Modern Gasworks Practice

688 MODERN GASWORKS PRACTICE THE PRESSURE THROWN BY A GASHOLDER The pressure thrown by a gasholder is dependent upon the weight of the floating portion and its diameter. Thus, if the total weight of the bell, including the weight of the water in the lutes, lis W tons and the diameter of the holder is D feet the pressure may be calculated as follows — 549 W Maximum pressure in inches of water = —- A single-lift holder usually throws from 3 to 4 inches of water pressure, while in the case of a telescopic holder the top lift will throw about the same amount, with each succeedinglift throwing about 2 inches. When dealing with multiple-lift holdets a figuie comparing very closely with the actual pressure thrown in practice may be obtained by considering the total weight and the mean diameter of the holder, althongh this is not strictly accurate. Theoretically, the weight of the bell, when the holder is fully inflated, acts on the greatest area, ie. the area obtained by consider-ing the diameter of the lowest lift. In practice, the pressure thrown by the inner lift alone is the chief consideration, owing to the faet that this governs the minimum pressure which the holder throws ; and, therefore, Bears a distinct relation to the pressure on the district. THE CONTAMINATION OF GAS IN HOLDERS It occasionally happens that coal gas, although found to be perfectly clean at the outlet of the purifiers, shows decided traces of sulphuretted hydrogen at the holder outlet. Fortunately. such an occurrence is comparatively rare in this country, but one London Company experienced considerable annoyance in this respect some few years ago. The pollution of gas in this way is the result of the spontaneous production of sulphuretted hydrogen in the gasholders, or, less commonly, in station meters. Although no positive reason can be ascribed for the formation of the impimty, it seems extremely probable that it may be due to the presence of certain micro-organisms, or to an electrolytic action set up by the iron plates. It is, of course, a recognized faet that certain bacilli characteristic of ordinary water possess the property of decomposing definite salts (such, as calcium sulphate, present in the water), with the evolution of sulphuretted hydrogen. A series of experiments con-ducted .on water taken from a gasholder tank seemed to corroborate the organic theory, the bacteria deriving their necessary food supply from dead iron organisms. In the case of the London Company, complete immunity from the trouble was obtained after the bacilli had been destroyed by means of a germicide, such as sulphate of copper. The French investigator, M. Guillet, has concluded that contamination is also caused by exposure of the gas to a clean iron surface resulting from the detacli-ment of rust. He has shown that in the presence of the hare metal the carbon disulphide in the gas reacts with. carbon dioxide and water, with the liberation of sulphuretted hydrogen. The remedy suggested in this case, however, is somewhat inconvenient, as the holder must temporarily :be put out of action. It entails the soldering of zinc sheets to the iron. plates at definite places, when galvanic action checks the formation of the sulphuretted hydrogen.