Traps are made in many forms, none of which combines every desirable feature. A trap with vertical drop at the inlet is considered best for the main intercepting trap, as it allows the incoming water to break up the scum and floating matter so that it will be carried out promptly by the flow. This form also presents a difficult place for sewer rats to climb, and is therefore favored for that reason also.

In regular fixture traps, open-neck bends, and the least surface possible, are favored. The Y and 1/8-bend connections in one fitting, and other fittings combining the virtues of the open bends of long-radius fittings, are used merely because they offer little chance of stoppage; but traps should have every part exposed to view in order to betray leakage. Tide-water traps are usually nothing more than simple, large, swinging cheek-valves. Some intercepting traps are provided with a swinging check. The tide-water feature is necessary only when high water or tides are likely to raise the water into which the sewer discharges so as to flood the cellar through fixture openings.

Siphonage. Traps introduce into plumbing the element of siphonage. This may be normal and desirable, as in the case of closets which discharge their contents by siphonic action, but unde-sired siphonage in fixture traps, and the means of preventing it, are prime factors in every plumber's work.

Ordinary siphonage can best be illustrated by a few simple with Legs of diagrams showing the principles involved. In Fig. 193 is shown a U-tube with legs of equal length, filled with water. If we invert the tube, as shown in Fig. 194, the water will not run out, because the legs are of equal length, and contain equal weights of water, which will pull downward from the top with the same force, tending to form a vacuum at A. Cohesion of the particles of water, together with equal atmospheric support of the water at the open ends of the tube, prevents any appreciable void space when the U is of short length. If one of the legs is lengthened, as in Fig. 195, so that the column of wafer is heavier on one side than on the other, the water will run out. The atmospheric pressure being practically equal on both legs, the greater weight of the water in the long end, through cohesion, pulls the water in the shorter tube up over the bend, in much the same way as an unbalanced chain would run over a pulley. The columns of water in the tube in this case may be likened to a piece of rope hanging over a pulley; when equal lengths hang on each side it will remain stationary; but if one end is longer and therefore heavier than the other, the whole rope will be drawn over by the longer and heavier portion.

Fie:. 193. U Tube

Fie:. 193. U-Tube.

Equal Length.

Fig. 194. U Tube Inverted.

Fig. 194. U-Tube Inverted..

Fig. 195. Inverted U Tube with Legs of Unequal Length.

Fig. 195. Inverted U-Tube with Legs of Unequal Length..

If the short leg of Fig. 195 be dipped in a vessel of water, as shown in Fig. 196, we then have the conditions necessary to form a common siphon. The atmospheric pressure, which before acted on the water at the bottom of the short leg of the tube, now becomes operative on the surface of the water in the vessel, and the flow through the tube will continue until the water-level in the vessel falls slightly below the end of the tube, admitting air and breaking the siphonic action. Gravity acts proportionally on the water of both legs of the U during siphonage, and the point of tension is therefore at the highest point of the bend.

If the bend should be pierced at the top, air-pressure would be established at both ends of each leg, and gravity would instantly empty the short leg into the vessel. It is in this manner that a crown vent to a common fixture trap breaks the flow and throws enough water back into the body of the trap to preserve the water-seal. Fig. 197 shows the principle of Fig. 196 applied to the trap of a plumbing fixture. If the bowl is well filled with water, so that when the stopper is removed from the bottom, the waste pipe for some distance below the trap will be filled with a solid column of water, siphonic action like that just described will take place and the trap will be drained. A sufficient amount of water runs down from the fixture and sides of the pipe above the trap to partially provide for the seal, its full restoration being assured when a crown vent is used, by water being thrown back from the short leg of the siphon (center leg of the trap) as shown in Fig. 198.

Fig. 196. A Common Siphon.

Fig. 196. A Common Siphon..

Fig. 197 Trap Fulfilling Siphon age Conditions.

Fig. 197 Trap Fulfilling Siphon-age Conditions..

Fig. 198. Siphoning of Trap Broken by Crown Vent.

Fig. 198. Siphoning of Trap Broken by Crown Vent..

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 discharge 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.