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Higher velocities, or greater drop per foot,
will increase the carrying capacity of a
drain. When designing fixture branches, keep
in mind that a slope/drop of more than
1/4 inch per foot may cause siphonage of the trap
seal.
Sizing Building Drains
The building drain in a sanitary system
must be of sufficient size to carry off all
the water and waste materials that may be dis-
charged into it at any one time. The minimum
allowable size is 3 inches for cast-iron pipe,
but sound practice prescribes a 4-inch pipe,
and most plumbing codes or ordinances require
4-inch pipe as a minimum. Increasing the size
beyond that computed as required (the minimum
of 3 inches still applies) does not increase
the efficiency of the drain. The passage of
liquid and solid waste through a horizontal
pipe creates a natural scouring action, which
is partially lost when the size of the drain
is increased above the necessary size. The
flow in too large a pipe is shallow and
slow, and solids tend to settle to the bottom.
The solids may accumulate to such an extent
that they cause stoppages in the line. The
optimum size of pipe should flow half full
under normal use. This will create an effi-
cient natural scouring action and still allow
capacity for peak loads.
The standard method used in determining
the size of a building drain is the Unit
System. Drainage fixture unit system values
for standard plumbing fixtures have been
established and some of the most common are
shown in table 7-2. Use the trap size listing
at the bottom of table 7-2 for estimating
drainage fixture unit (d. f u.) values for fixtures
that are not listed.
To select the correct size of pipe for a
horizontal sanitary drainage system, you must
first calculate the total volume of liquid
waste, expressed in drainage fixture units, that
the system will be subjected to. Assume, for
example, that a plumbing installation is to
consist of 20 water closets, valve-operated;
22 lavatories with 1 1/4-inch traps; 15 shower
heads in group showers; 20 wall urinals;
2 service sinks with standard traps; and 4
floor drains (2-inch). The total discharge,
Table 7-2.—Drainage Fixture Unit Values for Various
Plumbing Fixtures
7-4
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