Have
you ever experienced a "water failure"? That is, have you ever
turned on your faucet and found that no water came out of it?
If you get your water from a municipal water system, the
answer is probably "no."
We have power
failures all the time. Cable TV
goes out fairly frequently. Although less common, the phone
system goes down every so often, and it is now common to
get an "all circuits busy" message when making long-distance
calls. But the water in any city or suburb is always there.
Water pressure is very reliable.
A big reason for that level of reliability is the water
tower. You see water towers everywhere, especially if you
live in a flat area full of small towns. Each water system has
one or more towers. In this edition of HowStuffWorks,
we will look at how water towers work. The next time you drive
by a water tower, you will know exactly what it is doing!
Tower, Tank and Pump
Water tower in
Kill Devil Hills,
NC
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A
water tower is an incredibly simple device. Although water
towers come in all shapes and sizes, they all do the same
thing: A water tower is simply a large, elevated tank of
water. For example, take the water tower shown at the right.
This tower is located in Kill Devil Hills, near Kitty Hawk,
NC. It is about 165 feet (50 meters) tall and holds about 1
million gallons (nearly 4 million liters) of water.
Water towers are tall to provide pressure. Each foot
of height provides 0.43 PSI (pounds per square Inch) of
pressure. A typical municipal water supply runs at between 50
and 100 PSI (major appliances require at least 20 to 30 PSI).
The water tower must be tall enough to supply that level of
pressure to all of the houses and businesses in the area of
the tower. So water towers are typically located on high
ground, and they are tall enough to provide the necessary
pressure. In hilly regions, a tower can sometimes be replaced
by a simple tank located on the highest hill in the area.
A water tower's tank is normally quite large. A
normal in-ground swimming pool in someone's backyard might
hold something like 20,000 or 30,000 gallons (that's a lot of
water!), and a typical water tower might hold 50 times that
amount! Typically, a water tower's tank is sized to hold about
a day's worth of water for the community served by the tower.
If the pumps fail (for example, during a power failure), the
water tower holds enough water to keep things flowing for
about a day.
One
of the big advantages of a water tower is that it lets a
municipality size its pumps for average rather than
peak demand. That can save a community a lot of money.
Say that the water consumption for a pumping station
averages 500 gallons of water per minute (or 720,000 gallons
over the course of a day). There will be times during the day
when water consumption is much greater than 500 gallons per
minute. For example, in the morning, lots of people wake up at
about the same time (say 7:00 a.m.) to go to work. They go to the
bathroom, take a shower, brush their teeth, etc. Water
demand might peak at 2,000 gallons per minute at 7 a.m. --
there is a big cost difference between a 500-gallon-per-minute
pump and a 2,000-gallon-per-minute pump. Because of the water
tower, the municipality can purchase a 500-gallon-per-minute
pump and let the water tower handle the peak demand. At night,
when demand normally falls to practically zero, the pump can
make up the difference and refill the water tower.
In most towns, the water people drink comes from either a
well, a river or a reservoir (normally a local lake). The
water is treated in a water treatment plant to remove
sediment (by filtration and/or settling) and bacteria
(typically with ozone, ultraviolet light and chlorine). The
output from the water treatment plant is clear, germ-free
water. A high-lift pump pressurizes the water and sends it to
the water system's primary feeder pipes. The water
tower is attached to the primary feeders quite simply, as
shown in this diagram:
If the pump is producing more water than the water system
needs, the excess flows automatically into the tank. If
the community is demanding more water than the pump can
supply, then water flows out of the tank to meet the need.
Form and Function
Water towers come in all
shapes and sizes. Take, for example, this giant peach along
I-85 in Gaffney, South Carolina!
This water tower comes complete with leaf,
stem and that funny crease that peaches
have!
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Photo courtesy John C.
Wren
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In a city, tall buildings often need to solve their
own water pressure problems. Because the buildings are so
tall, they often exceed the height that the city's water
pressure can handle. Therefore, a tall building will have its
own pumps and its own water towers. In the following picture,
taken from the Empire State Building in New York City, there
are at least 30 small water towers visible on the tops of
these buildings!
Another interesting fact about water towers -- they can
affect your insurance rates! During a fire, the water
demand increases significantly and may greatly exceed the
capacity of the pumps at the water plant. A water tower
guarantees that there will be enough pressure to keep water
flowing through the fire hydrants. Fire insurance rates are
normally lower in a community in which the water system has
water towers.
The next time you are out driving around, especially if you
are driving through a series of small towns, take the time to
notice the water towers. Now that you know how they work, you
will be amazed by how many you see and by all the different
forms they take!
