There are three distinct systems of piping, known as the "two-pipe system," the "one-pipe relief system," and the "one-pipe circuit system," with various modifications of each.

Fig. 43 shows the arrangement of piping and radiators in the two-pipe system. The steam main leads from the top of the boiler and the branches are carried along near the basement ceiling; risers are taken off from the supply branches and carried up to the radiators on the different floors, and return pipes are brought down to the return mains, which should be placed near the basement floor below the water line of the boiler. Where the building is more than two stories high, radiators in similar positions on different floors are connected with the same riser, which may run to the highest floor, and a corresponding return drop connecting with each radiator is carried down beside the riser to the basement. A system in which the main horizontal returns are below the water line of the boiler is said to have a "wet" or "sealed" return. If the returns are overhead and above the water line, it is called a "dry" return. Where the steam is exposed to extended surfaces of water, as in overhead returns, where the condensation partially fills the pipes, there is likely to be cracking or "water hammer" due to the sudden condensation of the steam as it comes in contact with the cooler water. This is especially noticeable when steam is first turned into cold pipes and radiators, and the condensation is excessive. When dry returns are used the pipes should be large, and have a good pitch toward the boiler.

In the case of sealed returns the only contact between the steam and standing water is in the vertical returns where the exposed surfaces are very small (being equal to the sectional area of the pipes) and trouble from water hammer is practically done away with. Dry returns should be given an incline of at least 1 inch in 10 feet, while for wet returns 1 inch in 20 or even 40 feet is ample. The ends of all steam mains and branches should be dripped into the returns.

Systems Of Piping 100051

Fig. 44.

If the return is sealed, the drip may be directly connected as shown in Fig. 44, but if it is dry, the connection should be provided with a siphon loop as indicated in Fig. 45.

The loop becomes filled with water and prevents steam from flowing directly into the return. As the condensation collects in the loop it overflows into the return pipe and is carried away. The return pipes in this case are of course filled with steam above the water, but it is steam which has passed through the radiators and their return connections, and is therefore at a slightly lower pressure, so that if steam were admitted directly from the main it would tend to hold back the water in more distant returns and cause surging and cracking in the pipes. Sometimes the boiler is at a lower level than the basement in which the returns are run and it then becomes necessary to establish a "false" water line. This is done by making connections as shown in Fig. 46.

Systems Of Piping 100052

Fig. 45.

Systems Of Piping 100053

Fig. 46.

It is readily seen that the return water in order to reach the boiler must flow over the loop "A" which raises the water line, or seal, to the level shown by the dotted line. The balance pipe is to break the seal as the water flows over the loop, and prevent any siphon action which would tend to drain the water out of the return mains after a flow was once started.

Systems Of Piping 100054

Fig. 47.

One-Pipe Relief System. In this system of piping the radiators have but a single connection, the steam flowing in and the condensation draining out through the same pipe. Fig. 47 shows the method of running the pipes for this system. The steam main, as before, leads from the top of the boiler and is carried to as high a point as the basement ceiling will allow; it then slopes downward with a grade of about 1 inch in 10 feet and makes a circuit of the building or a portion of it.

Risers are taken off from the top and carried to the radiators above as in the two-pipe system, but in this case, the condensation flows back through the same pipe and drains into the return main near the floor through drip connections which are made at frequent intervals. In a two-story building the bottom of each riser to the second floor is dripped, and in larger buildings it is customary to drip each riser that has more than one radiator connected with it. If the radiators are large and at a considerable distance from the next riser, it is better to make a drip connection for each radiator. When the return main is overhead, the risers should be dripped through siphon loops, but the ends of the branches should make direct connection with the returns. This is the reverse of the two-pipe system. In this case the lowest pressure is at the ends of the mains so that steam introduced into the returns at these points will cause no trouble in the pipes connecting between these and the boiler.

Systems Of Piping 100055

Fig. 48.

If no steam is allowed to enter the returns, a vacuum will be formed, and there will be no pressure to force the water back to the boiler. A check valve should always be placed in the main return near the boiler to prevent the water from flowing out in case of a vacuum being formed suddenly in the pipes.

One-Pipe Circuit System. (See Fig. 48.) In this case the steam main rises to the highest point of the basement as before and then with a considerable pitch makes an entire circuit of the building and again connects with the boiler below the water line. Single risers are taken from the top and the condensation drains back through the same pipes and is carried along with the flow of steam to the extreme end of the main, where it is returned to the boiler. The main is made large and of the same size throughout its entire length; it must be given a good pitch to insure satisfactory results.

Systems Of Piping 100056

Fig. 49.

Systems Of Piping 100057

Fig. 50.

. One objection to a single-pipe system is that the steam and return water are flowing in opposite directions, and the risers must be made of extra large size to prevent any interference. This is overcome in large buildings by carrying a single riser to the attic, large enough to supply the entire building; then branching and running "drops" to the basement. In this system the flow of steam is downward as well as that of water. This method of piping may be used with good results in two-pipe systems as well. Care must always be taken that no pockets or low points occur in any of the lines of pipe, but if for any reason they cannot be avoided they should be carefully drained.