WOOD PIPE FOE CONVEYING IRRIGATION WATER. 



15 



SPILLWAYS. 



As a precaution against damage that might result from accidental 

 stoppage of the pipe and to facilitate quick emptying in case of acci- 

 dent, spillways should be provided near the intake to siphons on 

 irrigation systems where it is feasible to do so. 



AIR VALVES. 



At every summit of a wood-pipe line, an air valve or chimney 

 should be placed. This is to allow air to enter so as to prevent a 

 vacuum and liability of collapse when the pipe is emptied, as well 

 as to permit the escape of air that may 

 accumulate at such points. Of the various 

 types of air valves on the market one in 

 common use is illustrated by figure 4. A 

 valve of this kind remains open until 

 closed by internal water pressure, and by 

 means of an angle valve air that accumu- 

 lates while the pipe is in service may be 

 released by hand. 1 Where practicable, 

 iron pipes open at the top are carried to 

 a point above the hydraulic gradient in 

 preference to the use of air valves at sum- 

 mits. Air valves and chimneys are usually 

 connected to wood pipe by means of cast 

 saddles, which are held in place by steel 

 bands (PL III, fig. 1). 



BLOW-OFFS. 



Pig. 4. — A type of air valve. 

 (Patented.) 



Blow-offs are attached near the bottom 

 at low points of the wood pipes in a man- 

 ner similar to that of attaching chimneys, 

 and a sufficient number should be pro- 

 vided so that every section of the pipe line may be drained and 

 flushed out. Ordinary gate valves are usually employed for this 

 purpose, the size to use being dependent on conditions. In lines 

 where a large amount of silt is liable to accumulate, such valves 

 should be of large size. 



On the 84-inch pipe of the Pueblo, Rocky Ford Irrigation Co. the 

 6-inch blow-offs operating under a head of 75 feet would completely 

 clog up with grass, leaves, and debris. To clean the pipe it was 

 necessary to cut a number of holes through it. These were made 

 30 inches square. New blow-off gates of this size were designed 

 to replace the 6-inch ones originally used. 



1 For other designs of air valves see Jour. New England Water Works Assoc, 8 

 (1893-94), p. 27; Engin. News, 33 (1895), p. 234; Trans. Amer. Soc. Civ. Engin., 36 

 (1896), p. 23. 



