252 MODERN GASWORKS PRACTICE
type embraces many advantages, chief among these being the means of cleaning, both branches being accessible by the removal of a single flange, and that not a cumbersome one. The joints of bridge pipes should be made of similar material
to that employed for ascension pipes. In the “ Darwen ” pipe (Fig. 155) a large expansion and cooling chamber is provided at the top of the ascension pipe, whilst
the top of the bridge forms a water-supply tank which feeds a drip cock immediately above the rising pipe.
DIP PIPES
Dip pipes may be connected to the bridge pipe by means of either flange or socket joints ; the latter are the more common. The pipe is always connected to the hydratilic main by a flange. Although in many cases the seal end of the dip pipe is left square, it is now generally recognized that more effect-ive working ensues if it is turned to a knife edge, as seen. in Fig. 162. By this means less oscillation of the liquor takes place, the gas spreads .more effectively, and suspended tar vesicles are more easily removed. For the last-named reason
' some engineers prefer the serrated end as in Fig. 163.
Some care is necessary during the erection of dip pipes in order to ensure that the seal on each will be the same. Providing the hydraulic mains have been laid
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