Doodlebug Pump -- FAQ

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Submersible Pumps

Gemeral Questions About Submersible Pumps

1. How do Submersible Pumps work?
   
2. Can a submersible pump be run without an air tank?
   

Submersible Pumps

1. How do Submersible Pumps work?

   A submersible pump is a specialized type of centrifugal pump. It is relatively small in diameter with several small impellers coupled to an electric motor. Since a submersible pump sets in the water to operate, as opposed to being on the surface and drawing the water up, it is very efficient and is not limited by water depth.

   The one limitation on this type of pump is the diameter of a well. This restricts the operation of a submersible pump, so the impellers have to be stacked in order to produce sufficient pressure. The greater the pressure, the more the number of pump stages. A typical standard 4" submersible pump can vary in length from 24" to 48" long and measures 3.9" in diameter. This allows it to fit inside a 4” well casing. The motor turns at 3600 rpm. The total number of impellers required in a particular application will depend on the depth of the water and the pressure required.

   How are water flow and pressure related?

   In a submersible pump water flow refers to the rate water is produced and is expressed in gallons per minute (GPM) or gallons per hour (GPH). Pressure is the force that is produced. This pressure is usually stated in pounds per square inch (PSI) or feet of head (FT/HD). It is sometimes referred to as LIFT (as in vertical lift).

   Two useful conversion factor are:

   • 1 PSI = 2.31 feet of head
   • .433PSI = 1 foot of head

   For example: If the water level in the well is 100 feet below the surface, the pump must produce 100 feet of head to allow the water to reach the surface. (100 X .433 PSI = 43.3 PSI).

   This means the pump will use 43.3 PSI just to get the water to the surface. Then depending on how much pressure is required and how far the water must travel once it reaches the surface, additional pressure will be required.

   In our example if we needed 50 PSI to supply water to a building, we would need a total of 43.3 + 50, for a total of 93.3 PSI. This does not take into effect any friction loss in the delivery pipe or any vertical rise above the surface to the building. This additional pressure would have to be added to arrive at a total pressure requirement.

   How To Size a Submersible Pump

   Every application is different. Pump size and horsepower will depend on how much water(flow) and how much pressure you need (at the surface).

   The following list of questions need to be considered in order to select the correct pump.

   1. What is the size of the well casing?
   This determines what size of submersible pump you can use. The most common size for small domestic wells is 4”, however it could be 6" or larger. If the well casing was 6", then up to a 6" diameter pump could be used. In this example we will use a 4” well.
   2. What is the “static Level” of the water in the well when there is no activity (no pumping). In other words, if you were to measure the water level from the surface, what would it be?

   3. What is the Well "Draw down"? In a well, the level of the water rises to a certain point, this is called "static level". When the well is being pumped and the water level drops to a certain point and holds steady this is referred to as "draw down". This “draw down” level is important.

      When a well driller finishes drilling a well, he will next “develop” it. This generally consists of pumping out the well for a certain amount of time, Usually, several hours. This serves two purposes. First it removes all of the suspended material usually found in a freshly drilled well. Secondly, it allows the driller to determine how many gallons per minuite the well will produce. At the end of the pumping period the water will have dropped to a certain level (called the "cone of depression"). With this information you can determine how many gallons the well can produce. This critical information will be contained on the drillers log.

      After the well drilling is completed, the driller is required to be submit a copy of the drilling log to the State where the driller is licensed. He is required to keep a copy of it and generally required to give a copy to the well owner.

      If you can't get a copy of the drilling log and the pumping capacity is unknown you have to guess. This is not a good option. if you guess wrong the pump might not produce enough enough water, or it might produce more flow than your well can sustain. You might ask a neighbor to see what their pumping level is. If your both in the same aquifer and the wells are similar in depth it might give you a place to start.

   4. How much flow do you need? This question can be answered two ways. First you are limited to the number of gallons the well will produce.

      Or, if the well does not produce enough, you might consider pumping into a storage tank on the surface.

      Depending on how much water you need this might be a 1000 gallon or larger tank. You would then utilize a second Jet Pump to supply your water needs to the house at a higher flow then your well might normally produce.

      An average two bath 3 or 4 bedroom home would generally not require more than 12 GPM at any given time. This does not take into account any outside watering needs. If you have livestock or landscape, the water needs might be greater. In this example we will assume 12 GPM is adequate.

   5. How Much Pressure Do You Need?

      Usually 50 PSI is enough for most homes. It's enough pressure to shower and run every water appliance in your home.

      If we agee on 50 PSI, we still have to consider two other items. First the distance of the pump to the surface and second any additional vertical rise or drop from the well to your house.

      Remembering our formula for converting PSI to Feet of Head: (50 X 2.31 = 117.81 Feet of Head).

      Lets also assume the pump set depth is 100 feet below the surface: 117.81 + 100 = 217 81 Feet of Head).

      If there is any elevation gain or loss this figure must also be added or substracted to arrive at our final figure. Lets further assume the house sits 40 feet higher than the surface of the well (217.81 + 50 = 267.81 Feet of Head).

      There is also the friction loss thru the pipe to be taken into account. When water moves through the pipe between the pump and the house a friction loss is created. The friction must be calculated. Charts are available to determine the loss in PSI per 100 feet of pipe. If the pipe run represented several hundard feet, this loss would have to be considered in the pump selected. Water should not flow faster than 5 feet per second. If you exceed this amount, "water hammer" can occur. As a rough guide 1" Sch 40 PVC Pipe will flow up to 12 GPM and 1-1/4" Sch 40 will flow up to 26 GPM without having this problem. Usually the size of the pipe used is the same as the discharge of the pump. 1-1/4" is a standard size for residential wells.

   In our example:

   Our example pump would have to supply 12 GPM flow at a minimum of 218 feet of head. If we add an additonal 10 PSI loss for pipe friction, the final requirement would be: 218 + 23 (10 X 2.31= 23) for a total of: 241 Feet of Head at 12 GPM. This would supply 50 PSI to our house.

   If this seems a bit complicated or confusing, don't worry about it. That's what we're here for. If you e-mail or call us we can help you select the correct pump for your needs. If nothing else, let us double check your calculations. That way your sure you've got the right pump.

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   6. Can a submersible pump be run without an air tank?

      The short answer is yes. The reason for an air tank (also called a captive air tank) is to provide a delay to allow the use of a pressure switch. You can either set up a direct switch to turn the pump on and off (make sure it is sized correctly) or the pump can be controlled by a float switch (for tank fill applications). A third option is a pump start relay used in conjunction with a irrigation controller to turn the pump on and off.

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