. Bulletin of the Department of Agriculture. Agriculture; Agriculture. "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 hydra
. Bulletin of the Department of Agriculture. Agriculture; Agriculture. "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
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