168. The proper diameter of pipes which are to supply hot or cold water, depends upon several considerations:

1. The number and size of faucets that are likely to be discharging water at the same time.

2. The pressure or head of the water.

o. The length of the pipe.

If the pipe is crooked, making numerous bends or angles, due allowance must be made for the resistance arising therefrom.

A pipe of small bore, having great length, is likely to be noisy, if the pressure is great, being subject to singing noises and water hammer. This defect may be avoided by using a pipe of larger diameter, thus reducing the velocity of the moving water.

169. Horizontal pipes may be reduced in diameter as various branches are taken off. This is done only to economize in the cost of pipe, etc. An example of such reduction is shown in Fig. 62. In this the nearly horizontal distributing pipe is reduced from 1 inch to 1/2 inch as the branches are taken from it.

Fig. 62.

Suppose that the distributing main should enter at the opposite end so that the pantry sink branch would be taken off first, then it would only be reduced one size, that is, from 1 inch to £ inch, because its extreme end must equal that of the sink branch. It is well to have the distributing mains a little too large rather than too small; the annoyance of one faucet robbing another will then be avoided.

170. Vertical pipes, which descend from a tank, may be reduced in a similar manner, as shown in Fig. 63. The tendency of the water flowing from the tank A is, of course, to fall to the bottom of the vertical line of pipe, and flow out of the lower branch. Although the vertical line is decreased in size as it descends, it still follows that there is a greater pressure upon the lower branches than upon the higher, and to compensate for this difference in pressure, the sizes of the branches upon the different floors should be decreased as they descend. Thus, in the figure the top branch is 1 inch and the lowest 1/2 inch. By this system of distribution a nearly uniform supply of water can be given to each floor in a high building.

Pipes which rise from a service pipe in the basement and ascend to the upper stories, usually should not be reduced in diameter, until the last branch is reached. This is because the pressure grows less as the height increases, and to secure a satisfactory flow on the upper floors, the pipes must be large in diameter. Even if the head is so great that there is plenty of force on the upper floors, if the pipes be reduced in diameter, they will be liable to annoyance from the action of the faucets in the lower stories. If a faucet in the basement be opened, for example, the flow from a faucet on the top floor, which happens to be open at the same time, will be checked or even stopped.

Fig. 63.

171. The size of the corporation cock which may be attached to a street main is usually determined by the water department. The diameter of the service pipe should not be governed by the size of the corporation cock, however, but should be determined solely by the requirements of the building. If the quantity of water required is very large, the water authorities will, upon due presentation of the facts, usually allow a larger connection to be made to the water mains.

172. The following sizes of branches are commonly used in buildings where the pipes are not of great length. If the pressure is less than 20 pounds per square inch, the system may be rated as low pressure, and if above 20 pounds as high pressure:

Table 7