Modern Gasworks Practice

626 MODERN GAS WORKS PRACTICE For larger holders the necessary size of a single rolied section for the upright stanchion would not conduce to wards economy or comeliness ; accordingly, it is customary to build up the columns on the principle of the lattice girder, the horizontal cross girders being designet! on the same lines. As a general rule, it may be said that single rolled Steel joists may be used for the vertical columns so long as their depth is not found (after the necessary calculations for stability have been made) to exceed 18 inches. When joists are employed their total height from tank level shotild not be greater than 65 times their cross-sectional depth. THE PROPORTIONS OF GASHOLDERS The preliminary operations in designing a gasholder are to asceitain the required capacity ; and, this being done, to determine upon the diameter and height of the bell, the mimber of lifts, etc. In calculating the capacity of holders the space enclosed by the domed top is neglected, owing to the fact that the gas in this portion is inefiective from the point of view of storage. For preliminary calculations the size of the holder may be airived at by applying the simple formula— Capacity in cubic feet = -7854 X (diameter)2 X height. For a single-lift holder this calculation will suffice, but when a multiple-lift holder is under consideration the capacity of the separate lifts should be distinctly estimated by applying the above rule to each section, chie allowance being made for the cups. CONSIDERATIONS AFFECTING THE PROPORTIONS OF GASHOLDERS There are many factors by which the general proportions of gasholders are influenced. Chief amongst these are :— (a) The nature of the sub-soil, and, therefore, the design of the tank. (b) The ground space available for construction. (c) The pressure required to be thrown by the holder if boosting is not to be resorted to. If the ground space available is unlimited the constauctor is not hampered m having to curtail diameter, with. a consequent increase in height, althougli deep lifts may be necessary in order to obtain the desired pressure. Deep lifts give rise to more costly tanks, although a saving may be effected by arranging. for one lift less than the number which might ordinarily be inserted, thus economizing on the total number of cups and grips, these accounting for a considerable item in the outlay. AVhen the erection of a holder is contemplated, it is an excellent plan to consider two suitable propositions entailing different proportions, with a view to taking out quantities for both. The following figures, which. show, very approxi-mately, the weight of metal employed in the construction. of two types of gasholders of identical capacity (If million cubic feet), will serve to illustrate the point.