THERE are, as I have already indicated, two schools or systems of plumbing which may be characterized as the "complex" system shown in Fig. 32, and the "simple system" of Fig. 33, the first involving two or three times as much elaboration and expense as the second, and rendering it almost impossible for the prospective purchaser of a house to determine whether it is safely plumbed or not.
We have already explained some of the reasons why the simpler system is the best. A public sewer becomes very well ventilated and practically safe when its ventilation is effected by making every house drain and soil pipe a ventilating flue. The sewer then has, in cities, in addition to the usual public ventilating openings on the streets and elsewhere, also special 4-inch suction tubes every ten or fifteen feet throughout its entire length, assuming the houses on both sides of the street to average between 20 and 30 feet in width each.
The temperature in the houses is at all times either warmer, as in winter, or colder, as in hot days in summer, than the air of the street and of the sewer, and thus creates a constant and thorough sewer ventilation. We have shown somewhat exhaustively that if disease germs are brought into our houses in air currents, they come in the air outside of the sewers, either above or below ground, and not in the sewer air itself, and we know that whatever putrescible matter, excepting street washings, is to be found in the sewers, comes from the house drains themselves. Assuming, then, that the houses average say 26 feet wide and 50 feet high, the number of running feet of soil and drain pipe in each would average at best not less than 100 feet, and the interior surface of this house drain pipe being of iron and not so well scoured as the glazed inner surfaces of the sewer, would therefore contain more decomposing matter than that part of the sewer in the street which serves each separate house. It would be absurd, therefore, to insert a disconnecting trap and double or treble the amount of piping in a house for the mere purpose of excluding this extra drain pipe air, even if it were not demonstrated that this very complication increased rather than diminished the chances of its entrance. More than half of this complication is due to the absolutely and at all times worse than useless so-called "back vent" system, a system founded on misconception and perpetuated by ignorance, prejudice and humbug. Indeed, so far as the science of plumbing is concerned, this system is already a back number, for the leading authorities in plumbing and sanitary engineering have placed themselves squarely in opposition to it.
Of course it will be useless to study the various plumbing fixtures of a house and the proper methods of connecting them up with the piping until we know what that piping is to be, and accordingly our first duty is to tackle this "back vent" monster and destroy it, for being a thirsty creature, it will, if left on guard over us, be certain in time to lick the water seal out of our traps, or else, by gorging itself with grease, to lose all consciousness and abandon the trap seal altogether to its enemies.
Like the cholera germ, this most pernicious infliction cannot stand light. The science of hydraulics and pneumatics is fatal to it and shall form for it our club of extermination. The Hydraulics and Pneumatics of Plumbing.
Fig. 238. Diagram to illustrate the phenomenon of siphonage.
Siphonage. Our next drawing illustrates the principle of siphonage. A trap consists of a U-shaped bend in a pipe forming an inverted siphon, as shown on the left-hand side of Fig. 238. By filling this trap with water and turning it upside down we see that there is no greater weight of water in one leg than in the other, and therefore there is no tendency on the part of the water to run from one end of the tube or siphon more than from the other so long as the two legs are of the same length. They both pull down from the top with equal force and tend to form a vacuum in the bend. But if we lengthen one leg so that the water in it becomes heavier than that in the other, it will run out, while atmospheric pressure will force the water in the short leg up to the top and out of the tube, because though the atmospheric pressure at the bottom of the long tube is very slightly greater than that at the bottom of the short tube, the air column being a few inches longer, this extra pressure of air counts as nothing against the weight of the same number of inches of water column. Now, if the shorter leg of the siphon be dipped in a vessel of water, as shown in the illustration, the atmospheric pressure which before acted on the bottom of the water in the short leg is transferred to the surface of the water in the vessel and will act in emptying it down to the bottom of the short leg and illustrate the well known action of siphoning. The water in the vessel and in the short arm of the siphon constitutes a trap. The long arm is the outlet arm of the trap, and when water from the basin or any other fixture to which the trap is attached flows through the trap and down the long arm, it sets up this siphoning action, which will continue until the trap seal is reduced to a point slightly below the bottom of the short arm of the siphon, or so-called "upcast limb," of the trap, thereby breaking the seal. Frequently a sufficient amount of water trickles down from the fixture and sides of the pipe above the trap after the siphoning action to partially restore the seal. This direct action of the water of a fixture in breaking its own trap seal by siphoning is called "self-siphonage."
