In order at any time to ascertain the pressure of the gas in the metre, close the stop-cock which admits the gas, and open the aperture G and F, which will shew the level of the water on the scale board E. This being first observed, close the aperture G, and open the stop-cock, and the pressure of the gas in the metre will be indicated by the rise of the water on the scale board E, above its original height.
PART XII.
Gas Mains, and Branch Pipes.
The name of mains, is given in the strictest sense of the word, to the cast-iron pipes from two inches in diameter and upwards, placed under ground, for conveying the gas into smaller branch pipes; but in a more extended sense, the term is applied to every pipe from which smaller ramifications or branch pipes proceed.
All mains destined to convey coal gas should be proved, they should be submitted to the trial of sustaining a column of water 300 feet high, and the pipe should be rejected if the least moisture appears on any part of the side of the pipe whilst submitted to this trial. For although such a pipe may remain impervious to gas for some time, the imperfection or fissure which permits the water to issue through under such a pressure, speedily increases, in consequence of the moisture to which the main under ground must necessarily be exposed. A skilful workman who is in the habit of proving pipes will distinguish, with an astonishing degree of correctness, a faulty pipe, by the sound produced by blows of the hammer upon the pipe. The faulty part, when struck upon, produces a jarring sound very different from the clear sound which a blow of the hammer produces when the pipe is in a perfect state. By this means the workman also detects, by the ear, inequalities in the thickness of the metal of the pipe.
Fig. 14, [plate V.], represents a longitudinal section of two flanch pipes, and the mode of connecting them. a, and b, are the pipes with their flanches connected; they are joined together, and rendered air-tight, by first interposing between the flanches a coat of iron cement, and then screwing up the faces of the flanches by means of screw bolts and nuts.
The composition of the cement is as follows:
Take four ounces of flour of sulphur, and two of muriate of ammonia, and mix them intimately together. When the cement is wanted, take five ounces of the above mixture, and add to it six pounds of cast iron borings, and blend them intimately together in a mortar; wet the mixture with water, and when brought to a proper consistence, apply it to the joints with a wooden or blunt iron spatula.