260
MODERN GASWORKS PRACTICE
first of the above, one such being illustrated in Fig. 173. In most cases, however, these have proved more or less unsatisfactory, chiefly owing to sticking of the plugs or valves due to the accumulation of tar, or to the imperfect closing of the valves so that they are not gastight. Modem improvements of this type of anti-dips
Fig. 173.—Simple Throttle-tyeb Antt-j>tp.
Fia. 174.—Simmonds’ Anti-dip.
are the designs introduced by Simmonds and Davidson shown in Figs. 174 and 175. In Davidson’s system the base of the dip pipe is machined on the inside so
as to form a seating for the cast-iron cone suspended. from the rod passing through a stuffing box in the specially designed bridge pipe. The
cone is pulled up on to its seating and released by means of a weighted lever arrangement which. is operated from the charging floor. The stuffing box is packed with greasy packing. The advantages claimed for this type are that it permits of continuous draining away of tar and liquor, so that the main is practically empty, and it does not—as is the case with so many dry mains—curtail the capacity of the main, or involve the use of complicated appliances which are out of reach during working.
In Simmonds’ anti-dip the dip pipe is sealed in liquor in the usual way, and an aperture for the passage of the gas is provided in the side of the pipe above the liquor level. At this point a square valve face is cast on to the side of the pipe, the area of the gas outlet being equivalent to the cross-sectional area at the base of the pipe. The faced slide valve is operated by means of a rod passing through a stuflmg-box and connected to a counter-
weighted lever. The operating handle extends to the
charging floor, being held down during the period of gasmaking so that the seal is by-passed, and released whilst the retort doors are open in order to prevent