A trap is a siphon placed in the drain to catch some of the water discharged from a fixture and form with it a barrier for the entrance of air from the drains into the house. The water forms what is called the trap "seal." In addition to the water seal some form of mechanical closure has sometimes been added in the form of a valve, gate or ball, under the supposition that a water seal alone could not afford sufficient security. But the mechanical devices have now been found to create serious obstructions to the flow of the water, and to become, in time, corroded or in other ways more or less defective. At the same time it has been clearly shown that a sound water seal can be attained which affords entire security, and alone, so that the complication of a mechanical seal becomes entirely superfluous.

The four principal enemies of the water seals of traps are siphonage (which is the suction acting on trap seals produced by a partial vacuum in the waste pipe behind a falling plug of water discharged from a fixture), evaporation, back pressure and clogging by sediment accumulation. These enemies of trap seals will be examined in full detail in subsequent chapters.

Under the supposition that all traps are liable to lose their water seal through siphonage, momentum and back pressure, or allow of the transmission of gas bubbles under back pressure unless protected by special ventilation, the "back vent" law was passed requiring a separate vent pipe to be carried from the outgo of every trap into a special ventilation duct, independent of the soil pipe ventilator, up as far as to a point above its connection with the highest fixture. At the time when this provision was framed no simple and reliable self-cleansing water trap was known which could resist the severest tests of siphonage, momentum and back pressure which might be encountered in plumbing.

Fig. 62. Old English D Trap, styled the  Goose Trap, probably in honor of the man who invented it.

Fig. 62. Old English D Trap, styled the "Goose" Trap, probably in honor of the man who invented it.

Had such a trap been known, as is now the case, the back vent law would obviously never have been made.

The common round or "pot" trap, though not self-cleansing, can nevertheless be made large enough to be practically proof against siphonage, and it may be periodically cleaned by hand if it is found to clog with sediment; but no trap, and least of all the S trap, can resist the destructive effect on its seal of the rapid evaporation produced by the ventilating current required by this law. The seal is destroyed by evaporation in a very short time, varying with the rapidity and dryness of the current and the volume of water in the trap.

Special trap venting is now considered undesirable for many other reasons which give rise to dangers much greater than that which it pretends to remove, the danger from evaporation, for instance, being, as houses are now plumbed, much greater than that from siphonage. Traps are left in disuse, and subject to the danger of loss of seal by evaporation much oftener than is generally supposed. Thus they are unused in city houses which are left unoccupied during summer; in country houses which are unoccupied during winter; in hotels and apartment houses during the quiet seasons, or at times when they are only partly filled; in private houses in the spare chambers reserved for visitors; in business offices between the expiration and renewal of their leases; in schoolhouses and all public and private office buildings at times of vacation; in houses or chambers closed on account of the absence of their owners for travel, sickness, death or any other cause; in case of drought, or "cutoff" of water supply for repairs of pipes, rebuilding or other cause; in extra fixtures in houses, and in other places, and at other times which will, upon reflection, occur to the reader. In a few days after a trap has thus been abandoned to the influence of the ventilating current, the time varying with the dryness and velocity of the current and with the volume and amount of exposed surface in the body of the trap, its seal will be destroyed by evaporation.

Corrosion of well-flushed waste pipes of moderate length by sewer air is never to be feared with ventilated soil pipes. It is believed that there is no authenticated case on record of such corrosion. Indeed, the induction through the branch pipes of soil pipe air is by no means an advantage when ample pure air from the room is available after every flushing from a properly constructed fixture. When the main soil pipe is properly ventilated the diffusion of gases, the absorptive power of water for gases, and the frequent water flow through branch pipes afford them sufficient protection when the pipes are in use. When the pipes are not in use the waste matters adhering to their inner surfaces dry up, and it is believed that what decomposition then takes place goes on so slowly that its corrosive effect on the pipe is practically inappreciable. We have shown that besides the constant draught on the water seal by evaporation, the vent pipe increases the unsecured area of the trap, and if any portion of a trap not directly scoured by water passing through it is liable to collect sediment, and ultimately clog up, then the mouth of the vent pipe is also subject to this danger. It thus, in a measure, defeats one of its own objects - i. e., to provide a safe trap which shall be self-cleansing and contain no chamber or corner which shall not receive the full scour of the water passing through this trap. Now, the ventilating openings of traps having been so often found clogged and even completely closed by sediment, we see that this precaution is no certain protection against siphonage and momentum.