149. In planning any system of steam pipes, there are two things to be kept always in mind and which must be fully provided for; these are drainage, and the movement of the pipes by expansion. No heating can be done without condensation, and the water thus produced must be disposed of promptly and completely, and in a manner which will prevent interference with the steam supply.

150. The general arrangement of a steam main to supply several risers is shown in Fig. 58. The boiler a is set on the cellar or basement floor, and furnishes steam to the entire system. The steam main b, whose office it is to convey steam to the several risers c, through which it flows to the radiators d, d, etc., placed within the rooms to be warmed, is connected to the steam space of the boiler, and is so suspended from the floor joists by hangers that it will have a uniform fall from its highest point, which is immediately above the boiler, to its lowest point f. A pitch of about 1/2 inch in 10 feet is usually considered sufficient fall for the main. When steam is generated in the boiler it is forced into the steam main, from there into the risers, and thence into the radiators. The air which the pipes contain is forced out of the system to the atmosphere through air vents placed upon each radiator at the end opposite the steam inlet. As steam flows through the main and the risers, part of it will be condensed by heat being transmitted through the pipes to the air and to objects surrounding them.

Fig. 58.

This condensed steam will fall by gravity to the bottom of the steam main, flow to its lower end f, and enter the bottom of the boiler through the return pipe g.

151. The water of condensation from the radiators first accumulates in the base of the radiators until a sufficient hydrostatic head is formed to cause it to flow out of the radiators against the inflow of the steam. It then falls down the risers, through the riser connections, and into the steam main, also against the flow of the steam. If the riser connections to the steam main, or radiator connections to the riser, have too little pitch, or if the pipes are too small, the flow of the water of condensation through them will be resisted so much by the flow of steam that the water will not flow off as quickly as it is formed, the result of which will simply be that the water will accumulate in the pipe until it is entirely closed, and then water hammer will take place. The steam main should be made sufficiently large to prevent such a difference between the pressure in the boiler and that at the point f, as would cause the water to back up in the main and retard the flow of steam to any riser connection.

152. When a main or any horizontal steam-supply pipe has to be run a long distance, it becomes impracticable to grade it uniformly throughout its whole length, because the far end drops too low to be drained conveniently. In such a case the difficulty may be overcome by introducing vertical offsets, or relays, in the line of pipe. A relief pipe may then be attached at the foot of each relay to drain off the water of condensation.

153. The riser connections, or the nearly horizontal pipes between the steam main and the foot of the risers, in one-pipe systems, may be made as shown in Fig. 59. This permits the main to be kept away from the foundation wall sufficiently to allow it to be got at conveniently for screwing together and also for putting on coverings, etc.

The piece a serves as a spring piece, and permits both the main and the riser to shift slightly by expansion. In Fig.59 the spring piece is bent, to insure good drainage. The construction shown in Fig. 60 is sometimes used for the same purpose, the grade being secured by cutting the thread crooked at the end a. This is bad practice, because the teeth of the dies cut too deeply into the pipe on one side and weaken it seriously.

Fig. 59.

154. A riser should not be connected directly into the top of the main by a T, unless both pipes are very short. If the riser is long, its weight will cause the main to sag, and if the connections to the radiators above are rigid, the downward expansion will either bend the pipe or lift the radiators.

155. The connections to radiator branches, etc. should, if possible, be made with Y fittings. Plain T connections are objectionable in a one-pipe system, because the water of condensation runs down upon the interior surface of the riser, and is very apt to flow outwards into the branch, thus increasing the difficulty of draining it properly.