Pump motor life can be measured in cycles-starts and stops. A tank should be large enough to prevent a pump from overheating due to rapid cycling.
Selecting a tank suitable for a given pump application is accomplished by knowing the minimum run time, pump GPM and system operating pressure.
The Tank Sizing For Pump Cycle Control Chart example uses a 1 HP pump with an output of 15 GPM. From the Recommended Minimum Runtime data a 1 HP pump should run a minimum of two minutes between start and stop cycles. To run a 15 GPM pump for two minutes, 30 gallons of water must be pumped and stored (15 GPM X 2 = 30 gallons). This storage volume is referred to as drawdown, and is the usable water between pump cycles.
Next, it is necessary to determine what total tank volume (water and air) is necessary to store 30 gallons. This number will change depending on pump cut-in and cut-out pressures. As these numbers increase, the storage capacity of the tank decreases.
The Acceptance Factors Chart shows that a 30/50 PSI pressure setting will yield a 31% drawdown, where a 40/60 PSI setting allows only 27%.
For our example we'll use a 40/60 setting. Dividing the 30 gallons of stored water by the .27 Acceptance factor results in a total tank volume of 111 gallons necessary to properly protect this particular pump. Select the tank size closest to this figure.
TIMES