Fig. 276. Effect of Back Pressure.

Fig. 276. Effect of Back Pressure.

Fig. 277. Back Pressure resisted by posi sition of the Trap below the Fixture.

Fig. 277. Back Pressure resisted by posi-sition of the Trap below the Fixture.

Fig. 278. Effect of Back Pressure.

Fig. 278. Effect of Back Pressure.

With a pot trap, however, the power of resistance is much greater, since it contains water enough to rise under the influence of back pressure to a very considerable height in an inlet pipe. Now, so far as my experiments have shown, the severest back pressure that can possibly be brought to bear upon a water trap in a properly plumbed building having ventilated soil and drain pipes can be resisted by a column of water from 14 to 16 inches high. Hence, if a trap in such a building is placed under a fixture in such a manner that the bottom of its seal shall stand from 14 to 16 inches below the outlet of the fixture it serves, it may be considered perfectly safe against loss of seal or soil pipe air transmission by back pressure. For it will be found that if the column of water in a trap is high enough to resist this back pressure, it will entirely exclude the entrance of sewer gas or soil pipe air so compressed in the pipes. In other words, the air will not, under such circumstances, ever be driven through the water column in bubbles, as is sometimes feared. Hence in setting traps under kitchen sinks where back pressure from water falling from fixtures above is to be feared, the traps should always be placed low enough below the outlet of the sink to permit of the formation of a water column high enough to resist the back pressure. Otherwise the water may be blown out of the trap into the sink, and sewer air will follow. If such action follows every time any fixture above is discharged, the constant repetition of such injection of sewer air into the room may result in serious consequences.

Now, the main house trap is a frequent cause of heavy back pressure in the basement of a house owing to the friction caused by the resistance of its seal, and this furnishes another argument in favor of its abandonment.

In the experiments in back pressure made in my investigations for the Board of Health, the traps were tested on the basement floor (see Fig. 242) just above the horizontal run of the soil pipe. The tests were divided into (A) those in which the traps had no vertical extension of the inlet arm, and (B) those in which the inlet arms were extended.

An S trap having the ordinary 6-inch length of inlet arm above the seal was first tested. The first discharge of the water closet alone threw the water out of the trap, projecting it several feet in the air, and broke the seal. The experiment was often repeated with the same result. Fig. 278.

(A) A 4-inch pot trap lost its seal in four discharges (see Fig. 276). The top of the inlet arm stood 2 inches above the top of the seal.

An 8-inch pot lost 2 inches of its seal in seven discharges. The top of the inlet pipe stood 3 inches above the top of the seal.

The same trap lost 3 inches by fourteen discharges of the water closet and bath tub together.

(B) With a vertical extension (Fig. 277) or 16 inches, a 1-inch cast lead S trap retained its seal entirely whether tested with the discharge of the water closet alone or with water closet and bath tub together; but in all cases air was forcibly driven through the water forced up into the inlet pipe, because the volume of water in the trap was insufficient to outweigh the back pressure.

An S trap having 5 inches of seal without extension lost its seal in all cases, but with an extension of 16 inches the water was not thrown out under the severest discharges. With this trap, moreover, the large volume of water was with the extension sufficient to overbalance the pressure of the air, and no bubbles were driven through the trap. The same deep seal S trap was then tested after half its seal had been removed as by evaporation or other accident. In this case the trap acted exactly as did the ordinary shallow-sealed, ordinary cast lead S trap, and always allowed air to be driven through it.

A 4-inch pot trap with the 16-inch extension neither lost any of its water nor allowed any air to escape through its seal under any of the severest conditions.

The same trap with a 6-inch extension, bringing the top of the extension 8 inches above top of seal, lost its entire seal in two discharges of the water closet and bath tub together. The volume of the water in the trap was sufficient, but the pipe was not long enough to allow of the formation of a column sufficiently high to resist the air pressure.

An 8-inch pot trap with either a 9, 12 or a 16-inch extension lost no water and allowed no air to pass under either of the tests. But with a 6-inch extension the water was driven out of the trap.

A piece of 1-inch waste pipe, 12 inches long, holds about of a pint of water. A 15-inch piece holds a pint. Hence a trap used with such a waste pipe should have a capacity of not less than pint.

In our apparatus erected for this course we shall be able to illustrate "back pressure" in every degree of force, from an amount scarcely measurable up to that which will throw the entire contents of our tank upon the lecture floor, according as the valve just below the trap testing branch be left open or completely closed.