Building Design and Construction Handbook - Merritt - Ventech!

WATER-SUPPLY, SPRINKLER, AND WASTEWATER SYSTEMS 14.9
FIGURE 14.1 Up-feed water-distribution system for a two-story apartment building. (Reprinted
with permission from F. S. Merritt, ‘‘Building Engineering and Systems Design,’’ Van Nostrand
Reinhold Company, New York.)
14.6.3 Down-Feed Water Distribution
For buildings more than 8 to 10 stories high, designers have the option to pump
water to one or more elevated storage tanks, from which pipes convey the water
downward to plumbing fixtures and water heaters. Water in the lower portion of an
elevated tank often is reserved for fire-fighting purposes (Fig. 14.2). Generally, also,
the tank is partitioned to provide independent, side-by-side chambers, each with
identical piping and controls. During hours of low demand, a chamber can be
emptied, cleaned, and repaired, if necessary, while the other chamber supplies water
as needed. Float-operated electric switches in the chambers control the pumps supplying
water to the tank. When the water level in the tank falls below a specific
elevation, a switch starts a pump, and when the water level becomes sufficiently
high, the switch stops the pump.
Usually, at least two pumps are installed to supply each tank. One pump is used
for normal operation. The other is a standby, for use if the first pump is inoperative.
For fire-fighting purposes, a pump must be of adequate size to fill the tank at the
rate of the design fire flow.
When a pump operates to supply a tank, it may draw so much water from a
public main that the pressure in the main is considerably reduced. To avoid such a
condition, water often is stored in a suction tank at the bottom of the building for
use by the pumps. The tank is refilled automatically from the public main. Because
refilling can take place even when the pumps are not operating, water can be drawn
from the public main without much pressure drop.
Figure 14.2 is a simplified schematic diagram of a down-feed distribution system
of a type that might be used for buildings up to 20 stories high.