USE OF CONCRETE PIPE IN IRRIGATION. 



29 



entrance to the pipe is commonly protected by iron bars set on an 

 . angle in front of the pipe entrance. The bars may be spaced 6 inches 

 or more apart. Such protection will prevent boards and large tree 

 branches entering the pipe. 



AIR VENTS. 



When air is drawn into a pipe carrying water, being lighter than 

 the water, it tends to collect at the highest points of the line. Such 

 accumulations of air lessen the water-carrying capacity of the pipe 

 and may obstruct it altogether. To guard against occurrences of 

 this kind means should be provided to allow the entrapped air to 

 escape into the atmosphere. One of the safest and best means of 



Fig. 7. — Showins 



long screen at junction of open ditch and pipe line. 

 matic register at left of picture. 



Weir box and auto- 



doing this is by the insertion of standpipes at all points where air 

 is liable to collect. These standpipes need not be large for the 

 average sizes of pipe. They are not only automatic but continuous 

 in their action„ For lateral pipes a galvanized-iron pipe 1 inch 

 in diameter and covered with a perforated cap and protected by a 

 substantial post makes a cheap and serviceable air vent. It is also 

 good practice to get rid of the air as soon as it enters the pipe by 

 inserting standpipes near each intake, care being taken in all cases to 

 extend the air pipe far enough above the pipe so that its top will 

 be well above the hydraulic grade line, or in other words, above the 

 head to which water will rise. 



Where standpipes can not be used the confined air may be released 

 by the use of automatic air valves which can be inserted on pipes 

 under medium and high pressures. One of the simplest of these is 

 a spherical rubber ball which is pressed tightly against the circular 



