By Ken Cerpovicz

Summary: The evolution of water tanks over the years has improved how they’re used in the field. With that has come difficulty for some on exactly what tank is needed for a particular application. But by using a simple scientific principle and proper sizing information beforehand, the installation of water tanks doesn’t have to be a perplexing issue.

Since its 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. In this article, you’ll get 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 following examples is the fact it’s 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 a 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:

Pump systems
It’s 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. 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 (hp), pump output in gallons per minute (gpm) and pressure switch setting in pounds per square inch (psi)—cut-in and cut-out. The below procedure will guide you through a sizing scenario using a 1-hp pump. The pump will be operating at 15 gpm with a pressure switch setting of 40 psi cut-in and 60 psi cut-out. The acceptance factors have already been calculated in Chart 1. The figures are based on a tank precharge H (initial air charge) set at 2 psi below pump cut-in pressure.

According to the “Recommended Minimum Runtime” notation, a 1-hp pump should operate for a period of at least two minutes between motor starts. To run a 15-gpm pump for two minutes, 30 gallons of storage is necessary. The acceptance factor indicates that 27 percent of a given tank’s volume will be usable water between 40 psi and 60 psi. Dividing 30 by an acceptance factor of 0.27 results in a tank with a volume of 111 gallons.

Volume assist
Often, a plumbing system’s flow requirements aren’t satisfied for a number of reasons. Insufficient line size, unforeseen fixture additions, excessive peak demand, and various types of line restriction all affect water flow capability and/or requirements. If a given water line or pump is incapable of supplying the necessary volume a system needs, pressure will suffer under flow conditions. A pre-pressurized tank can be used to store water under pressure to assist in meeting the aforementioned demands. Let’s take a look at a common problem involving insufficient line size and a bank of flush valves: Four 1.6-gallon per flush (gpf) flushometer-type valves are installed on a ¾-inch water line with a static pressure of 50 psi. In this example, the possibility exists for all four to be operated simultaneously. Assuming the valves need 30 psi to operate properly, what size tank is necessary to satisfy this peak demand?

As shown, the tank should be located centrally in reference to the fixtures. This will help prevent flow imbalance due to uneven pressure drop. It’s necessary to set the tank air charge to the minimum pressure required by the fixtures—in this case, 30 psi. A check valve will also need to be installed prior to the bank of fixtures to hold the pressurized water in the tank.

Reverse osmosis
Pressurized storage is necessary in reverse osmosis (RO) systems due to the fact this type of filtration yields filtered water at a very slow rate. Using a basic RO setup for reference, we’ll explore the sizing principles involved. Because of the minimal pressure requirement at the RO faucet, it’s possible to take advantage of a low tank precharge pressure to obtain a large storage of pressurized water. In this example, utilizing a 14-gallon tank, we’ll assume a 60-psi feed pressure from the house, and an RO system that closes feed flow when the storage tank pressure is two-thirds that of the feed line. Using the charts provided, it’s possible to determine the volume of usable water in the tank when the feed shuts off.

It should be noted that tank precharge affects the RO faucet outlet pressure. Although this requirement is relatively low, it’s important to take into consideration any pressure drops between the tank and faucet. The addition of an in-line post-filter combined with the slight elevation increase will reduce available pressure at the faucet.

Conclusion
As the above three sizing scenarios have shown, choosing a pre-pressurized tank for a job need not be guesswork. Provided the right information is available and applied correctly, one equation will cover any sizing necessary. Knowing which data to gather and how it relates to Boyle’s Law will allow you to determine the correct tank size for any application.