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WATER CONDUCTORS AND ACCESSORIES 



on suddenly, the standpipe furnishes the water quickly without 

 waiting for the velocity in the long pipe line to pick up, and thus 

 greatly aids the regulation. 



To be most efficient, the surge tank should be located as near 

 the power-house as possible, and if operating under atmospheric 

 pressure, its height should evidently be above that of the high- 

 water level in the forebay or storage pond. It is obvious, however, 



FIG. 68. Pressure Variations with Stand Pipe. 



that such an open design would not be feasible for high-head devel- 

 opments, and in such cases a closed standpipe is usually provided, 

 the increased air pressure being obtained by the static head. In 

 many plants both open and closed surge tanks are provided, the 

 open type being installed at the upper end of the pipe line, where it 

 passes over the brow of the hill above the power-house, while 

 closed air chambers 1 are installed just outside the power-house. 

 For pipe lines several miles in length it is also advisable to provide 

 equalizing reservoirs at intervals along the pipe line, so that changes 

 in the velocity of the water column will be as gradual as possible. 

 The differential surge tank consists of a standpipe of about the 

 same diameter as the conduit, freely connected to it, and a storage 

 tank of larger dimensions, surrounding the standpipe and con- 

 nected to the conduit by a properly restricted passage. In a 

 simple tank, the level of the stored water, following a demand for 

 more power, represents the accelerating level which is urging more 

 water from the forebay, and measures the head acting on the 

 water wheel. In the differential type, owing to the resistance 



1 For the use of air tanks for pipe line regulating purposes, see Proceed- 

 ings American Society of Civil Engineers for August, 1917. 



