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Answering Your Water Pump Questions

with RPS Engineer Mike

How do I calculate what size pump I need?

How do I calculate what size pump I need?

 

Figuring out what pump to put down your well can be a confusing task for a landowner. With the many different horsepower options, and different gallons per minute ratings, sizing a pump can seem a little daunting! Let's dive in, give our techs a call if you need more help at any point 855-560-5670. 

 

Step 1: Find Total Dynamic Head 

The first thing you will want to try and figure out is your total dynamic head, or TDH. The TDH is the measurement of how many vertical feet of lift your pump will be experiencing in total. This is a combination of the lift from the static water level to the top of your well casing, and the lift that comes after the well casing. For example, if you have a 300’ deep well and your water level is 200’ from the surface, the lift out of your well is 200’. Then let's say you are pumping to a storage tank that is 30’ higher in elevation compared to the well, this would put your TDH at 230’. 

 

Another factor that contributes to TDH is pressure, so if you are pumping to a pressurized tank (such as for a household) you will have to add more data to determine your TDH. Take the previous example, and say instead of a storage tank we are pumping to a 60psi pressure tank. Every 1 psi is equal to 2.31’ of vertical lift. If you are pumping into a 60psi pressure tank, then you will have to take 60 and multiply it by 2.31, which will give you 138.6’ of extra vertical lift that your pump will have to be able to handle in order to pump into a pressure tank. With the previous example, we have 200’ of lift from the static water level, 30’ of lift to get to the pressure tank, and an extra 138’ of lift added by the pressure tank itself. That would put the TDH of our example at 368’. 

 

Step 2: Look At the Pump Curves

Now you have the TDH of your specific situation. The next step will be to look at the pump curves of different pumps and see if you can find one that will work in your situation as well as give you the gallons per minute that you want. Below is an example of our pump curves for our RPS 25GPM (gallons per minute) pumps.

 

Here you can see the performances of our different pumps graphed as lines on this chart. The way to read the chart is to find your TDH on the vertical axis of the chart and follow it to the right until you run into one of the pump lines. If you go down to the horizontal axis of the chart you will see the gallons per minute that that pump is capable of doing at your TDH. Continuing with our example, with a TDH of 368’ we can follow the graph and see that with a 2HP pump with a 25GPM pump end, you would get around 19-20GPM into the pressure tank. If you keep following the line from your TDH you will see that you could also use a 3HP pump and get a higher GPM, around 28GPM with the 3HP pump. 

 

Pumps from different companies can have different performances, as well as pumps with different pump ends on the same HP motor. A 2HP 25GPM pump will have a much different TDH limit than a 2HP 10GPM pump will. If you are unsure if the pump you are looking at will work in your well, be sure to check the pump curves. Or talk to one of our knowledgeable and friendly RPS water pump techs!

 

Previous article How do I choose HP for my pump?
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