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.
The building drain in a sanitary system must be of sufficient size to carry off all the water and waste materials that may be discharged 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 efficient 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