THE FLOW OF WATER IN" CONCRETE PIPE. . 53 



Where the usual types of inlet and outlet structure are employed, 

 with but little construction and consequent expense incurred for con- 

 servation of entry and velocity heads, it is recommended that the 

 figures in Table 5 be used. 



AIR IN PIPE. 



Like all other lines, a concrete pipe must be protected by air valves 

 against the accumulation of air at "summits" on the line. If this 

 is not done the capacity will be reduced. 



In addition to the air at summits it has been found that the 

 capacity of a siphon pipe may be reduced by accumulations of air at 

 the intake end. This accumulation may take place several hundred 

 feet down the incline from the pipe entrance and manifest itself by a 

 periodic "blowing back" of compressed air. The writer has seen 

 two instances where the resulting conflict between air and water has 

 burst the pipe. 



Pipes taking water directly from reservoirs are ; of course, not 

 subject to these troubles, the depth above the intake being, as a rule, 

 sufficient to insure filling of the pipe with water alone. 



Mr. A. E. Ashcroft, of Vernon, British Columbia, Canada, in speak- 

 ing of some 36-inch siphon pipes, says: 1 



A large number of air valves at the upper ends of the pipes have been effective in 

 reducing vibration and thumping when the pipes are discharging only partially full. 

 (See also Mr. Finkle's discussion, p. 94.) 



The best air outlet is probably a " chimney'' rising above the hy- 

 draulic gradient. These vents may be from 1 to 36 inches in diam- 

 eter, depending on the size of the pipe line, and small ones should be 

 so assembled that they may be taken apart, as debris collects in such 

 vents and must be periodically removed. Moritz suggests the area 

 of the relief pipe ,be one-twentieth of that of the pressure pipe. 2 



FRICTION LOSSES. 



The loss of head necessary to overcome internal resistances within 

 the pipe is proportional to the length of the pipe, but is independent 

 of the static pressure in the pipe. 



In order to determine the size of pipe and the loss of head necessary 

 to overcome the frictional resistances in the conveyance of a given 

 quantity of water, three estimate diagrams and four tables have been 

 prepared. Examples of typical pipe problems are given. The fac- 

 tors of safety given below should be considered in each problem. 



FACTORS OF SAFETY. 



As in all conservative design or estimating, factors of safety must 

 be used in order to take care of the divergence, due to unforeseen or 

 abnormal conditions, from a general average. 



1 Pro. 4th An. Conv., W. Can. Irg. Assoc, Ottawa, 1911, p. 76. 



2 Working Data for Irrigation Engineers, p. 70. 



