30 
BULLETIN 906, U. S. DEPARTMENT OF AGRICULTURE. 
opening when the pipe is full of water, but which falls down when 
the water is lowered by the collection of air. Another kind operates 
by means of a float and lever, the lowering of the float causing the 
air valve to open and discharge the entrapped air. 
RELIEF STANDS. 
Where steel or wood pipe is used to carry irrigation water, it is not 
customary to make provisions for relief of pressure except in long 
lines that are laid on heavy grades. Concrete pipe, however, unless 
reinforced is only intended for 
low heads and provision must 
be taken to prevent high pres- 
sures. 
Where a pipe is laid down 
a long grade, and is large 
enough to carry all the water 
running less than full, there 
will be little or no head on 
the pipe when outlets are pro- 
vided at frequent intervals 
with their tops always open, 
but as it is usual to provide 
shut-off gates to divert water, 
pressures may increase to such 
an extent as to break the pipe 
when all gates are closed un- 
less proper precautions are 
taken to guard against such 
conditions arising. 
The ordinary open diversion 
box is fitted with gates that 
divert water by closing the 
gate on the outlet pipe only, the gate being kept tight by water 
pressure against it. In this case the water can not back up in the 
pipe and form a dangerous head, since the water in the box will 
overflow, but where the pipe will withstand 20 to 30 feet head, 
and it is desirous to force water along a lateral that runs up 
hill 15 feet or more, it is not feasible to construct the ordinary 
diversion box, as such a box would have to be too high to be con- 
veniently accessible. In this case, pressure gates may be provided 
that close the inlet pipe providing there is an open stand above the 
gate that will relieve the pressure. Another plan is to provide an 
overflow stand that will maintain a constant head at the point of 
diversion, the excess water spilling over into the main. Two vertical 
Overflow and relief stand. 
