PRIVATE WATER SYSTEMS - Pumping Installation - Pump Capacity

Pump Capacity

The pump capacity required in a home or on a farm is determined based on the calculated peak demand for the farm or home. If the pump does not have sufficient capacity, the pressure of water delivered to every outlet drops until the pump can meet the de- mand. Therefore, it is very important to be able to predict present and future peak demands to insure satisfactory water service for many years to come.

For predicting the capacity needed, studies by the USDA show there are basically two types of water usage intermittent and sustained uses

Three tables that are soon to follow show the most common uses in and about the home and farm along with the amount of demand each use puts on the pump. Column one of the tables, labeled peak demand allowance (PDA), includes demands for both intermittent and sustained uses. These figures are lower than the figures in column two, labeled individual fixture flowrates (FFR),because studies have shown that many uses do not occur simultaneously, so the actual pump demand is not the total maximum flow rate of each fixture. In fact, these USDA studies show that home uses show demands of 1/4 of the fixture flow rates, and sustained demands develop a total demand of 1/2 fixture flow rates unless the demand is sustained for more than four hours.

To determine the required pump capacity list all of the present and foreseeable home; lawn, garden, and miscellaneous; and farm watering uses and the demand for each from the tables that appear upon selecting one of the highlighted words.

Next, for the uses in each category having the highest PDA, replace the demand allowance figure with that use's fixture flowrate. For example, if all three areas are considered, the 10 gpm for floor cleaning would replace the 5 pgm in determining the farm demand.

Then determine what capacity pump is needed by totaling the demand allowances for whatever uses have been listed in all three categories. Be sure the fixture flow rate has been inserted for the use requiring the greatest fixture flow (this is to insure that the pump will supply ample water for the greatest single fixture demand).

If the pump is to provide some fire protection to the home or farm, at a pump capacity of at least 10 gpm will be necessary. A pump of this capacity along with a 1/4 inch fire nozzle will produce a stream of water that will reach 30 feet high. While this will not be effective against a large or very hot fire, it is effective in putting out small fires or hosing down buildings near a fire. A pump with a capacity of 25 gpm and a 3/8 inch or 2 1/4 inch fire nozzles will produce a stream of water 40 feet high or 65 feet horizontally.

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