Modern Gasworks Practice

■680 MODERN GASWORKS PRACTICE (a) The life of a steel tank is short in comparison with that of the masonry tank. (b) The exterior of a steel tank is afiected by corrosion, tirus fairly frequent painting is essential. This periodical expense is not incurred with the buried tank. (It should be noted, in this connection, that the inside surface of the plates, being always under water, is not affected by corrosion, except to a certain extent at and above the surface of the water.) (c) The crown-rest framing for the holder bell will be more costly, owing to the impracticability of employing a dumpling. (d) The guide standards of the holder will be longer. The Design of Tanks When the erection of a gasholder is contemplated, trial holes must be bored down on the site in Order to determine the suitability of the ground for foundations. A good foundation is absolutely essential, not only to provide for the satisfactory working of the holder, but to ensure economy in construction. Where brick tanks have been already bullt, care should be taken that the ground in their vicinity is not violently disturbed. Instances can be pointed to where such, tanks have been cracked by the driving of piles near by. Good ballast may occasionally be obtained fiom the.subsoil, and in this case it may prove more economical to provide a concrete tank, even th.ough the holder be comparatively small. As regards the foundations for steel tanks, the circular slab is usually levelled off flush all over, but some designers prefer that the concrete should slope up to a height at the centre of 4 or 5 inches above the level at the periphery. It is said that in this manner, if there is any sinking of the ground underneath, the bottom will assume a level bedding. It would seem, however, that if any rise is to be given this should be allowed at the periphery (where the whole of the weight of side sheets and standards is- concentrated) rather than at the centre. As in the case of the masonry tank, the gas inlet and outlet pipes are led up through the floor of the steel tank, but they should be constructed from wrought iron and with the open ends bell-mouthed. The floor plates themselves are comparatively thin and should be rectangular, except for those in the outer row, which are made to the circular shape of the tank and are usually slightly thicker. Outside floor plates are usually T% inch to | inch, with. the curb row | inch thick. The bottom row of the side sheeting is composed of the heaviest plates in the structure, the thickness of the plate depending upon the depth of the tank, but in exceptional cases being as much as 2| inches. These plates are connected to the tank floor by means of a bottom curb which usually consists of a single angle placed internally (as shown in Fig. 422) or extemally, or a double angle may be employed. By far the most common practice, however, is to provide the internal angle alone. Tn the treatmeirt of gasholder tanks it is generally assnmed that a certain amount of restraint is exercised by the bottom curb and floor plates in preventing the circum-ferential Stretching of the bottom row when the tank is under load. This, no doubt,