60S
MODERN GASWORKS PRACTICE
when at work, no deposit of any kind can collect, tlius leakage from one valve chamber to another
is precluded.
Hydraulic centre valves, in which water is the sealing medium, are also employed, but the ordinary type possesses the disadvantage that it will only operate the purifiers in one combina-tion, i.e. any three boxes at work with one out of action. The construction of the centre valve is somewhat complex, but an idea of the principle may be obtained from the diagrammatic representation given in Fig. 370.
Common types of rack and pinion valves are shown in Figs. 371 and 372, whilst the ordinary single water valve is seen in Fig. 373. These water or hy-draulic valves are of simple construction and consist of a rectangular cast-iron box with a mid-feather extending to within a short distance of the bottom. When water is admitted to the box the gas way is stopped.
Fio. 370.—Diagram showing Principle of Operation of Centre Valve.
and in order to open the gas passage the water is merely run of? from the drain pipe at the base. The depth seal employed in these valves is usually the same as that in the seal of the purifier cover.
Fig. 371.—Rack and Pinion Valve.
Fig. 372.—Rack and Pinion Gas Valve.
So far as the relative advantages of the various types of valves are concerned, the following points may bo stated-