139. The main disconnecting trap, or main drain trap, as it is sometimes called, is a very important detail in a house-drain age system. Its office is to prevent gases in the city sewers or cesspools from entering the house drains. These gases are usually considered to be more dangerous than those generated within a house-drainage system.

The requirements of a good main disconnecting trap are:

1. It should, under all conditions and at all times, maintain a perfect disconnection between the gases in the street sewers and those in the house drains.

2. It should, under all ordinary conditions, be self-cleansing, so that solid matter may not accumulate in it and evolve gases.

3. It should, in all cases, be accessible for cleaning-out purposes and for inspection, the handholes being maintained positively gas-tight, preferably by means of screwed joints (metal to metal), not by a bedding of putty.

140. In Fig. 55 is shown a trap so constructed that, when set properly, the inlet lip e is about 2 to 3 inches above the normal water-line of the trap. The liquid sewage, by falling into the trap well, that is, the space just over the water in the inlet side of the trap, will force any solid matter in the well down under the tongue and through the trap, as shown by the arrows.

The space between the lip e and the water-line also provides for a small head of water which is required to cause a flow through a to the sewer, and by so doing prevents water from backing in the house drain b. The solids carried by the water in b are easily deposited in the trap well, from which they are just as easily forced by the small cascade falling upon them, under the tongue and through the trap w.

Fig. 55.

In order to obtain easy access to a and b, or to the trap itself, the fresh-air inlet c is joined to a T branch, and a 4 or 5 inch brass trap screw, the socket of which forms a sleeve for calking, is secured into the hub of the trap, as at m. This gives access to the main house drain and trap well.

Another handhole, or access plate, is shown at n; this is for easy access to the pipe a. It is very essential that this handhole be hermetically sealed, because the pipe a will constantly be loaded with sewer gas, and if the plate should not be bedded tight, sewer gas will flow into the cellar unobserved. A screwed cap having metal-to-metal contact is preferable for closing the handhole n.

Care should be taken to allow a space all around the pipe where it passes through the wall, particularly over it, so that the wall may settle without touching the pipe. The iron sewer connection a is continued through the wall to a distance of 5 or 10 feet before it joins the fireclay pipe, so that if any slight leak should occur in the fireclay pipes or their joints, the leakage will not affect the building.

The fireclay sewer pipe should never, on any account, be continued into the cellar.

141. When a main disconnecting trap must necessarily be located outside of the building and underground, it is customary to build a brick manhole around it for ease of access. If the manhole is remote from windows, doors, etc., it is customary to provide it with a perforated cover so that the fresh air may easily enter. In such a case, the handhole on the trap inlet is left open and the manhole floor is cemented water-tight and made flush with the handhole.

A manhole should be at least 2 feet 6 inches in diameter at the base, the walls being built of brick and 8 inches thick.

It should be closed on top by a flag of pavement at least 3 inches thick, and the iron cover or grating, as the case may be, should be sunk flush into the stone. The opening in the stone should be at least 15 inches in diameter, or square, and the plate which covers it should have a 1-inch bearing.