General recommendations are:
All tanks be certified under Water Systems Council Standards for size and pressure.
All tanks be coated or made of material to resist corrosion.
Hydropneumatic tanks have a working pressure rating in excess of the maximum pressure, but not less than 75 psi.
All tanks be constructed of materials and/or coatings which are non-toxic.
All tanks be provided with a means of draining.
Atmospheric storage tanks be covered.
Tank size should have the capacity of draw down (gallons between cycles of high and low shut-off at desired pressure setting) at twice the GPM of pump discharge rate.
Useable tank volume is the amount of water that can be drawn off between high and low pressure switch setting. This volume varies with basic tank size, switch settings and degree of supercharge.
In limited capacity source systems, the storage capacity must be larger in order to have a satisfactory system.
For the two-pump system discussed earlier, the open storage tank should have a capacity which is at least equal to one day’s average consumption requirements. Larger storage volume will be necessary to meet special demands, such as fire fighting.
Storage capacity can be increased by using extra hydropneumatic tanks to store capacity needed to satisfy system demands.
For low yielding wells, the well casing itself may be used to store water. This storage is that water in the well casing between static level and maximum pumping level, which is about one gallon per foot for 6-inch diameter wells.
Each of these methods is an attempt to make best use of water from a limited capacity source. The desired rate and duration of use will dictate the tank size required. The system capacity is the important consideration.
For surface water sources, the hydropneumatic tank size would be determined in the same way as for an adequate source. Water treatment exposure time may require larger tanks.
The reservoirs or cisterns, which receive surface runoff should be large enough to supply the average daily demand over a drought period of maximum length.