Sanitary sewer systems should be sized and laid out to permit use of the smallestdiameter pipes capable of rapidly carrying away the wastewater from fixtures without clogging the pipes, without creating annoying noises, and without producing excessive pressure fluctuations at points where fixture drains connect to soil or waste stacks. Such pressure changes may siphon off the liquid seals in traps and force sewer gases back through the fixtures into the building. Positive or negative air pressure at the trap seal of a fixture should never be permitted to exceed 1 in of water.
Flow in Stacks. The drainage system is considered a nonpressure system. The pipes generally do not flow full. The discharge from a fixture drain is introduced to a stack through a stack fitting, which may be a long-turn tee-wye or a short-turn or sanitary tee. The fitting gives the flow a downward, vertical component. As the water accelerates downward under the action of gravity, it soon forms a sheet around the stack wall. If no flows enter the stack at lower levels to disrupt the sheet, it will remain unchanged in thickness and will flow at a terminal velocity,
limited by friction, to the bottom of the stack. A core of air at the center of the stack is dragged downward with the wastewater by friction. This air should be supplied from outdoors through a vent through roof (Fig. 14.9), to prevent creation of a suction that would empty trap seals.
When the sheet of wastewater reaches the bottom of the stack, a bend turns the flow 90 into the building drain. Within a short distance, the wastewater drops from the upper part of the drain and flows along the lower part of the drain.
Slope of Horizontal Drainage Pipes. Plumbing codes generally require that horizontal pipes have a uniform slope sufficient to ensure a flow with a minimum velocity of 2 ft / s. The objective is to maintain a scouring action to prevent fouling of the pipes. Codes therefore often specify a minimum slope of 1⁄4 in / ft for horizontal piping 31⁄2 in in diameter or less and 1⁄8 or 1⁄4 in / ft for larger pipes.
Because flow velocity increases with slope, greater slopes increase pipe-carrying capacity. In branch pipes, however, high velocities can cause siphonage of trap seals. Therefore, use of larger-size pipes is preferable to steeper slopes for attaining required capacity of branch pipes.
See also Art. 14.20.