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 Systems
It 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.
ND
