application.
Terms:
TDH = Total Dynamic Head
GPM = Gallons per Minute
PSI = Pounds per Square Inch (Pressure)
Static Water Level = The depth to water in the well from the top of the well casing.
Pumping Water Level = The depth to water in the well from the top of the well casing while the pump is running.
Draw Down - The difference between the static water level and the pumping water level.
Friction Loss - The amount of friction produced while water is being pumped through pipes and fittings. Friction
loss should be a part of your TDH calculation, if possible.
Calculations:
TDH = PSI * 2.31
PSI = TDH / 2.31
1 - The first question would be are you replacing a pump or installing a new pump in a new well? In either case, it would be beneficial to know how many gallons per minute the well can produce. Usually the well driller would have this information. If you choose a pump that is too large, then you will have the potential of over-pumping the well causing the pump to run dry.
2 - In either case within Item 1, decide how many gallons per minute you require and determine how much pressure is required at the top of the well. If the system will be used as part of an irrigation system, then you should try to determine how many GPM per head and how many heads per zone. At the same time, if the system will be used for irrigation, you want to try to size the pump to where you only have only 4 to 6 starts per hour, maximum.
3 - So now, let’s say you require 20 GPM with 60 PSI at the top of the well. 60PSI will equate to 138.6’ TDH, 60 * 2.31, and let’s say you have a static water level of 100’ and a pumping water level of 145’. Your Total Dynamic Head will be 145’ + 138.6’ to equal 283.6’ TDH. At this point, you will look at the pump curve for the pump that you are considering purchasing. The curve will show you what horsepower pump to choose for the duty points that you require. See “How To Read A Pump Curve” in this forum.
4 - So now we have chosen a pump for the sample application in Item 3. Now, let’s say the pump will primarily be used for irrigation and we have a total of 1,500’ of 2” PVC pipe going out to our zones. We now need to know what the potential friction loss will be for the amount of pipe in the system. See “How Does Friction Loss Effect Pump Performance” in this forum. We now have to add the potential friction loss to our TDH calculation. In this case with 20GPM through 2” PVC we will lose .86’ per 100’ of pipe, so we then multiply .86 by 15 and will get an additional 12.9’ of TDH required. In most cases this amount will be insignificant, but, if we had 1500’ of 1 ¼” PVC then we would be looking at 6.39’ per 100 feet of pipe. This would then add an additional 95.85’ to our TDH calculation, in which case, may put us into a higher horsepower pump. This does not include any fittings such as tees, 90’s or 45’s etc.
Summary: It is very import that the values listed above are taken into consideration. If a pump is chosen where it runs to the extreme right or left of its curve, then eventually the pump motor will burn up due to overloading, in which case, the pump manufacturer may not honor the warranty due to the fact the pump was misapplied.
