211. The main supply pipes which are laid in the streets are called mains. They are made of cast iron.

The branches which conduct gas from the mains to the house are called service pipes or services. They are usually made of cast iron or wrought iron.

The pipes which convey the gas from the meter to the various parts of the building are called distributing pipes.

All vertical pipes are distinguished as risers or drop pipes, according to the direction of the flow of gas within them. The flow is upwards in a riser, and downwards in a drop pipe. All of the piping inside a building is usually plain wrought iron, with screwed joints.

212. Size Of Pipes

Size Of Pipes. The capacity of each pipe must be great enough to supply all of the burners which receive gas through it, when every burner is in full operation. Allowance must also be made for all heating and cooking apparatus, not only for that which is decided upon, but for all that is liable to be required.

Service pipes should never be less than 3/4 inch in diameter, because of the liability to chokage, and it is advisable to make the diameter at least 1 inch if the pipe is of iron. For small cook stoves, the supply pipe should be at least 3/4 inch in diameter, and larger stoves should have pipes 1 to 1 1/2 inches in diameter.

In computing the quantity of gas required for lighting purposes, one burner may be reckoned as consuming 5 cubic feet of gas per hour, unless otherwise stated in the specifications. The quantity actually required by burners of modern and improved construction, however, differs so much from that of the common forms, that it is impracticable to compute the volume of gas required by merely noting the number of burners.

Having ascertained the probable maximum quantity of gas required in cubic feet per hour, the necessary diameter of the pipe can be found from Table 10. If the length of the proposed pipe exceeds the maximum length given in the table, then the diameter chosen should be the next size larger. If the pressure of gas exceeds 2 inches of water, the principal pipes may be reduced in diameter one size. If the pressure is less than 1 inch of water, then all the pipes must be made one size larger, and in case of very long pipes, the diameter will require to be increased still more.

The pressure of the gas is assumed in the accompanying table to be about 2 inches of water. It should be understood that the quantities given are those which the pipes will deliver at the burners without objectionable fall of pressure.

Table 10. Capacity Of Wrought-Iron Gas Pipes

213. The use of the table is shown in the following example: