Quick Answer
To determine the size of a first flush diverter for your tank, you'll need to know the storage capacity of your tank and the average rainfall in your area. A general rule of thumb is to size the diverter to handle 1-2 inches of rainfall per hour. You can calculate the required size based on your specific needs.
Calculating Tank Size
When selecting a first flush diverter, it’s essential to consider the storage capacity of your tank. As a general guideline, a 2-inch rainfall event should fill the tank to no more than 20-30% capacity. For example, if your tank has a 1,000-gallon capacity, the first flush diverter should be sized to handle a flow rate of 2-3 gallons per minute (GPM) for 2-3 minutes to flush out the initial sediment-laden water.
Understanding Rainfall Intensity
Rainfall intensity is measured in inches per hour (in/hr), and it varies depending on your location. To determine the required size of the first flush diverter, you can use a rainfall intensity chart or consult with a local water harvesting expert. As a general rule, a 2-inch rainfall event is considered moderate, while a 4-inch event is considered heavy.
Diverter Sizing Formulas
To calculate the required size of the first flush diverter, you can use the following formula: Diverter size (inches) = Tank capacity (gallons) x Rainfall intensity (in/hr) / 8.33 gallons per minute (GPM). For example, if your tank has a 1,000-gallon capacity and the rainfall intensity is 2 in/hr, the required diverter size would be: 1,000 gallons x 2 in/hr / 8.33 GPM = 240 inches. To convert this to a more practical measurement, divide by 12: 240 inches / 12 = 20 inches.
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