A more common form of siphonage, however, is illustrated in Fig. 239, where the seal of the trap is broken by the discharge of some fixture other than the one to which it is attached, and usually in a story above it. Here the discharge of a water closet in the upper story destroyed the trap seal below; the falling column of water from the upper closet rarified the air in the soil pipe behind it as it went. To fill this partial vacuum following the water plug air tended to press into the soil pipe through every opening. The friction of the rough sides of a tall stack of soil pipe, even though it be open at the roof, will often cause more resistance to air in its attempt to fill this partial vacuum than will the inertia of the water in any fixture trap below. In the case shown by the picture the outer air found a much easier access to the interior of the soil pipe through the trap seals than by any other way,
Fig:. 239. Trap Siphoning.* and so it broke these seals and thus opened a free entrance to soil pipe air into the house. The seal of the upper closet would be emptied by the same action.
*From "Dangers to Health. A Pictorial Guide to Domestic Sanitary Defects," by T. Pridgin Teale. M. A. Pub. by J. J. Churchill, London.
On the left hand side we see the action of siphonage on a lavatory trap caused by the discharge of a bath tub above.
Fig. 240. Diagram to Illustrate Back Venting. (From Bayles.)
Three Methods of Protecting Trap Seals. Three methods have been employed with a view to preventing the destruction of the seal by siphonage. The most natural method and the one which has been used now for about a quarter of a century is to ventilate each trap by connecting it with a special ventilating pipe constructed for, the purpose.
Mr. James C. Bayles describes this method in 1878 in his "House Drainage and Water Service," as one would describe a method at that time not long established, using a cut which I have reproduced here (Fig. 240). It is, however, now generally admitted that the vent pipe shown by Bayles in the cut could not afford even a temporary protection on account of its small size.
Incidentally it may be said that Mr. Bayles rightly objects to the use of a main house trap, saying that in his judgment such a trap does vastly more harm than good. Among other reasons he gives for this is that it hinders the ventilation of the sewer. "When the pressure upon the air confined in the sewer," he says, "is increased from any cause, it should have an outflow through every house drain. When from any cause a partial vacuum is created in the sewer, every house drain should be an inlet for air. In other words, we should allow the sewers to breathe through the main waste pipe of every house, besides giving them as many breathing holes in addition as can be provided." The waste piping of houses can now be done without difficulty so as to secure permanently tight work.
The second method of guarding against the loss of seal by siphonage is to make the body of the trap so large that a sufficient quantity of water will always adhere to its sides after siphoning to restore a seal. This is the principle of the pot or cesspool trap.
The third method is to construct the trap of such a form as to render it both antisiphonic and self cleaning at the same time.
The first method adds enormously to the cost and complication of the work and gives rise to greater dangers than those it was designed to cure. Nevertheless it has become popular with many, and is responsible for the so-called "trap-vent" law, once excusable because nothing better was for some time known, but now worse than absolutely indefensible in the light of our present knowledge, as inviting the entrance into our homes of sewer-gas, now that simple methods are known for keeping it out.
In regard to the practical working of trap back venting two things have been made clear. First, that it is not always efficient in preventing siphonage even when new, and very frequently fails when old. And, second, that it is always more or less active in destroying the trap seal through evaporation.
The second method is both inexpensive and simple, and is much more efficient and reliable in resisting siphoning action than the first. It has, however, the serious disadvantage of involving the use of cesspools or centres of putrefactive decomposition in the house, and brings, in the aggregate, a vast amount of pollution into the public sewers tending to frustrate our best efforts in the direction of their complete purification. They are also liable to be converted by grease accumulation into ordinary S or siphoning traps and thus entirely lose their original power of protection.
The third method is the simplest and least expensive of all, and has demonstrated itself to be perfectly reliable and satisfactory. Nothing but ignorance and selfish private interest has stood in the way of its exclusive adoption.
Let us now examine these three methods carefully in detail since the question is not only one of the most important and interesting ones in the whole domain of sanitary plumbing, but its investigation will throw light upon every other part of our subject.