SELF - SIPHONAGE takes place when the waste water flows through the trap "full bore" from the fixtures. As usually constructed, wash basins have outlets far too small in proportion to the size of the waste pipe and trap. The strainer cuts off a considerable portion of the passageway, and the hairs, lint and other sediment which soon invariably collect on the strainer cut off another large portion, leaving an outlet sometimes not more than equivalent to a -inch or 5/8-inch pipe. The waste pipe and ordinary S trap are usually 1-inch or 1-inch in diameter, so that the stream of water admitted to flush and scour them is reduced to less than a quarter or a sixth of their capacity, and we wonder why our trap and waste pipes are never thoroughly scoured. An S trap is no more self-cleaning when the water stream admitted is less than a quarter or a sixth of its capacity than would be an ordinary pot trap having a stream of the same size relatively to itself passing through it. In other words, a 1-inch S trap attached to a -inch waste flowing "full bore," or to a waste having a -mch inlet, is no more self-scouring than would be a 3-inch pot trap attached to a 1-inch waste also flowing full bore, and therefore such an S trap becomes practically then a "cesspool" trap.

This is a consideration of great importance, but one which appears generally to be lost sight of, and our plumbing laws are defective in not recognizing it. It is one of the principal causes of the collection of sediment in branch wastes and ordinary S traps which have a false reputation of being universally self-scouring. They are evidently only so when properly set, and when the waste water from the fixture is permitted to flow through them rapidly and "full bore." It cannot pass rapidly "full bore" unless the outlet and strainer give an open waterway as great or greater than that of the waste pipe and trap. It should be greater in order to allow something for friction and sediment. A trap having a very deep seal retards too much the rapidity and strength 'of the water current passing through it; hence it should be made as shallow as possible consistent with other requirements.

Self Siphonage And Momentum 283

Fig. 274.

Assuming, now, that we have properly set our trap in such a manner that a rapid stream of water passes through it full bore wherever the fixture is used in a legitimate manner, we shall find that, where an S trap is used, a siphon is formed by the outflowing waste water, which, without ventilation, breaks the seal of the trap. Hence the vent must be placed either at the crown or near enough to it to break the longer leg of the siphon. The momentum of the falling water assists the action of self-siphonage, and it is necessary for perfect protection against these two forces to vent very near the crown.

To make a practical test of the truth of this I had a wash basin fitted up with an outlet large enough to fill a 1-inch waste pipe and S trap full bore. Through this outlet the basin was emptied in 2 seconds, and the seal of the trap was completely destroyed by the siphonage and momentum of the falling water. The trap was an ordinary cast lead trap, having the usual seal of 1 inches, and was connected with a waste pipe of the same size with the bore of the trap. With a waste pipe very much larger or smaller than the bore of the trap the seal is not so easily broken by self-siphonage, for obvious reasons.

Sediment Collection in S Traps Converting Them

Into Cesspools.

In order to ascertain if the smallness of the outlet of a fixture could actually convert an S trap into a cesspool trap, as I have above asserted, and to study the effect of ventilation in removing deposits from cesspools, I have made a number of experiments on sediment collection and removal. I believe all unprejudiced and well-informed sanitarians now admit that the special vent pipe is no longer to be recommended as a protection against siphonage, for the reasons I have mentioned. All admit, however, that the main stacks of soil pipe should be thoroughly vented at head and foot. The object of this is to dilute the gases of decomposition to such an extent as to render them as harmless as possible, and then to remove them from the premises. Liberal ventilation hastens somewhat the oxidation of the foul matters in the pipes, but not enough to form an active agent in removing solid impurities.

There has been nevertheless a great deal of misunderstanding and idle theorizing on this subject among writers and practitioners in sanitary plumbing. There are advocates of indiscriminate venting who profess a preference for air pipes even to a thorough water scour, the most radical ones going so far as to affect for the latter comparative indifference, saying, "If compelled to choose between oxygen and suds, we should give the former preference every time."*

Let us now, therefore, abandon theories and authorities and seek for facts to guide us in forming an independent judgment on this very important question. The only question, then, now in dispute is: Do traps and branch waste pipes require the application of special vent pipes to prevent an accumulation within them of solid deposits and corrosive gases?

* James C. Baylies in the "Sanitary Engineer."


The first experiments were made on solid and the second on gaseous impurities. Under the first heading it was necessary to determine, first, if and to what extent the removal by oxidation of the refuse matters in our waste pipes goes on under a ventilating current under the varying conditions possible in practice; second, at what rate the accumulation of sediment or solid deposit goes on under the same circumstances; third, to what extent a water scour is able to prevent and remove solid deposits without the aid of the special vent pipe; and fourth, to what extent traps and branch waste pipes are self-ventilated without the aid of the special vent pipe in good plumbing practice.