(c) Must be so designed that the residuum will not clog or affect the operation of the machine and can conveniently be handled and removed.
(d) Must be so arranged that during the process of cleaning and recharging the back-flow of gas from the gasholder or other generating chambers will be automatically prevented.
27. GASHOLDERS.--(a) Must be of sufficient capacity to contain at least 4 cubic feet of gas per 1/2-foot burner of the rating. This is to provide for the requisite lighting period without the necessity of making gas at night, allowance being made for the enlargement of burners caused by the use of cleaners.
(b) Must be provided with suitable guides to direct the movement of the bell throughout its entire travel.
28. PRESSURE RELIEFS.--Must in all cases be provided, and must be so arranged as to prevent pressure in excess of 100-tenths (10) inches water column in the mains.
29. PRESSURES.--Gasholders must be adjusted to maintain a pressure of approximately 25-tenths (2.5) inches water column in the mains.
[CHAPTER V]
THE TREATMENT OF ACETYLENE AFTER GENERATION
IMPURITIES IN CALCIUM CARBIDE.--The calcium carbide manufactured at the present time, even when of the best quality commercially obtainable, is by no means a chemically pure substance; it contains a large number of foreign bodies, some of which evolve gas on treatment with water. To a considerable extent this statement will probably always remain true in the future; for in order to make absolutely pure carbide it would be necessary for the manufacturer to obtain and employ perfectly pure lime, carbon, and electrodes in an electric furnace which did not suffer attack during the passage of a powerful current, or he would have to devise some process for simultaneously or subsequently removing from his carbide those impurities which were derived from his impure raw materials or from the walls of his furnace--and either of these processes would increase the cost of the finished article to a degree that could hardly be borne. Beside the impurities thus inevitably arising from the calcium carbide decomposed, however, other impurities may be added to acetylene by the action of a badly designed generator or one working on a wrong system of construction; and therefore it may be said at once that the crude gas coming from the generating plant is seldom fit for immediate consumption, while if it be required for the illumination of occupied rooms, it must invariably be submitted to a rigorous method of chemical purification.
IMPURITIES OF ACETYLENE.--Combining together what may be termed the carbide impurities and the generator impurities in crude acetylene, the foreign bodies are partly gaseous, partly liquid, and partly solid. They may render the gas dangerous from the point of view of possible explosions; they, or the products derived from them on combustion, may be harmful to health if inspired, injurious to the fittings and decorations of rooms, objectionable at the burner orifices by determining, or assisting in, the formation of solid growths which distort the flame and so reduce its illuminating power; they may give trouble in the pipes by condensing from the state of vapour in bends and dips, or by depositing, if they are already solid, in angles, &c., and so causing stoppages; or they may be merely harmful economically by acting as diluents to the acetylene and, by having little or no illuminating value of themselves, causing the gas to emit less light than it should per unit of volume consumed, more particularly, of course, when the acetylene is not burnt under the mantle. Also, not being acetylene, or isomeric therewith, they require, even if they are combustible, a different proportion of oxygen for their perfect combustion; and a good acetylene jet is only calculated to attract precisely that quantity of air to the flame which a gas having the constitution C_2H_2 demands. It will be apparent without argument that a proper system of purification is one that is competent to remove the carbide impurities from acetylene, so far as that removal is desirable or necessary; it should not be called upon to extract the generator impurities, because the proper way of dealing with them is, to the utmost possible extent, to prevent their formation. The sole exception to this rule is that of water-vapour, which invariably accompanies the best acetylene, and must be partially removed as soon as convenient. Vapour of water almost always accompanies acetylene from the generator, even when the apparatus does not belong to those systems of working where liquid water is in excess, this being due to the fact that in a generator where the carbide is in excess the temperature tends to rise until part of the water is vapourised and carried out of the decomposing chamber before it has an opportunity of reacting with the excess of carbide. The issuing gas is therefore more or less hot, and it usually comes from the generating chamber saturated with vapour, the quantity needed so to saturate it rising as the temperature of the gas increases. Practically speaking, there is little objection to the presence of water-vapour in acetylene beyond the fear of deposition of liquid in the pipes, which may accumulate till they are partially or completely choked, and may even freeze and burst them in very severe weather. Where the chemical purifiers, too, contain a solid material which accidentally or intentionally acts as a drier by removing moisture from the acetylene, it is a waste of such comparatively expensive material to allow gas to enter the purifier wetter than need be.