Modern Gasworks Practice

THE STORAGE OF GAS o w to them since. It is estimated that treatment of this kind costs only one-fiftieth of the outlay demanded for painting. GASHOLDER TANKS Gasholder tanks as now erected may primarily be classified under three head-ings (1) Buried tanks, the top of which is approximately level with the prevailing groimd line. (6) Tanks entirely aboveground, in which the top of the concrete foundation is on. a level with the ground line. (c) Semi-buried tanks which are constructed partly below ground and which are “ banked up ” level with their upper edge with the material taken from the excavation. There is, in addition, a fourth, but uncommon, type, in which the floor of the tank is constructed upon piers or arches above ground-level, the space below being employed as a storehouse or for some similar purpose. With. one or two possible exceptions, it may be stated that types (a) and (c) are almost solely constructed from brick, concrete, or reinforced concrete, while steel, reinforced concrete, or cast iron are employed in all tanks aboveground. From the point of view of modern practice, cast iron may be dismissed from consideration forthwith. It was employed largely some three or four decades ago at a time when rolied steel plates were in comparison extremely costly and could not be obtained of sufficient thickness for large tanks. The mimber of brick tanks which are to be found to-day, particularly in connection with the older gasholders, is chiefly the outcome of the faet that in years past it was not deemed advisable to construct cast-iron tanks above a certain capacity ; and,, as the cost of steel was still prohibitive, the only alternative was that of exeavating a hole and lining it with a shell capable of retaining water and of sufficient stability to withstand the forces acting against it. Brickwork and Concrete Tanks Tanks composed of brickwork are, in general, of two descriptions :— (b) Puddled tanks. (c) Rendered tanks. The main difference between the two types is in the means introduced for ensuring that the tank shall be water-tight. Brickwork, per se, is not impervious to water, hcnce recourse must be had to some means of making it so. Water-tightness is ensured either by jacketing the tank on its exterior surface with a layer of puddled clay (hence the expression “ puddled tank ”), or by coating the interior of the tank with. a thin rendering (about -J- inch to 1 inch tliick) of neat cement or rieh mortar, in which case it will be a “rendered tank.” A half-section of a typical puddled tank, showing how the work of excavation is lessened by leaving a “ dumpling,” is shown in Fig. 417. In constructing a tank of this description, it is usual to build x x