Quick Answer
The drawdown of a pressure tank is calculated by determining the amount of water volume the tank loses under a given flow rate. This can be estimated using the tank's capacity, the flow rate, and the duration of the drawdown. A common method is to use the formula: drawdown (gallons) = (flow rate (gallons per minute) * time (minutes)) / tank capacity (gallons).
Understanding Tank Capacity
To calculate the drawdown, first determine your tank’s capacity. Typical residential tanks range from 20 to 100 gallons. For example, a 50-gallon tank has a capacity of 50 gallons. Keep in mind that larger tanks can provide longer drawdown periods but are often more expensive and take up more space.
Calculating Flow Rate and Time
Next, calculate the flow rate of your well pump in gallons per minute (gpm). A common flow rate for residential wells is between 5-20 gpm. The time of the drawdown can be measured in minutes. For example, if you’re running a shower for 5 minutes at a flow rate of 7 gpm, the time is 5 minutes and the flow rate is 7 gpm.
Applying the Formula
Now, apply the formula to calculate the drawdown: drawdown (gallons) = (flow rate (gallons per minute) * time (minutes)) / tank capacity (gallons). In our example: drawdown = (7 gpm * 5 minutes) / 50 gallons = 0.7 gallons. This means the tank will lose approximately 0.7 gallons during the 5-minute shower.
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