Modern Gasworks Practice

RETORT-BENCH APPURTENANCES 275 The size of the foul main must depend upon the combined areas of all pipes leading into it. Theoretically, the cross-sectional area slioiild be equivalent to the sum of the areas of the latter pipes, but this is never carried out in practice, firstly because mains of such capacity are unnecessary owing to the condensation rapidly taking place, and because the foul main is delivering gas more or less continuously, whereas there is a stidden rush of gas into each. hydraulic main as charging takes place. Generally speaking, the following sizes for foul mains are those prevailing, and may be safely applied :— Capacity of Bench pek 24 Hours. (a) 30,000 to 100,000 cubic feet . (6) 200,000 to 500,000 cubic feet : (c) 1 million to million cubic feet (d) 2 million to 3 million cubic feet Size of Foul Main. If one. If two. 6 inches to 9 inches. — 12 inches to 15 inches. — 18 inches. 14 inches. 24 inches. 18 inches. Fig. 193.—Expansion Joint job Foul Matn. Another rule is to make the foul main (if it delivers all the gas) of 125 per cent, the area of the main connections in the works. Retort benches in the past were invariably supplied with a cast-iron foul main, but the modern practice is to construct the latter from lap-vvelded steel plates or solid drawn. tubes. When the foul main is of greater diameter than 12 indies, steel should, as a general rule, be used, and in such cases it will usually be slightly cheaper than cast-iron. A further re-conimeiidation. is that owing to the thinner metal walls employed in the steel pipe the outward radiation of heat is greater, hence the cooling effect on the gas is very much more pronounced. As the foul main is situated above the retort beuch, it is liable to undergo considerable expansion when the heats are raised ready for work; accordingly some means must be introduced for the prevention of breakage. With. the cast-iron main, the lead joints at the spigots will be sufficient to cope with any change of length, and flange joints should, therefore, be avoided, unless some form of expansion joint is inserted. The steel foul main, having no flexible joints to yield to movement, should— if of any length—always be pro vided with an expansion joint similar to that shown in Fig. 193. In the usual way one of these joints will be necessary for each. 100 feet of pipe. In modern work the foul main is usually supported on the cross-stays, or from ,'them by chains or steel straps. RETORT-HOUSE GOVERNORS (Ehe retort-house governor is an auxiliary which is more or less essential in the larger works. It dillers from the ordinary station, type of governor (which forms the last piece of apparatus through whicli the gas passes before leaving the works) in. that whilst the latter is employed with the object of reducing pressure, the retort-house governor is interposed for the purpose of reducing the intensity of vacuum created by the exhauster. This, however, is not its primary function.