138 



WATER CONDUCTORS AND ACCESSORIES 



Concrete pipes are in use for heads up to 150 feet. They are 

 very smooth, being in this respect nearly on a par with wooden- 

 stave pipe, and thus offer little resistance to the flow of water. 



FIG. 67. Cross-section of Concrete Pipe. 



They are especially adapted for use where raw material such as 

 sand, stone or gravel and cement are available locally, in which 

 case the pipes are generally manufactured on the job where they 

 are used. 



2. WATERHAMMER AND SURGE TANKS l 



Waterhammer. When the gates at the lower end of a pen- 

 stock are closed and the water column suddenly checked, the 

 pressure immediately rises and may reach very high and destruc- 1 

 tive values if not provided for or prevented. This rise of pressure 

 is known under the name of " waterhammer." 



When the gate begins to close the pressure rises first at this 

 point, and a pressure wave or vibration begins to travel towards 

 the upper end of the pipe. If the pipe is absolutely rigid, the 

 velocity at which it would travel would have been about 4650 feet 

 per second, or the same as that of sound. On account of the 



l See also sections on " Water Conductors " and " Governors." 



