Pressure Tank Selection - by Nelsen Corporation

In order for the system to operate satisfactorily, the proper compressed air-to-water ratio must be maintained. Since air is soluble in water, the air charge in a plain tank must be constantly replenished by some type of air volume control as in “A”, Fig. 2, or the compressed air must be separated from the water as shown in “B”, “C”, “D”. The Wafer, “B”, floats up and down on the water and, thus reduces air-to-water contact area, while the Diaphragm, “C”, and Bladder, “D”, hermetically seal the air from the water.  Diaphragm and Bladder type tanks are pre-charged with air at the factory. An air charging valve is provided in these types of tanks to allow the installer to change the pre-charge pressure. Pre-charging a tank increases the amount of water that can be withdrawn between operating cycles of the pump. To select the proper pressure tank, follow the instructions below.

1. Determine the peak demand for a seven minute period, which is the average time of higher water usage by such devices as automatic washers and showers. The Peak Demand is found by reading down the column in Table 2 under the number of bathrooms. For example, the Peak Demand for a home with 1½ bathrooms is 70 gallons.
2. Follow the same column down to the “Minimum sized pump requirement”, which is 10GPM, or 600 GPH for the 1½ bathroom example.
3. Check to make sure the well and the pump selected have sufficient capacity to meet or exceed the Peak Demand rate. If they do not, go to Step 4. If they do have sufficient capacity, select the tank from Table 3. Continuing with our 10 GPM example and assuming a pressure switch setting of 30 – 50 lbs per in2, it is found that an 80 gallon Plain steel tank would be required, compared to only a 45 gallon size in the Wafer type or 40 gallon for the bladder or diaphragm types. For farms or other installations requiring water in addition to household use, the extra gallons needed during the seven minute Peak Demand period must be estimated using Table 2. The additional gallons are then added to the amount found in Table 2, above. For instance, if the 1½ bathroom house, in the example above, had as estimated increase usage of 35 gallons during the peak demand period, the total water needed during the period would be 35+70 or 105 gallons. Divide 105 by 7 to get the use rate of 15 gallons per minute. Enter Table 3, for 30 – 50 lbs per in2 pressure, to find the tank size required, which is 150 gallon Plain steel, 80 gallon Wafer, or 75 gallon Bladder or Diaphragm types. If a standard tank is not available in the indicated size use the nest larger standard size.
4. If the well and pump do not have the capacity to meet the Peak Demand, an extra large pressure tank, or a 2-pump system may be necessary. In a 2-pump system, the well pump pumps into a storage tank at a rate which does not exceed the well capacity. This pump is controlled by a switch actuated by water level in the storage tank. A second pump, of the centrifugal “booster” type, pumps from the storage tank to the pressure tank. It is actuated by a pressure switch. Consult your representative on specific installations requiring supplemental water storage.