DISTRIBUTION OF WATER 



271 



pipes, up to 72 inches diameter, the cost increases more rapidly than the 

 diameter, being roughly proportional to its l'5th power, and the cost 

 of a twin line is about 30 per cent, greater than that of a single line. 



Pipe Line Accessories. Where the summit of a pipe line approaches the 

 gradient line, and generally at the highest point of each vertical bend, an 

 air valve should be provided to discharge any air which might tend to 

 accumulate. This valve usually takes the form of a ball float which 

 falls as air accumulates and allows it to escape. Fig. 119a shows a 

 small valve of this type, while Fig. H9b shows the type of valve fitted 



FIG. 119. Air Valves. 



to the 42-inch supply pipe lines of the Loch Leven power works. 1 In 

 the latter case the float, working between guides, carries the air-valve 

 spindle at its upper end, the upper portion of the valve casing serving as 

 an air vessel to reduce shocks in the pipe. 



Such valves also serve to admit air to the pipe line and to prevent the 

 formation of a vacuum in case of a rapid eflux of water following a 

 fracture at some lower point of the line. This is of importance in the 

 case of large steel pipes which are usually too thin to withstand an 

 external pressure of any magnitude without collapsing. 



To guard against the discharge of water backwards from the main in 

 the case of a fracture occurring in an inverted syphon below the level of 

 the discharge reservoir, or in any similar position, a reflux valve is 



1 " Proc. Inst. C.B.," vol 187., 1911-12, pt. 1, p. 28, 



