The direct action of the water of a fixture in breaking its own trap seal by siphonage, is called self-siphonage. A more common form of trap siphonage in defective work, is where two or more fixtures connect with the same waste pipe, as shown in Fig. 199. In such cases, the seal of the lower fixture is more apt to be broken by the dis charge of the upper. The falling column of water leaves behind it a partial vacuum in the soil pipe; and the outer air tends to rush into the pipe through the way of least resistance, which is often through the trap seal of the fixture below The friction of the rough sides of a tall soil-pipe, even though it be open at the roof, opposed to the flow of air through it, will sometimes offer more resistance than the trap seals of the fixtures, with the result that the seals are broken, and gases from the drain are free to enter the building.
Kinds of Traps. The kinds of fixture traps are innumerable. They can be divided into two general classes—those that seal with water only, and those that have a mechanical seal as an adjunct to that of the water. These may be again divided into plain and anti-siphoning classes.
The trap having no concealed partitions and with all its walls exposed to view, is best. If the water leaks through the wall, its defectiveness is evident, and the annoyance from the leak suggests repairing.
Of the simple water-seal fixture traps, the open-walled drawn lead is used for ordinary work. It can be had with equal-length arms or with extended inlet or outlet, so as to reach from fixture to floor or wall without a piece of intermediate pipe. The form shown by full lines in Fig. 200 represents a full "S" pattern. When the ends are bent as per dotted lines A and C, the trap is called a running trap; when the ends are at D and C, it is said to be a hal f-S or P trap; when the ends are set as at D and E, it is called a i-S trap. F is a clean-out screw for emptying and cleansing. The distance represented by X should, in a trap for ordinary purposes, be to 2 inches, according to size. Frequently this distance, which constitutes the water-lock, is much reduced; and sometimes the trap is unsealed by the plumber stretching its bends in order to reach some faulty roughing-in.
In buildings where the plumbing may be left unused for weeks from time to time, as is likely in rented houses, deep-seal traps, or those with mechanical seals also, should be used. This point is not so important in de tached houses or those rented to one family only at a time, since, when a family moves out, there is no one to suffer. But in flat buildings, where some of the flats may be vacant for a time sufficient for an ordinary seal to be broken while other families are living in the house, deep-seal traps are more essential.
Fig. 201 shows what is termed a bag trap, made to bring the inlet and outlet in the same vertical line. These traps are inter changeable with any others with straight-line outlet—for instance, as shown in Fig. 204.
An open-wall trap partly cast and partly tubing, generally made of brass, is shown in Fig. 202, the vent connection to wall being at A. This form of trap generally has a swivel-joint at B, which is below the water line, so that the body may be swiveled to meet roughing-in openings in any direction within two diam eters of the line of fixture outlet. The bag form shown is most convenient for D-shape or standing waste bowls which present the outlet comparatively near the wall. The regular "S" of this type suits bowls with center outlet, and will reach a wider range of variation in roughing-in.
Fig. 203 shows a common lead drum or pot trap, most convenient to the plumber. It is furnished without openings, and the plumber makes bends, and wipes-in his inlet and outlet at points in the circumference most con venient to reach the fixture opening. A is the screw-top clean-out; and B, the wrench-face for turning it. The trap is furnished, when desired, with nickel-plated brass flanged cover, as shown at C, to screw on at the floor level. F is ordinarily the outlet, the inlet being wiped-in near the bottom to give it the water-lock. This is not proper, however, as it puts the sewer air against the clean-out cover, which might leak gases into the building without betraying any evidence of its defectiveness by water leakage. To be strictly correct, F should be the inlet, and the outlet, in the shape of an off set, or that of an inverted P-trap with out the trap-screw, should be wiped in near the bottom in a way to retain the proper seal and thus bring the sewer air against the water-seal instead of the clean-out cover.
Traps that retain their seals by means of interior weirs are of doubtful character, even at their best; none but well-tested cast-brass traps of such a pattern should ever be installed. Fig. 204 is a section of a flask or Atlas trap, with vent, usually made of cast brass and de pending upon two interior weirs to form the seal, one retaining the water, and the other dipping into the water to prevent sewer air from get ting into the house through the fix ture. If the water weir of such a trap becomes defective, there is no evidence except odors by which the occupants may discover it. If the dipping weir is defective the value of the water seal is nil. In either ease the trap is no barrier to the admission of drain air to the house.