WATER CONDUCTORS 115 



The effective head at one point in a pipe will differ from 

 that at another point upstream or downstream from it, by an 

 amount corresponding to the losses and, of course, to any work 

 done or received between the two points when a machine, such 

 as a turbine or pump, is placed in the pipe line. Considering 

 only the losses, it follows that the effective head must decrease 

 in the direction of the flow by an amount equal to the head lost. 

 Therefore, although either the pressure, elevation, or velocity 

 may increase in the direction of the flow, the sum of them must 

 continually decrease so that an increase in one of these items 

 must always be accompanied by a corersponding decrease in 

 on 3 or both of the others. 



In regard to the head to be used in computing the efficiency 

 of an installation or a turbine, the turbine testing code of the 

 turbine builders specifies the following: 



" For the purpose of computing the plant efficiency the 

 total or gross head acting on the plant is to be used, and is to 

 be taken as the difference in elevation between the equivalent 

 still-water surface before the water has passed through the racks, 

 to the equivalent still-water surface in the tailrace after dis- 

 charge from the draft tube. When the water in the forebay in 

 advance of the racks flows with sufficient velocity to make its 

 velocity head an appreciable quantity, the actual elevation 

 of the water surface shall be increased by the amount of this 

 velocity head. The same process shall apply to the point of meas- 

 urement in the tailrace; that is, the velocity head at the point 

 of measurement in the tailrace shall be added to the actual ele- 

 vation of the surface, the sum being considered the equivalent 

 still-water elevation. 



" In computing the efficiency of the turbine, the losses through 

 racks, in the intake to the penstocks, and in the penstocks shall 

 not be charged against the turbine; nor shall the head necessary 

 to set up the velocity required to discharge the water from the 

 end of the draft tube be charged against the turbine. 



" The net or effective head acting on turbines equipped with 

 casings is to be taken as the difference between the elevation 

 corresponding to the pressure in the penstock near the entrance 

 to the turbine casing, and the elevation of the tail water at the 

 highest point attained by the discharge from the unit under test, 

 the above difference being corrected by adding the velocity 



