Building Design and Construction Handbook - Merritt - Ventech!

WATER-SUPPLY, SPRINKLER, AND WASTEWATER SYSTEMS 14.19
leg to overflow and create a discharge siphon; and the blowout (Fig. 14.3e), used
with a flushometer valve, which projects a strong jet into the up leg to produce the
discharge. Blowout-type water closets are generally reserved for use where clogs
due to solids in the bowl are common, such as in penal institutions, stadiums, or
arenas. Because of the large amount of water consumed during the flush of a blowout
type of water closet, these types of fixtures are not used to the extent they once
were. Siphon-jet type water closets are the most common type of water closets
specified.
As part of the Energy Policy Act of 1992, all water closets manufactured after
January 1, 1994, for use in the United States were required to have a maximum
water use of 1.6 gallons per flush. Blowout water closets were required to have a
maximum water use of 3.5 gallons per flush and urinals were required to have a
maximum water use of 1.0 gallon per flush.
Air Gaps. These should be provided to prevent backflow of wastewater into the
water supply (Art 14.6.4). At plumbing fixtures, an air gap must be provided between
the fixture water-supply outlet and the flood-level rim of the receptacle.
Building codes usually require a minimum gap of 1 to 2 in for outlets not affected
by a nearby wall and from 1 1 ⁄2 to 3 in for outlets close to a wall. Table 14.3 lists
minimum air gaps usually used.
In addition to the usual drain at the lowest point, receptacles generally are provided
with a drain at the flood-level rim to prevent water from overflowing. The
overflow should discharge into the wastewater system on the fixture side of the
trap.
Floor and Equipment Drains. Floor drains should be installed at all areas where
the possibility of water spillage occurs. Common areas that are provided with floor
drains include restrooms, mechanical rooms, kitchens, and shower and locker
rooms. Equipment that requires piped discharge from drains or relief devices, such
as boilers, require recessed-type drains of adequate size, preferably with a funnel
receptor. Large commercial kitchens often require deep, receptor floor sinks to
receive indirect wastes from kitchen equipment.
14.8 WATER DEMAND AND FIXTURE UNITS
For each fixture in a building, a maximum requirement for water flow, gal/min,
can be estimated. Table 14.1 indicates the minimum flow rate and pressure required
by code. The maximum flow may be considerably larger. Branch pipes to each
fixture should be sized to accommodate the maximum flow and minimum pressure
the fixture will require. Mains serving these branches, however, need not be sized
to handle the sum of the maximum flows for all branches served. It is generally
unlikely that all fixtures would be supplying maximum flow simultaneously or even
that all the fixtures would be operating at the same time. Consequently, the diameters
of the mains need be sized only for the probable maximum water demand.
In practice, the probable flow is estimated by weighting the maximum flow in
accordance with the probability of fixtures being in use. The estimate is based on
the concept of fixture units.
Fixture unit is the average discharge, during use, of an arbitrarily selected fixture,
such as a lavatory or water closet. Once this value is established, the discharge
rates of other types of fixtures are stated in terms of the basic fixture. For example,