GAS PIPING The work of piping for gas is so closely allied to that of plumbing, since iron pipe has come into general use, that a brief notice of this branch is not out of place in connection with matters pertaining to plumbing. Coal gas is only about one-half the specific weight of air. The weight of natural gas is somewhat less than that of coal gas. The distribution of pressures which prevails in a closed system—the pressure of the fluid being equal at every point—should not be lost sight of in considering the ordinary method of distributing gas over a city or through a building in closed pipes. Although it would be true that in an open vessel the pressure of illuminating gas would by reason of its low specific grav ity be greater at the top of the vessel than at the bottom, this is not the case in a closed system in which a fixed pressure is main tained.
The most economical pressure at which to consume gas is five tenths of an inch water pressure.
As no town is strictly level, and the friction of the pipe requires some head of pressure to overcome it, the pressure in the mains is car ried above the point at which the best results are obtained. This is generally counteracted by not turning on the full amount at the burner. In towns varying greatly in the level of different portions, it is economy to use an automatic governor to reduce the pressure. This is true of exceedingly tall buildings, too. But in the tall building, one governor for the whole is not enough; the supply to the upper floors should be controlled by a governor situated on one of the upper floors.
Large pipe should not be notched into joists in the middle of their length; it weakens the joists. All pipes should be laid with a decline, toward the meter when possible, otherwise in such a way that they will drain toward a fixture or drip. The meter should be placed in a position easily accessible, and where it may be read without the use of an artificial light. It is connected in the house main on the street side of the first branch. A dry meter—the kind now almost universally employed—is shown in Fig. 99.
Different meters vary but little in the arrangement of the dials. In large meters, there are as many as five or more dials; but those used for dwelling houses usually have but three. Fig. 100 shows the
common form of index in a dry meter. The small index hand D, on the upper dial, is not taken into consideration when reading the meter, but is used merely for testing. The three dials, which record the con sumption of gas, are marked A, B, and C; and in each, a complete rev olution of the index hand denotes 1,000, 10,000, and 100,000 cubic feet, respectively. The index hands do not move in the same direction. When the hands are pointing upward, A and C move from left to right, while B moves in the opposite direction. Annex two cyphers at the right of the figures indicated when taking the statement of a meter. The left-hand index shown in Fig 100 reads 48,700. Suppose, after being used for a time, the hands should have the positions shown in the right-hand dial. This would read 64,900; and the amount of gas used during the interval would equal the difference in the readings: 64,900 — 48,700 = 16,200 cubic feet. Meters so invariably register in favor of the consumer after being in use only a few weeks, that the companies are by law permitted to set them 2 per cent fast when new. The route chosen for gas pipes should be the warmest consistent with convenience and economy. Coal gas will freeze—that is, the moisture in it will, in severe weather, form a network of frost that checks or stops the flow. Coal gas and natural gas are practically fixed. There is little trouble from condensation, even from coal gas, after it reaches the residence. There is sufficient reason, however, to incline the pipe and to avoid trapping any portion so that it will not drain. If a pipe runs through a cold place, a drip should be put in at some convenient point where it can be emptied if necessary. No offsets should be made in a way to favor choking the pipe by the products of corrosion falling down vertical parts. No fixture or bracket opening should be less than I-inch; no rising main less than i-inch. All openings for fixtures should have straight threads, and the pipe or fittings should be well secured, perpendicular to the wall passed through, so that they will not wobble, push in, or pull out. Ceiling drops should be cemented in the joint at the line, so that they will not unscrew when the cap is removed or a fixture taken down.