The primary function of the pressure tank in a pumped water system is to protect the pump motor. This is accomplished by storing water under pressure so the pump does not have to come on every time there is a small, intermittent demand for water. The amount of usable water stored in a pressure tank is called the drawdown, and is the amount of water that can be drawn from the tank between the time the pressure switch cuts out, turning off the pump, and it cuts back in, turning on the pump. We showed how to calculate drawdown in the March 2008 issue of this publication. Figure 1 depicts what goes on inside a captive-air pressure tank as the water is drawn out and the pressure drops.
Pressure tanks are sized according to the flow capacity of the pump, with enough drawdown to allow the pump to be off for a minimum of 1 minute between run cycles, to let the motor cool, as recommended by the motor manufacturers. A 10-gpm pump would require a pressure tank with 10 gallons of drawdown, and so on.
There are two types of pressure tanks – captive-air tanks (also called pre-charged, diaphragm or bladder tanks) and conventional tanks (also known as hydro-pneumatic, galvanized, ASME and epoxy-lined tanks). Using the term “hydro-pneumatic” to describe a conventional tank is a bit of a misnomer. All pressure tanks used in the ground water industry are hydro-pneumatic, meaning they contain water (hydro) and air (pneumatic). In this article, we will use the terms “captive-air” and “conventional” to identify the two different types of tanks.
The largest commonly available captive-air tank has a total capacity of 119 gallons. The size of a pre-charged captive-air tank is limited by federal highway regulations, which require special permits to transport pre-charged pressurized tanks having a capacity 120 gallons or more.
The drawdown or usable water available from a 119-gallon tank, operating in conjunction with a 30/50 pressure switch, is 36.8 gallons, according to Boyle’s Law. Following the 1-minute-runtime-between-cycles rule mentioned above, a 36-gpm pump would be the largest pump you could use with such a tank. What should you do, then, if you have a larger pump? There are several options.
Conventional tanks are available in virtually any size, and are a viable option. They are, however, expensive, and require the use of an air-charging system to replace the air that is absorbed into the water. For this reason, the use of multiple captive-air tanks is gaining in popularity. It is not uncommon to see six or eight captive-air tanks lined up in row to provide the necessary drawdown for a large pump.
For instance, six 119-gallon tanks would provide enough drawdown for a 200-gpm pump (6 X 36.8 = 220.8 gallons). The size and number of tanks you end up with will depend on economics and available real estate for your tanks. It might turn out that eight of the more common and less expensive 85-gallon tanks with a drawdown of 26.4 gallons each would be cheaper than six 119-gallon tanks, but be sure to figure in the cost of the manifold system in your total.
Multi-tank SystemsIt is very important that a multi-tank system be plumbed properly for it to work properly. There are two common mistakes made on these systems – locating the pressure switch in the wrong place, and under-sizing the interconnecting piping. The two basic rules to follow: 1) make sure all the tanks see the same pressure while the pump is running, and 2) make sure the pressure switch sees the same pressure as the tanks. To comply with the first rule, use a large enough manifold pipe to assure the flow velocity does not exceed 6 feet per second. This will keep the pressure drop to a minimum from one end to the other, and will help assure uniform pressure in the entire tank system.
To comply with the second rule, place the pressure switch as close to the center of the tanks as possible. Figures 2 and 3 illustrate proper pressure-switch placement.
When properly designed and installed, the use of multiple residential-type pressure tanks in a larger, high-capacity system offers a viable alternative to a single, large, conventional pressure tank. The next time you need more tank capacity, give this alternative some consideration.