"USE OF CONCRETE PIPE IN" IRRIGATION. 



51 



The greatest difficulty with overflow relief standpipes and over- 

 flow orchard hydrants is the trouble caused by entrapped air. If 

 the sheet of water flowing into the outlet stand entirely covers the 

 opening into the underground pipe air is carried down with the sheet 

 of water and is not able to escape, due to the aperture being closed 

 with water. In many cases more than three or four overflow hy- 

 drants can not be operated at one time, due to air trouble. If 

 the hydrants or relief stands are fitted with gates at the connection 

 to the feed pipe and the valve opening is large enough to take the 

 full head of water, there will be little trouble with air. Such valves 

 are sometimes difficult 

 to adjust, however, 

 especially where a 

 portion of water is to 

 be distributed 

 through the hydrant. 



The way to pre- 

 vent air troubles in 

 this type of orchard 

 hydrant is to con- 

 struct the outlet from 

 the spillway large 

 enough so that the 

 entire space will not 

 be filled with water 

 at any time. If there 

 is a clear space above 

 the outlet feed pipe 

 and the aperture from valve to feed pipe is never entirely covered 

 with water, air will be able to escape. 



The spillway pipe can also be set at an angle so that water wasting 

 into the outlet pipe will never entirely fill it (fig. 35). If the spill- 

 way pipe is of small diameter and set inside a large diameter pipe 

 it is necessary to have the outside pipe very much larger than the 

 spillway pipe. The Lemon Heights Land Company near San Diego 

 has a large number of overflow stands that are made by using the 

 main hydrant of 20-inch pipe and the spillway of 4-inch pipe. The 

 spillway pipe was originally set vertically, but there was so much 

 trouble from air that all stands were changed by setting the spillway 

 at an angle of about 45 degrees with the vertical. A notch was cut 

 in top of the spillway pipe so that water would flow through it with- 

 out entirely filling it. There has been no trouble from air since the 



Fig. 35. — Overflow distributing hydrant. Wasteway pipe 

 set on angle to prevent air troubles. 



