For years, the gravity tank has graced urban skylines,
housing water and harnessing an energy known as simple gravity. Prior to the proliferation of
fire pumps, the wooden rooftop water tanks were long considered the first line
of defense against high-rise building fires. They ensured both an adequate
quantity of water and adequate pressure.
There are about 200 gravity tanks still servicing Chicago buildings, and fire
sprinkler contractors can expect a call when there’s a problem. These are
typically freeze-ups in the 2" filler line or a circulating hot water line
that didn’t get drained. Or an ice plug may form in the service riser. Or a
rupture will occur between the roof and the underside of the tank if a heater
failed to run, and the result is a pretty big mess.
But all is repairable, and a simple service call is all it takes for the entire
system to be back up and running, and that is big news lately in Chicago. Last
July, their city council unanimously passed an ordinance to keep gravity-fed
wooden water tanks from being haphazardly torn down. The ordinance imposes a
90-day demolition delay on all gravity tanks, a time frame that the city can
extend at its discretion.
New York has made no such rescue efforts. The spokeswoman for the NYC Landmarks
Preservation Commission, Elisabeth DeBourbon, has stated: “we only designate
entire buildings as landmarks. We don’t designate features of buildings.”
For combination sprinkler/standpipe systems, water pressure (provided entirely
by gravity) is determined by the height of the tank above the particular floor
of sprinklers. If you need 15 psi at the highest fire sprinkler, then the
tank’s water level must be 35 feet above that sprinkler. The gravity tank is
vented and requires no amenities other than a level switch or float valve
(inside the tank), which sends a signal to a domestic pump in the basement to
lift water into the tank until it is filled.
The greater the pump capacity, the shorter the time needed to refill the pump.
A second float valve near the top of the tank serves as the domestic pump
shutoff. The tank also contains a roof hatch with access to an inside
maintenance ladder. Installation of new tanks must conform to requirements
outlined in NFPA 22, Standard for Water Tanks for Private Protection.
In addition to fire pumps, what has put an end to new installations of gravity
tanks are concerns regarding maintenance, structural integrity and the
reluctance of employers to have anyone working at high elevations. No one, it
seems, wants a new gravity tank. But while they appear to be artifacts of a
bygone era, those still in service provide an essential fire protection and
life-safety service. In New York City, assuredly the last vestige for gravity
tanks, it is estimated that wooden tanks are present in 90% of structures more
than six stories high.But they won’t last forever. The redwood tanks,
particularly those harvested of wood from China’s east coast, have a projected
50-year life expectancy. Many of those were erected in the 1950s and will be
the next to go. And they will be missed.