What size submersible pump do I need?
There are always two questions we ask when someone says that they want us to size a submersible pump. We also ask the same questions, when they say their submersible pump doesn’t work. What GPM do you want? What is your TDH?
Sometimes the pump can’t do what you want it to do. That’s not the pump’s fault. You need to know what you want and then pick the pump based on what your needs are. GPM stands for Gallons Per Minute. This is how much flow you want from the pump. It is entirely contingent on what the well can provide. TDH stands for Total Dynamic Head. It takes the entire losses and pressure requirement for a job into consideration.
You will need a Friction Loss Chart to calculate the head loss in your piping system.
Add that to your Pressure Requirement in Feet. To figure this multiple the desired pressure by 2.31.
The last part of TDH is your Pumping Level this is a little trickier to figure. It is made up of three things.
Elevation is the height from the well outlet to the point of usage, in most homes, the pressure tank. There may be no elevation in your problem because like my own house you may go from a pitless adapter to the basement where the tank is kept. Or if you don’t have a basement you may come out of the top of the well and go right into the tank.
Standing Water Level or static water level as some call it is from the well outlet to the surface of the water when the pump is off.
Once you turn the pump on you must then take into consideration the Drawdown. This is usually determined by test pumping the well at the time it is drilled. This is measured from the standing water level to the working water level. Most wells are going to have drawdown. The amount of drawdown is dependent on the well recovery rate. If you have lots of water then your drawdown will be minimal. If you don’t have lots of water then you may experience quite a bit of drawdown. In some parts of North America the recovery rate of a well is only 1 gpm. Obviously, since most houses use more water than this, you could have a very big problem. Most Well Drillers will drill a 6” well and they will drill it deep. There is 1-½ gallons of water per foot of 6” well casing. Say you have a 500’ well and the pump is hanging at 450’ with a static water level of only 50’ that would mean that you have 400’ of water at 1-½ gallons of water per foot or 600 gallons of reserve water to draw from. Of course you will need a larger horsepower pump to get the water from the bottom of the well, so to protect the pump it would be advisable to put a flow restrictor in line say just above the pump to keep the pump in it’s flow range. As an example, a 10-gpm pump can pump up to 16-gpm. This is not good for the pump, however. To prevent this, use a flow restrictor, which will keep the flow at say 12-gpm at the most, and your pump will last much longer.
Add all of these together and you have your TDH. Once you have your TDH and GPM you may go to the pump curve or chart and choose the right pump for the job.