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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 



Fig. S. — Overflow and relief stand. 



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 



