A trap is a device or fitting used to allow the free passage through it of liquids and solids, and still prevent the passage of air or gas in either direction. There are two kinds of traps used on plumbing fixtures known as syphon traps and anti-syphon traps. The simplest trap is the syphon trap - a horizontal pipe bent as shown in

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Fig. 14.

Fig. 14. This forms a pocket which will retain enough liquid to prevent air or gas from passing. The dip or loop is called the seal, and should never be less than one and one-half inches. This type of trap is what is known as a running-trap. This is not a good trap to use, and it is only capable of withstanding a very low back pressure.

The trap most generally used is what is known as the S trap, as shown in Fig 15. When this trap is subjected to a back-pressure, the water backs up into the vertical pipe, and naturally will withstand a greater pressure than the running-trap type - about twice as much.

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Fig. 15.

The trap shown in Fig 16 is what is known as a P trap, and in Fig 17 as three-quarter S trap, and has the same resisting power as the S trap.

A trap may lose its seal either by evaporation, self-syphonage or by suction. There is no danger of a trap losing its seal in an occupied house from evaporation, as it would take a number of week's time, under ordinary conditions, to evaporate enough water to destroy the seal.

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Fig. 16.

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Fig. 17.

A trap can be syphoned when connected to an unvented stack, and then only when the waste pipe from the trap to the stack extends below the dip, so as to form the long leg of the syphon as in Fig. 18.

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Fig. 18.

When two fixtures are installed one above the other, with nnvented traps and empty into one stack, the lower trap can be syphoned by aspiration. The water emptying into the stack at the higher point in passing to the trap inlet of the lower fixture, creates a partial vacuum which sucks the water out of the trap at the lower point. To prevent this, what is known as back-venting is resorted to, back-venting not only protects the trap against syphonage, but relieves the seal from back-pressure, by equalizing the pressure on both sides of the seal. All revent pipes must be connected to vent pipes at such a point that the vent opening will be above the level of the water in the trap.

In Fig. 19 two basins are shown connected to soil pipe with S traps and back - vented into the air-vent pipe, both connecting into the attic into an increaser, which projects through the roof. This drawing is given to illustrate the proper back-venting to prevent syphonage of basin traps, and when it is necessary to run separate stacks for wash basins, such as are sometimes installed in bedrooms, the main waste stack must be two inches in diameter and the vent pipe one and one-half inches, either cast iron or galvanized wrought iron.

Non-syphon traps are those in which the seal cannot be broken under any reasonable conditions. Some water can be syphoned from the best of non-syphon traps made, but not enough to destroy their seal. The commonest non-syphoning trap is known as a drum trap, which is four inches in diameter and ten inches deep. Sufficient water always remains in this trap to maintain its seal, even when subjected to the severest of tests.

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Fig. 19.

Fig. 20 shows a trap, which is the type generally used to trap the bathtub. This trap is provided with a brass trap-screw top for clean-out purposes, made gas and water tight against a rubber gasket. A trap of this kind would not be suitable for a lavatory, its principal fault being that owing to the enlarged body they are not self-cleaning, affording a lodging place for the depositing of sediment.

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Fig. 20.

The non-syphon trap to be used is one in which the action of the water is rotary, as it thoroughly scours the trap and keeps it clean, such as is shown in Fig. 21. This trap depends upon an inner partition to effect this rotary movement, and is so constructed that its seal cannot be broken by syphonic action and is permitted by health and sanitary departments, where it is impossible to run a separate vent pipe to the roof.

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Fig. 21.

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Fig. 23.

One of the oldest traps is the Cudell trap, as shown in Fig. 22. The rubber ball being of slightly greater specific gravity than water rests on the seat and forms a seal when the water is not flowing through the trap. This ball prevents the seal of the trap being forced by back-pressure, and acts as a check against back flow of sewerage should drain stop up, and provides a seal if water is evaporated.

Fig. 23 shows the old Bower trap. The water seal is maintained by the inlet leg, extending down into the body below the outlet. The bottom of this trap is glass, brass or lead, whichever is desired, and can be unscrewed from trap and thoroughly cleaned.

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Fig. 22.