114 WATER CONDUCTORS AND ACCESSORIES 



4 to 12 inches depending on the grade and the pressure of the water. 

 A lining serves several purposes. It holds the rocky material in 

 place ; it prevents seepage if the rock is porous ; and finally it de- 

 creases the friction which is of greatest importance in tunnel work, 

 as it permits a higher velocity with a correspondingly reduced sec- 

 tion. The velocity may be obtained from Kutter's formula, and the 

 values for n may be taken as 0.014 for lined tunnels and 0.028 

 for unlined. The safe velocity is from 10 to 15 feet per second. 



While the circular cross-section would be most advantageous 

 from the hydraulic point of view, it is usually given a horseshoe 

 shape (see Fig. 59) as this has been found to be the easiest to 

 excavate. In order to permit quick construction, especially of 

 long tunnels, one or more adits or openings are usually provided 

 at certain intervals so that the work can proceed from several 

 headings at the same time. 



Pipe Lines. Pressure pipes must be used for conveying the 

 water from the upper level at the forebay or dam to the wheels at 

 the power-house. These may be constructed of steel, wood, and 

 sometimes, although rarely, of concrete. The particular kind 

 to use depends upon the head and the corresponding pressure. 



Head. The total or gross head, as ordinarily understood, 

 is the difference between the elevation of the water in the fore- 

 bay and the tailrace. It must be distinguished from the net or 

 effective head acting on the turbine, the difference between the 

 two being equal to the head lost on account of friction in the 

 penstock, etc. 



The net or effective head at any point on the pipe line is 

 equal to the sum of the pressure head at the point considered, 

 plus the elevation head at the point above a datum plane plus 

 the velocity head in the pipe. Thus 



where 



h = effective or net head in feet; 



p = pressure head, this being equal to the pressure in pounds 



per square foot, at the point in consideration, divided 



by 62.4; 

 z = the elevation of the point above any arbitrary datum 



plane, in feet; 

 v = velocity at the point in feet per second. 



