How to Size a Pressure Tank
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Since it's inception in 1964, the pre-pressurized potable water tank has assumed various designs and has proven useful in a wide variety of open-system applications. Originally used in well installations, this technology gained widespread acceptance and was soon applied in a variety of applications requiring pressurized water storage. With this flexibility, however, can come confusion as to installation scenarios and related sizing. Below, we'll take a look at proper use and related sizing techniques in such applications as pump cycle control, volume maintenance, and water treatment systems.
Common to the below examples is the fact that it is necessary to determine both the amount of tank storage needed and the pressures necessary to obtain that storage. Simply stated: All sizings use the same principle with different values. Referred to as Boyle's Law of Ideal Gases, this principle will be used to determine how much water is stored in tank at a given pressure. This multiplier will be referred to as the Acceptance Factor. Keep this equation in mind as a foundation to size any pre-pressurized tank:
It is common knowledge in the water systems industry that cycling a pump motor rapidly will often lead to failure. To prevent this, most pump manufacturers normally recommend a minimum pump runtime of 2 minutes. In order to achieve the runtime, the pre-pressurized tank must store a given volume of water between pump cycles. Doing so requires the pump to run for a specific period to fill the tank. In general, larger pumps require longer intervals between start cycles. When sizing a pre-pressurized tank for the above purpose, the following information should be known: Pump horsepower, pump output in gallons per minute (GPM), and pressure switch setting in psi (cut-in & cut-out). The below procedure will guide you through a sizing scenario using a 1hp pump. The pump will be operating at 15 GPM with a pressure switch setting of 40psi cut-in and 60psi cut-out. The Acceptance Factors have already been calculated in the chart provided. The figures are based on a tank precharge (initial air charge) set 2psi below pump cut-in pressure.
According to the "Recommended Minimum Runtime" notation, a 1hp pump should operate for a period of at least 2 minutes between motor starts. In order to run a 15GPM pump for 2 minutes, 30 gallons of storage is necessary. The Acceptance Factor indicates that 27% of a given tank's volume will be usable water between 40 and 60psi. Dividing 30 by an acceptance factor of .27 results in a tank with a volume of 111 gallons.