Plumbing 



223 



Ordinary *T 

 nap, w/'fA 

 Bacft-venf. 



Fig. 40. Section showing three simple trap forms 



receptacle above it overflowed the bend, and was supposed to leave enough 

 water at all times to keep the seal closed. There are two things, however, 

 which menace this evaporation and syphonage. As every trap has more or less 

 connection with the air, unless it be used at frequent intervals the dangers 

 from evaporation are great. This is particularly noticeable of a house stand- 

 ing idle; its traps require immediate 

 attention when it is opened. A consid- 

 erable flow of water is apt to syphon 

 the trap, there being no interval of air 

 to check it. It is this fact which was 

 at the bottom of the vent law, which 

 was to the effect that: "All traps must 

 have a vent pipe of suitable size con- 

 nected at or near the crown of the trap, and extending either separately up to the 

 roof or connected with the soil-pipe line above the highest fixture." The back 

 vent is a remedy against this evil, but it is claimed by some that it involves other 

 evils of its own. How this back vent works may be seen in Fig. 38. As long 

 as the water maintains a level above the point T nt> gas can escape, and its 

 normal condition is that of V, which forms the water seal. Water flowing from 

 the basin raises the level of the seal above point S, causing it to overflow into the 

 waste pipe (D), and if the amount of water be considerable and there be no back 

 vent syphonage is liable to occur, as already stated. With the back vent (E), the 

 water receding from the crown of the trap (P) is followed down the waste by 

 the air, which breaks the syphon and leaves the seal at its normal height. 



Even this trap is not altogether sure, as the vent pipe may become choked with 

 grease or other matter so as to become useless. To obviate this an improved 

 "S" trap has been devised (see Fig. 40). The "jug trap" shown in the same 

 cut is an unvented lead contrivance, which is reasonably safe from the fact that 

 the bulk of the water held in it is hard to syphon. 



In order to avoid the extra expense of piping occasioned by the back-vent 

 system (see Fig. 42), the manufacturers have placed on the market several 

 so-called non-syphoning traps (see Fig. 41). A consists in the use of a glass globe 

 through which the workings of the trap 



Fig. 41. Types of the non-syphoning trap 



may be seen. B has a glass bottom, 

 and the rubber ball, while driven down- 

 ward by the outflow of water, resumes 

 its position as this ceases and closes 

 the mouth of the pipe. This is sup- 

 posed to break the syphonic action and 

 also to prevent the backing up of water 

 from the waste pipe. C and D have 

 extending "lips" which are supposed to cut the syphonic flow. All traps should 

 have a clean out (generally located in the bottom) so that solid matter may be 

 removed when necessary. It is well to inspect these frequently. 



The grease trap is a special form of trap used for kitchen sinks, which pre- 

 vents the hot grease from getting into the main and in hardening choke the passage- 



