Modern Gasworks Practice

RETORT-BENCH APPURTENANCES 259 seal 0-54 inch, again assuming a ratio of areas of 12 to 1. But on all systems of hydraulic mains some form of liquor overflow, whether it be by weir valve or tar tower, is imperative; accordingly the above formula has little bearing on the con-ditions of seal in the hydraulic main. If a permanent overflow is provided, then the liquor outside the clip pipe can never rise above a certain level; consequently, the difference in level of the liquid in and outside the pipe during the application. of a vacuum will be brought about solely by the fall in level of the liquid inside the pipe. Hence, if a vacuum of 1-inch. is applied to the hydraulic, the liquid in the dip pipe will fall 1 inch. Consequently, before the seal is broken a “ draw ” eq u.i valent to the original seal can be applied. ANTI-DIPS AND DRY MAINS Since with modem methods of carbonization there is frequently a vacuum in the ascension pipe, so that air is drawn forward wheu the retort doors are open, and, furthermore, it has been shown that the safety-seal function of the hydraulic is, with light seals, practically valueless, there is strong reason to suppose that the liquor seal could be dispensed with altogether. In order, liowever, that the in-drawing of air, with its consequent deteriorating efEect upon the quality of the gas, may be precluded, the device known as the anti-dip has been introduced. Primarily, it consists of a receptacle similar to the hydraulic main, but witliout the permanent liquor seal, means being provided for closing the inlet to the main during the time whcn the retort doors are open. The chief claims which can be made for the dry main are the following:— («) The liquor (or tar) seal is abolished, consequently any deterioration which the gas may suffer, due to the bubbling, is avoided. (&) There is no continuous fluctuation of pressure or vacuum in the ascension pipe, and consequently in the retort. In faet, oscillation is absent. (c) No air is pulled in during the time the retort doors are open, as is often the case with the hydraulic seal. (rf ) Little, if any, pressure in hydraulic main and retort, therefore less scurf formed. (e) If the main gets out of level the consequences are not very serious. On the other hånd, the appliance for making and breaking the seal entails the use of fairly costly apparatus, which inereases the original expenditure and the wear and tear. Anti-dips now in use may be classified under three distinet heads, namely:— («) Those where, by means of some form of plug or valve, the free-way in the ascension pipe is completely closed. This may be done by a butterfly or plug valve in the arch. pipe, or by closing the open end of the dip pipe by means of a niovable plug. (6) Types in which the liquor is raised at will, so as to seal the dip pipes. (c) Types in which the liquor is lowered at will so as to unseal the dip pipes. The earliest forms of anti-dips were mainly constructed on the principle of the