Modern Gasworks Practice

262 MODERN GASWORKS PRACTICE in tlie hydraulic. When the retort doors are opened, however, the pressure gas is admitted to the displacement chamber. This causes a depression of the liquid in the chamber, with. a corresponding rise in level in the hydraulic, the latter rise being sufficient to seal the dip pipes, thereby closing the free-way of the ascension pipes. The difference in level will, of course, be the same as the difference in pressure of the pressure gas used and the vacuum produced by the exhauster. In this system it will be noticed that all retorts in the bench are simultaneously sealed, whether their doors are open or not. Ä wheel is attached to the three-way valve, and the change is effected by a chain passing round the former and hanging down to the stage floor. It will be seen that the only working part, the three-way valve, never comes into contact with the newly made gas and tarry vapours. Within recent years there has been. a growing tendency to operate the hydraulic main with the dip pipe entirely unsealed. This practice bas mainly been adopted with certain systems of vertical retorts, altliougli it may be equally well applied to the horizontal bench. Two considerations enter into the reckoning, namely, that the yield of gas per retort and per ton of coal is undoubtedly enhanced by the elimination of the seal; but, on the other hand, unless extreme care is exercised the inert constituents in the gas may reach alarming proportions. As an example of an effective method of working horizontal retort benches with the dip pipes unsealed may be quoted the system in use at the Old Kent Road works of the South Metropolitan Gas Company. In this instance, the liquor in the liydraulics is maintained at a level just coinciding with. the level of the bottom of the dip pipes. When the retort doors are closed the vacuum on the foul main is adjusted to from 3- to 6-tenths, whereas when the doors are open the conditions are reduced to level gauge. A retort-house governor is fitted to every four benches, and this is operated by a wire reach ing down to the stage level and connecting with the necessary adjusting mechanism. TAKE-OFFS FOR TAR AND LIQUOR For many years the weir valve was by far the most common method. in use for disposing of the liquor and tar from the hydraulic main. As before explained, the gas, liquor, and tar were removed through a common outlet, a tar shield usually being provided, so that liquor formed the greater part of the liquid remaining in the hydraulic. The three products of distillation then flowed together to the foul main, from the bottom of which the liquids were drawn ofE through seals and run to the storage wells, where separation, due to gravity, took place. In. spite of the introduction of more effective appliances, the weir valve is still in fairly common use, and forms a ready and reliable means for the regulation of seals. The con-stiuction of the valve is simple, and is shown in detail in Fig. 178. Another means for the disposal of tar and liquor is that introducing the principle of the tar box, in which the gas outlet is distinct from that of the liquids and seal regulation is effected by the height of overflow of the latter. The principle intro-duced is in reality that of the U-gauge, one limb being the hydraulic main and the other limb the tar-overflow pipe. The arrangement will be readily understood by