Quick Answer
Drawdown volume is calculated as the volume of water in a tank that is discharged before the system can refill it. It's usually expressed in gallons or liters and depends on the tank size, pressure switch setting, and system demand. The formula involves the tank's effective size and the pressure switch's drop pressure setting.
Calculating Drawdown Volume for Gravity Fed Systems
To calculate drawdown volume for gravity fed systems, you’ll need to know the tank size in gallons (or liters), the pressure switch setting in pounds per square inch (PSI), and the system demand in gallons per minute (GPM). The formula is: drawdown volume = (tank size / 8.34 pounds per gallon) x (pressure switch setting / 2). For example, a 500-gallon tank with a 30-PSI pressure switch setting and a 10-GPM demand would have a drawdown volume of: (500 gallons / 8.34 pounds per gallon) x (30 PSI / 2) = 45 gallons.
Calculating Drawdown Volume for Pump Fed Systems
For pump fed systems, the calculation involves the tank size, system demand, and the pump’s flow rate. The formula is: drawdown volume = (tank size / system demand) x (pump flow rate / 2). For instance, a 1000-gallon tank with a 20-GPM demand and a 20-GPM pump flow rate would have a drawdown volume of: (1000 gallons / 20 GPM) x (20 GPM / 2) = 100 gallons.
Important Considerations
When calculating drawdown volume, it’s essential to consider the system’s pressure switch setting, as it directly affects the drawdown volume. A lower pressure switch setting will result in a higher drawdown volume. Additionally, the system’s demand and pump flow rate should be closely matched to avoid over-pressurizing the tank, which can lead to tank failure.
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