120 WATER CONDUCTORS AND ACCESSORIES 



its outlet, but for a pipe with submerged discharge it would be 

 the lower water level instead of the point of discharge. 



The slope or drop in elevation along the pipe corresponds 

 to the friction loss, so that, for example, the vertical distance 

 between D and E would be equal to the head lost on account of 

 friction between these two points. 



If the pipe is laid so that it rises above the hydraulic gradi- 

 ent AC, as at F, the pressure in the pipe at this point will 

 be less than that of the atmosphere by a head corresponding to 

 FG; thus negative. If no air could enter the pipe it would act 

 as a siphon and the flow would continue as usual, provided the 

 distance FG did not exceed about 25 feet, the theoretical limit 

 of vacuum being 34 feet. 



Air is, however, always present in the water and will collect 

 at the summit near F and the pressure will approach atmospheric, 

 in which case the gradient would shift to AF and the discharge 

 would only be that due to the vertical head between B and F 

 instead of between B and C. The remainder of the pipe from F 

 to C would merely act as a channel to deliver the flow. 



From the above it is evident that the pipe line should be laid 

 well below the hydraulic gradient, and much trouble may be 

 avoided, if from the outset a profile of the proposed route is 

 prepared and the hydraulic gradient carefully calculated and 

 drawn in. 



Size of Pipe Line: In determining the size of a pipe line or 

 penstock the first thing to consider is the number of pipes and 

 necessarily also the amount of water which each must be able to 

 carry. As to the number, this should preferably be equal to the 

 turbine units, as this secures a greater flexibility in the operation 

 of the plant. It further does away with the large Y-distributing 

 joints at the bottom of the penstocks, as well as with large size 

 gate valves and heavy plate thicknesses. 



In determining the most economical pipe-line installation for a 

 hydro-electric plant, several factors in addition to the primary 

 consideration of the grade or route must be studied. In general, 

 these must have direct relation to the earning capacity with 

 respect to the first cost. Usually the pipe-line investment repre- 

 sents one of the principal items of the initial cost of the generating 

 station. Especially is this apparent in connection with those 

 installations where the pipe line is long and subject to high pres- 



