Workingknowledge

WATER TOWERS

Tall Task

Drive past a water tower, and it appears to be a silent, passive giant. But it is the central player in a high-pressure balancing act.

Most municipalities obtain their water from a reservoir or well, purify it at a treatment plant and send it to a pump house that fills one or more elevated tanks. The pumps alone are strong enough to push water throughout a town's network of pipes, but the system's pres-sure—at your sink—would fluctuate as usage rose and fell and could drop too low to reach spigots during high demand. "A water tower acts like a capacitor. It maintains constant pressure on the lines and provides backup supply when demand exceeds pump output," explains Malcolm Jackson, a general manager for Utility Service Company, Inc., in Perry, Ga., which provides tank services nationwide.

A tank is sized to hold about a one-day supply plus reserves, notes Kevin Gallagher, vice president of sales at Caldwell Tanks in Louisville, Ky. Pumps may operate around the clock, filling a tank overnight when demand is low. When most of the population jumps into the shower in the morning, the tank is drawn down to augment what the pumps provide.

Water towers are prominent on flat land, but tanks are everywhere, resting (and often hidden) on high ground in hilly locales and perched on rooftops in cities. A tower or hilltop tank is typically situated 100 feet above the highest users. Each one of those feet creates 0.433 pound per square inch of pressure—43.3 psi for a difference of 100 feet. Pressure in street pipes in a valley far below could rise much higher; towns insert pressure-reduction valves on lines where pressure reaches 80 psi or more, which could damage a building's plumbing seals and valves, notes Anthony O'Malley, principal of the Larkin Group, water engineering consultants in Kansas City, Mo.

The friction of water running inside pipes affects the balancing act, too, reducing system pressure by 3 to 5 psi or more. Friction rises as the square of speed, so water during high demand that flows twice as fast as water during low demand creates four times the friction loss. "Old pipes, especially if they have a lot of sediment or encrustation," O'Malley says, "can cause some real friction problems." — Mark Fischetti

SHAPES deceive. Virtually all tanks are cylindrical to equally distribute water weight among support columns. Municipalities may add facades for aesthetics or advertising. Clockwise from top left: Forcalquier, France; New York City; Kuwait; Honolulu, Hawaii.

□ HEAVY LOAD: A common tank may hold one million gallons of water, which weighs 8.3 million pounds. Large concrete footings must bear this burden as well as transverse loads from 100-mile-per-hour winds against the tank (the standard storm rating). The earth at a suitable site must therefore be very dense, if not rock. On plains where the ground is largely glacial till, such as in North Dakota, steel pilings to anchor footings may have to be driven 100 feet into the ground.

□ STERILIZE: Tanks are emptied annually and disinfected with a spray of chlorine and possibly ammonia. Sulfur dioxide may be added to the runoff to neutralize an environmental threat from chlorine. Most towns of any size have more than one tank for ample supply and pressure, which allows for maintenance rotation.

□ NO PRESSURE: Firefighters who open hydrants in the spring are not relieving excess pressure, as often rumored. They are making sure hydrant valves work and are flushing lines of possible pressure-reducing sediment buildup.

□ CELL SALES: Many municipalities are leasing tank-top space to telecom providers, because cell phone transceivers provide excellent service when they hover 100 to 150 feet above callers—the typical height of water towers.

Exhaust fan port

Ve_nt Maintenance manhole -Roof

Telemetry

TANK is typically a steel cylinder joined to a roof and bottom. A vent allows air to enter and escape as the tank empties or fills, so a vacuum or high pressure does not distort the roof. An exhaust fan expels fumes during regular sterilization and interior painting.

PUMP forces water up a riser, and water exits through the same T-joint to town pipes. New designs include a higher, angled fill pipe to better mix contents, reducing the possibility of stagnant tank water. Telemetry monitors tank water level and pressure in pipes and tells pumps to turn on or off.

Overflow pipe -

SITE is dictated by topography; a town's tanks must be above the highest user to create sufficient pressure. For flat land, a tower provides elevation. In hilly areas, a tank rests on high ground. In cities, rooftop tanks augment basement pumps.

FLAT LAND

Exhaust fan port

Overflow pipe -

SITE is dictated by topography; a town's tanks must be above the highest user to create sufficient pressure. For flat land, a tower provides elevation. In hilly areas, a tank rests on high ground. In cities, rooftop tanks augment basement pumps.

FLAT LAND

From water treatment plant

T-valve

Have a topic idea? Send it to [email protected]

From water treatment plant

T-valve

Have a topic idea? Send it to [email protected]

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