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 anv time. If there 
is a Clear Space above FlG " 35 — ° V ! rfl0W di 7 str / buti ^ Wrant. Wasteway Pipe 
1 set on angle to prevent air troubles. 
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 
