The lavatory trap shown in Fig. 206, has an interior weir as shown at A; but the wall is doubled in such a way as to betray defectiveness by water leakage. It is made of cast metal, and is furnished with either glass or metal dome. The strong point claimed for this trap is the cleansing effect obtained by the flange extension of the exit, as shown at A, deflecting some of the water, which, together with the swirling effect produced by the tangential inlet, makes the trap self-cleansing.

Fig. 205.. Bath Trap with Submerged Inlet..

Fig. 206. Lavatory Trap with Interior.

Flanged Weir. Weir is Double-Walled to Betray Leakage.

Of the traps having a mechanical seal supplementing the water-lock, Fig. 207 is a specific type. The mechanical valve D is a rubber ball, lighter than an equal _bulk of water, playing in the cup C. It acts by flotation, and presses up against the inlet A with a force equal to the dif-erence in weight of the ball and the water it displaces. The body is generally made of lead; and the cup of glass, with screw-joint and gasket at F. This trap is proof against backwater; and, in case the waste line becomes choked below, will prevent a fixture from flooding even when others are discharged at a higher level. It has, however, several faults that counterbalance its merits. The inlet is open to the same criticisms that an interior wall of any other trap would be; the annular space at R accumulates filth; and the mechanical seal is worthless when most needed - that is, in the absence of the water-seal.

Another mechanical seal trap, shown in Fig. 208, is the exact opposite of the previous example. The ball sinks by gravity, and effects a mechanical seal even when the water seal is absent. This trap is not so easily siphoned as a plain trap. It has a clean-out screw, and can be had with vent opening.. Air from the sewer side acts against the clean-out cap through which access is had to the ball, and there are interior walls to become defective with little chance of discovery in practice.

Fig. 207. Trap with Mechanical Seal Acting by Flotation..

Fig. 208. Trap with Mechanical Seal Acting by Gravity..

A combined mechanical and water-seal trap is shown in Fig. 209, in which D is a hollow, flexible ball inclosing a metal ball D', thus giving a resilient seating surface that finds its place by gravity in water. The arrangement is proof against back-water, and the mechanical seal is positive without the aid of water. A represents the basin; B, the basin coupling; C, the valve seat; F, a glass cylinder body; and GG, a clamp with thumb-screw G1, for clamping the cylinder body in place. This trap holds a large amount of water, and is not likely to become unsealed from lack of use, as part of the seal is protected by the ball, and should the water evaporate, the mechanical seal is still effective. There are no interior walls through which the trap could lose its seal without betraying the fact by leakage. Generally speaking, mechanical seals in fixture traps cannot be depended upon. Anti-siphoning traps are a blessing in instances where pipe ventilation is difficult. It would be better to have none of them, however, than to attempt to supplant pipe ventilation by their use to any great extent.

It would be impossible here to consider the whole list of traps individually in an adequate manner. What has been said should be enough to enable one by careful study to decide each case intelligently upon its merits. Many special traps are deserving of more favor than is generally shown them. It is the fear of seeming to indorse the horde of cheap competitive articles that causes many to ignore alike the good and bad. This fear is well grounded. The wolves will creep in if the door is opened at all.

Loss of Traps Seals. Traps may lose their seals in six ways - by waving out, by capillary action, by leakage, by evaporation, by siphon-age, and - if the use of an unusual term be permissible - by impella-tion. The first, with its cause, has been described (see page 163). The last, like waving out, is caused by air-pressure, but on the house side instead of the sewer side of the trap. It occurs most frequently in intercepting traps where the fresh-air inlet has been connected too far from the trap, thus allowing heavy discharges of water and storm floods to compress the air between the fresh-air inlet and the trap. This action is of little consequence when so caused, as there is abundance of water to re-establish the seal. Its mention, however, suggests that a portion of the pipe is left unventilated by connecting the inlet too far from the trap. This error is usually made with good intention, because the foul-air outlet and fresh-air inlet are often made in the trap proper and are therefore too close together to pipe to the surface directly. There is a singular instance on record, of a trap having its seal broken by pressure on the house side - not from pressure of air in the pipe, but of that in the room into which the trap seal opened. This was a water-closet in a tight, unventilated compartment in a private house. Odors were often present which no one could account for. The job was new and first-class. The house was well built - too well. After many others had failed to diagnose the trouble, a plumber with some philosophy in his make-up examined the job. He stood in the hall, and slammed the closet-room door. It failed to latch, the room being so tight that the air-pressure kept it from seating on the rabbet of the frame. The door, of course, was instantly thrown partly open again by expansion of the air, and the plumber caught a glimpse of the water in the closet-bowl bobbing up and down. By repeating the experiment and measuring the depth of water between times, he discovered that, as suspected, the sudden closing of the door of the small, tight room was thrusting the water down in the bowl and causing enough to flow over into the soil pipe to break the seal. The trouble was remedied by cutting 1/2 inch off the door at the bottom.

Fig. 209. Trap with Combined Water-Seal and Gravity-Acting Mechanical Seal..

Fig. 210. Foreign Matter (Lint. Strings..

etc.) Causing Capillary Loss of Trap Seal.

Evaporation has been described elsewhere. Leakage of seals has been mentioned in conjunction with types of fixture traps. Siphon-age of traps is simple. The conditions necessary to start a common siphon being established in a waste pipe, the seal will be drawn out. The discharge of water from a fixture will siphon its trap (self-siphonage), if no provision against siphonage is made. The crown vent pipe, as described, breaks the siphon in a trap when its fixture is discharging, and prevents other fixtures from siphoning or waving out the seal. Capillary loss of seal occurs through hair, lint, and strings hanging over the weir of the trap. Dipping into the seal on one side, and ending in the pipe on the other, water will climb through or between such matter by capillary force, and will drip by gravity into the pipe. This is indicated by the tangled lines at R, Fig. 210, representing capillary material hanging over the outlet neck D of the trap. The trap indicated is for a lavatory with horn overflow bowl, V being the overflow connection, 1 the waste, B the crown vent, and 0 the outlet. Traps are sometimes locally vented at V.

Materials forming a porous coating on the inner walls of the trap through chemical action or otherwise, are now and then responsible for the loss of water-seal by action of a capillary nature. The shape of a trap may favor the accumulation of matter that will lead to capillary loss of seal. This is one reason why the plain, open-wall, cylindrical-bore traps are best. It is found that no matter how the trap is shaped, its cavity is, as a rule, not used except at the points which conform to the simplest, most direct course - as before said. Other shapes, then, present needless fouling surface and space for accumulation of matter that interferes with the proper service of the trap. Departure from the shape mentioned is necessary to secure an unvented trap that cannot be siphoned. Any trap that must necessarily be connected so as to put the air of the sewer side against the gasket of the clean-out cap, should not be used.

A difficulty common to venting the general run of plumbing fixtures, is presented by the fact that to crown-vent the trap prohibits sufficient immediate vertical rise of the crown vent to get above the fixture overflow-level, without making an offset in the vent, which, in case of stoppage of the waste, favors choking of the vent in the offset by matter floated into it as a consequence of the stoppage. A plan providing for flushing of the vent at will, is shown in Fig. 211, a sanitary tee branch being placed in the vent above the level of the sink or lavatory back, as shown at A, and closed by nickel-plated trap-screw cover B at the face of the finished wall. In this way, by removing cover B, a wire can be run through to the trap-screw clean-out, and the offset portion thus cleaned; and, if necessary, it can be flushed by injecting water at B with a hose or funnel.

Fig. 211. Installation in which Provision is Made for Flushing and Cleaning Offset Vent Whenever Neces-. sary..