670
MODERN GASWORKS PRACTICE
tightness ’ is ensured by the use of a water seal as heretofore. Simply explained, the holder (Fig. 415) consists of an outer cylindrical stell very similar in appearance to the present holder bell, but with the exception that the roof is the only moving portion. The contrivance is, in faet, little more than a cylindrical tank, the roof of which is, by means of guide rollers, permitted to rise or fall within the tank. Gas
in the ordinary way is admitted beneath the roof, which ascends and descends in accordan.ce with the prevailing pressure in the interior. The guidance of the floating roof is such that the smallest amount of clearance is permitted between it and the internal periphery of the cylinder. The means employed for the prevention of gas leakage consists in pumping water upon the exterior of the roof so that it runs down to the portions above the guide rollers, and seals up the clearance between cylinder and dorne. Some portion of the water is, of course, continually passing through, this clearance into the interior of the holder, whence it is removed by way of a seal and returned to the suction of the pump.
THE LEERDAM “ DRY ” GASHOLDER
A novel form of gasholder, designed on somewhat similar lines as the Nuremberg type, has recently been erected at Leerdam in Holland. The gasholder is merely a tank or reservoir of polygonal cross-section, consisting of a bottom, sides, and travel-ling roof. It is 52 feet in diameter and 45 feet high, with a capacity of 70,000 cubic feet. The joint between the moving roof and the stell is maintained gas-tigh.t by a tar seal, the bottom of the tank containing an annular trough. which acts as a collector for any tar which leaks through the seal. This tar is returned again to the roof seal by a pump. The travelling roof is guided by rollers acting in conjunction with the shell, and around its periphery there is an annulus for the reception of the sealing tar, the bottom of the annulus being formed of wooden wedges with springs, making a loose sliding frt with the shell. The annulus is divided into sections, so that should