A', B', C', and show fittings that look in the same direction as those shown by A, B, C, D of the same figure, but are on pipes that run down from the horizontal pipe. By varying the positions of the marks representing the drop fittings to suit, the diagram can be made to indicate open ings pointing in any direction desired.
All large risers should be exposed to view; and it is desir able to keep all piping accessible as far as possible, so that it may be easily reached for repairs if necessary. When it is necessary to trap a pipe, a drip with a drain-cock must be put in; but this should always be avoided under floors or in other inaccessible places. Where possible, it is better to carry up a main riser near the center of the building, as the distributing pipes will then average smaller, the timbers will not require so much cutting, and the flow of gas will be more uniform throughout.
Unless otherwise directed, outlets for brackets should be placed 5i feet from the floor, except in the case of hallways and bathrooms, where it is customary to place them 6 feet or more from the floor. Upright pipes should be plumb, so that nipples which project through the walls will be level; the nipples should not project more than I inch from the face of the plastering. Laths and plaster together are usually abo' t inch thick, so that the nipples should project about 11 inches from the face of the studding. Drop- or side-ells are used where possible for bracket openings. Gas pipes should never be placed on the bottom of floor timbers that are to be lathed and plastered, because they are inaccessible in case of leakage or altera tions. Fig. 104 illustrates some lines of gaspipe in a frame build ing, from which may be gleaned graphic ideas of how to fasten pipe securely in place.
Coal gas, and natural gas of some locations, has a strong odor that betrays leakage. Some natural gas is devoid of odor, in which case leakage is very dangerous, as there is no way quickly to detect its presence. For natural gas work, 10 pounds' air-pressure should fail to develop the slightest leak in the pipe, although the street pres sure is usually even less than eight ounces. For lighting gas, the street
pressure is seldom over 18 tenths water-pressure, and a 5-pound test is ample. These tests should be made with a mercury gauge, 2 inches height of column being considered as one pound pressure. A job may be considered tight when the mercury column not only does not drop, but does not even get flat at the top in from fifteen to twenty minutes' trial.
Every gas company has rules as to the number of lights allowed to be supplied from each size pipe, and the relative lengths of pipe permitted of each size. The following table gives sizes of gas pipes for different numbers of burners and lengths of runs, as usually installed : No restrictions are observed in selectin g fixtures for coal or natural gas. Coal gas carries enough carbon with it to produce a lighting flame when burned at the ordinary flame temperature. When the jet is lighted, the hydrogen is consumed in the lower part of the flame, producing sufficient heat to render incandescent the minute particles of carbon carried by it. The hydrogen, in the process of combustion, combines with the oxygen of the air, forming an invisible vapor of water, while the carbon unites with the oxygen, forming carbonic acid, or is set free as soot.
Various causes tend to render combustion incomplete. There may be excessive pressure of gas, lack of air, or defective burners. An excess of pressure at the burners causes a reduction of the amount of illumination; on the other hand, if the pressure is insufficient, the heat of the flame will not raise the carbon to a white heat, and the result will be a smoky flame. It therefore follows that for every burner there is a certain pressure (usually of an inch water-pressure before meu tioned) and a certain corresponding flow of gas, which will cause the brightest illumination.
There are a great variety of burners upon the market, among which the single-jet, bat's-wing, fish-tail, Argand, regenerative, and incandescent burners are the principal types.
The burner, Fig. 105, is the simplest kind, having but one small hole from which the gas issues. It is suitable only where a very small flame is required.