Siphon Traps

Traps are fittings used to prevent the passage of air or gas through a pipe without materially affecting the flow of sewage. To successfully perform the functions for which they are intended, traps must be so constructed or protected by vent pipes that they cannot be siphoned or have their seals forced by back pressure under any conditions that obtain in a well constructed drainage system; furthermore, they should be self-scouring at each flush of the fixture to which they are connected, and should contain sufficient depth of water to withstand loss by evaporation for a long period of time without breaking the seal.

There Are Two Types Of Fixture Traps Commonly Used

siphon traps and non-siphon traps. The simplest type of trap is a siphon trap (Fig. 26). It consists of a downward dip in a pipe that fills with water and thus prevents the passage of air. As this water seals the pipe to the passage of air or gases, it is referred to as the seal of the trap. The seal of this form of trap is formed by the column of water a, and is seldom over 1 3/4 inches in depth. It is not a good trap for the reason that it is only capable of withstanding a back pressure of .063 pound per square inch. If greater pressure is applied the water will back up sufficiently in the horizontal inlet to allow drain air to blow through the seal as indicated. The height b to which water raises in the inlet end of the trap determines the amount of back pressure required to force the seal. The form of siphon trap most generally used is shown in Pig. 27. When subjected to back pressure the water in this trap backs up in the vertical inlet leg and reaches a height b of 3 1/2 inches before drain air can blow through. This water column will withstand a back pressure of .126 pound per square inch, or double the back pressure a running trap will stand. Siphonage of Traps - The seal of siphon traps may be siphoned in either of two ways - first, by self-siphonage, and, second, by aspiration caused by the discharge of other fixtures.