Siphonage and Back Pressure. T HE trap vent pipe was, as I have said, originally supposed to afford a reliable cure for siphonage, and under that supposition the trap-vent law was made, and is in operation in the majority of cities and towns which have any plumbing laws at all.

For the purpose of testing the efficiency of the trap vent when it is new and clean, and therefore at its best, Mr. Hubbard has had the apparatus erected which you see in Fig. 241. The cut shows you the entire apparatus, including the parts not visible in the room. On the floor of the attic space above this lecture hall is our large supply tank, having a capacity up to the overflow of 40 gallons of water. From the bottom of this tank descends a 2-inch iron and glass pipe with two branches at a height of three feet from the floor for taking the traps to be tested. The piece of glass pipe is 4 feet long and has been inserted in order to enable you to follow the course of the water and study the manner in which air mixes with it in its descent from the tank under varying conditions of the tank valve vent. The valve in the tank is a 2-inch standpipe valve, and the top part of the 2-inch waste from the tank is vented by an inch pipe, descending to within five feet of the floor and, for the present, corked up at the lower end. Below the trap branches is a throttle valve by means of which we shall be able to illustrate the effect of "back pressure," as will hereafter be explained. Below the floor of this hall the 2-inch waste passes through the room below and discharges into a 25-gallon tank below the floor, as shown.

Fig. 241. Apparatus for Trap Testing: Erected at the North End Union for this Course of Lectures.

Fig. 241. Apparatus for Trap Testing: Erected at the North End Union for this Course of Lectures.

The tank empties through a 3-inch pipe, which connects with the soil pipe at the basement floor, being trapped on its way with a 3-inch running trap.

Now we may suppose that our tank represents a bath tub and the 2-inch iron pipe its waste, and that in the story below two basin wastes enter this waste through the two trap branches which you see, and that our traps are intended to serve these two fixtures. So far we have conditions corresponding to those in actual practice with new work where the rooms are very high.

The siphoning action may, however, if we wish, be made considerably more severe than is found in ordinary practice by corking up the top of this standpipe valve of our tank, which corresponds with the standpipe outlet valve of a bath tub, and also corking up the vent pipe coming from the top of the 2-inch tank waste, so that no air can enter the waste except through the fixture traps to be tested. Though in practice it often happens that house owners will close up the overflow outlets of their fixtures in the fear of "sewer gas" with corks and putty; and snow and frost will frequently close up the vent opening.

In order to represent "back-vent" piping we have here also some speaking tubing and bends, which we shall apply to the crowns of the traps to be tested and by this means investigate the effect of friction in retarding the action of this back airing.

By means of this apparatus we shall be able to determine (1) whether or not seals of traps in common use can be broken even when newly and fully vented and in accordance with the law, under conditions which can be and frequently are encountered in plumbing practice; (2) what effect corrosion, incrustation and various forms of clogging in the pipes have upon the traps seals; (3) the relative power of various forms of traps in resisting siphonage, and (4) whether any form is capable of resisting the severest possible siphoning action that can be encountered in plumbing practice.

Before, however, making our experiments with this apparatus, which will require light in the lecture room, we will complete our lantern slide work, comparing our plant with those used in my previous experiments for the Boston City Board of Health and at the Massachusetts Institute of Technology and elsewhere, pictures of which will be shown on the next two slides.

This cut (Fig. 242) shows the Board of Health apparatus. It is composed of ordinary piping erected exactly as it is in regular practice in house plumbing. A vertical stack of 4-inch soil pipe was erected without bends from the outlet above the roof to the horizontal run under the basement floor, a distance of 70 feet 9 inches. The soil pipe was run up straight in this manner in order to furnish the conditions for the severest possible tests for siphonage and back pressure. At the same time it formed the arrangement most commonly met with in practice. The unbroken fall of the water through such a pipe evidently creates the most powerful compression of the air in advance of it and the greatest rarification behind it.

Fig 242 Apparatus Used in the Experiments Made by the Author for the Boston City Board of Health.

Fig 242 Apparatus Used in the Experiments Made by the Author for the Boston City Board of Health.

Just below the fourth floor was placed a large cistern 44 inches long by 16 inches wide and 15 inches high up to the overflow, inside measure; or of 46 gallons capacity, as against 40 gallons capacity, which we have here. The cistern served to illustrate the action of a bath tub, by having a 1-inch discharge pipe at its bottom trapped with a Bower's large sized trap, and entering the soil pipe just above the entrance of the water-closet waste. The water-closet used was a plunger closet, at that time popular. To expedite its filling a large service pipe from the cistern was used, and the water was allowed to fill the closet through a brass compression-cock. The water-closet was supplied with a regular overflow pipe so that, when full, its capacity was always the same, i. e., 4 gallons. The plunger of this closet having no overflow, its operation produced as powerful a siphoning action as is possible with any plumbing fixture, and indeed the use of plunger closets is partly for this reason gradually diminishing everywhere.