Modern Gasworks Practice

12 MODERN GASWORKS PRACTICE A plentiful supply of water under control is, of course, an economic asset, effect-ing a considerable annual saving. Many of thelarger works obtain a cheap supply of water by sinkbig an artesian well. (e) The distance of the site from the centre of the area of greatest consumption is a further point. The works need not necessarily be erected within its own limit.« of supply, although it almost invariably is. Owing to high cost of land near the centre of towns, the gasworks is usually situated on the outskirts, or what were the outskirts when the works was erected. In such, cases, of course, the additional length of trunk mains must be balanced against the saving in cost of land, and the sale of residuals and cartage costs must in general be considered. For long distance transmission, modern methods of “ boosting ” or high pressure distribution may be employed, permitting the use of smaller and tlierefore cheaper trunk mains. Need-less to say, the high pressure must be governed down for ordinary purposes. It often happens (after considerable growth) thai storage plant has to be erected further out of the town, so that the central site may be occupied for manufacture and purifi-cation. A minor factor requiring consideration is the possible danger of pollution to the water supply by percolation, particularly in chalky districts ; and when such things as gasholder tanks are emptied, care should be taken to see that the water, which frequently contains cyanides, is treated or run away where it can do no harm. CONSIDERATIONS AFFECTING SIZE OF WORKS Before much headway can be made with the actual definition of the site boun-daries, some idea'as to the extent of the ground required must be arrived at, this, of course, being dependent upon the quantity of gas likely to be oonsumed immp.-diately and durmg the next fifteen or twenty years. It is as well to be generous in the purchase of land, particularly if it is cheap; and preference sliould be given to a site which is capable of extension. A square or rectangular site is the most suitable for laying out buildings and plant. As regards the quantity of land to be purchased, experience is really the only rehable guide; so much. depends upon the nature of the district to be supplied, whether residential or manufacturing, the dass of inhabitant, presence or absence of electrica! competition, and the price at which gas is intended to be sold. In this respect sales of gas, unlike water, cannot be satisfactorily calculated by an allowance of so mucli per head of population; for, although every individual may be credited with a desire for water, the same cannot always be said of gas. As a matter of fact, in the larger cities the average consumption per head per annum is from 6,000 to 10,000 cubic feet, whilst in the smaller towns it may be from 2,500 to 7,500 cubic feet. Fifteen years ago it was a general rule to base the former at 2,000 cubic feet per head and the latter at 1,600 cubic feet. Taking the country as a whole, the average consumption per consumer (not per head) at the present time works out at almost exaotly 30,000 cubic feet per annum. The folio wing table, which relates to the tatest figures available at the time of publication, gives an idea as to how matters stand to-day