WOOD PIPE FOR 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 antl: J 
released by hand.t. Where practicable, eA 
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 (PI. ITT, fig. 1). 
BLOW-OFES. 




a 
Blow-offs are attached near the bottom 
at low points of the wood pipes in a man- : 
ner similar to that of attaching chimneys, Fic. 4—A type of air valve. 
and a sufficient number should be pro- oe ne 
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 lable 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 débris. 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. 
1For other designs of air valves see Jour. New England Water Works Assoc., 8 
(1893-94), p. 27; Hngin. News, 33 (1895), p. 234; Trans. Amer, Soc. Civ. Engin., 36 
(1896), p. 23. 

