HYDRAULICS 377 



the pipe that rise above the line will be less than the atmos- 

 pheric pressure. Air always tends to collect in the nighest 

 point of a siphon, and means must be provided for its removal, 

 in order to keep up the flow. This is effected by means of an 

 air pump or air valve. Such means of removing the air should 

 be provided for whenever circumstances make it unavoidable 

 to place part of a pipe above the hydraulic gradient. 



CAST-IRON PIPES 



The thickness of a cast-iron pipe may be computed by the 



(P+p')d 

 following formula : t = h .25 



in which t is the thickness of pipe, in inches; p, the static 

 pressure, due to the head above the pipe, in pounds per square 

 inch; d, the diameter of pipe, in inches; and p', the allowance 

 for water hammer (shocks caused by opening of valves). 

 The following are values of p' for different diameters: 

 Diameter of Pipe Value of p' 



Inches Pounds per Square Inch 



3 tolO 120 



12 110 



16 100 



20 90 



24 85 



30 80 



36 75 



40 to 60 70 



EXAMPLE. Determine the thickness of a cast-iron pipe 14 in. 

 in diameter to withstand a pressure of 130 Ib. per sq. in. 



SOLUTION. Here, d=14 and /> = 130. The value of p' 

 corresponding to a diameter of 14 in. is a mean between the 

 values corresponding to the diameters 12 and 16, or 105. Sub- 

 stituting these values in the formula, 



t = (-.25' 



Weight of a Cast-Iron Pipe Line. To ascertain by a rapid 

 approximation the weight, in tons (2,000 Ib.), of a cast-iron 

 pipe line, the following formula may be used: 

 T=28mt(d+t) 



