Quick Answer
To calculate the drawdown capacity of a bladder tank, divide the total volume of the tank (in gallons or liters) by the desired duration of discharge (in hours) and the system flow rate (in gallons per hour or liters per hour). This will give you the maximum drawdown in inches of water. Consider the tank's height and diameter to ensure sufficient volume.
Understanding System Requirements
To calculate the drawdown capacity of a bladder tank, you need to know the total volume of the tank, system flow rate, and desired duration of discharge. The total volume of the tank is typically provided by the manufacturer and can be used directly in calculations. The system flow rate is determined by the pump’s capacity and is usually measured in gallons per hour (GPH) or liters per hour (LPH).
Calculating Drawdown Capacity
To calculate the maximum drawdown capacity, divide the total tank volume by the desired duration of discharge and system flow rate. For example, if you have a 500-gallon tank, a system flow rate of 20 GPH, and a desired discharge duration of 8 hours, the calculation would be: 500 gallons / (20 GPH * 8 hours) = 3.125 gallons per hour drawdown. This represents the amount of water that can be drawn from the tank without exceeding the maximum recommended drawdown capacity.
Considering Tank Geometry
When calculating the drawdown capacity, it’s essential to consider the tank’s geometry, specifically its height and diameter. A taller tank with a smaller diameter may have a lower drawdown capacity than a shorter tank with a larger diameter, even if the total volume is the same. This is because the available volume for drawdown decreases with depth. To ensure sufficient volume, consider the tank’s geometry and adjust the calculation accordingly.
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