64 BULLETIISr 376, U. S. DEPARTMENT OF AGEICULTUEE. 



the water surface at the intake, gave the loss of head between the 

 intake and gauge No. 1. These losses are shown in column 4. 

 Column 7 gives the friction loss between a point 3 diameters down 

 the pipe from the intake and gauge No. 1. This column is based 

 upon the friction loss per foot within the pipe (column 5) multipUed 

 by the number of feet (column 6) back from gauge No. 1 to the 

 3-diameter point. Column 4 less column 7 gives the computed loss 

 of head (column 8) due to velocity and entry heads combined be- 

 tween the intake and the 3-diameter point. Theoretically column 8 

 should approximate column 11, which is the sum of columns 9 and 

 10, but in most cases the velocity of approach was sufficient to make 

 the entries in column 8 much smaller than those in column 11. 



Referring to these two colunms: Tests 1 and 2 were conducted on 

 a concrete pipe where the water entered a 16-inch standpipe from an 

 8-inch pipe and left the opposite side in an 8-inch pipe. The obser- 

 vations show that no head was lost within the standpipe. Tests 3 

 and 4 were on pipes in an installation similar except that the water 

 left the standpipe in a 12-inch pipe at right angles to an 8-inch pipe 

 through which it entered. Tests 5 and 6 were on the pipe shown in 

 Plate XII, figure 3. Here the velocity of approach in the canal 

 acted directly on the intake opening, greatly reducing the loss of 

 head. Tests 7 and 8 were on a similar pipe, but in this instance the 

 canal turned an abrupt right angle just before entering the pipe, 

 causing a violently turbulent condition which probably introduced a 

 large error in the observed head at the intake. 



AIR IN PIPE. 



In speaking of a pipe that did not show sufficient carrying capacity 

 Moritz states : ^ 



Examination showed that air imprisoned in the pipe was causing the difficulty. 

 This was overcome by inserting a |-inch wrought-iron standpipe in the top of the pipe 

 about 15 feet below the intake. In this way the air was, to all appearances, entirely 

 removed, and the carrying capacity was raised to 1.54 cubic feet per second, an 

 increase of about 60 per cent. 



Pipes taking water directly from reservoirs are, of course, not sub- 

 ject to these troubles, the depth above the intake being, as a rule, 

 sufficient to insure fiUing of the pipe with water alone. 



Siphon pipes and, in even greater degree, pipe chutes are often 

 reduced in carrying capacity by entrained air. In his investigations 

 on wood-stave pipes the writer has observed that air troubles are 

 minimized under the following conditions: (a) Low velocity in 

 channel approaching inlet; (b) inlet end set well below the hy- 

 draulic gradient, as a rule with top of pipe at about same elevation 

 as bottom of the canal above the inlet; (c) intake chamber designed 

 to minimize eddies. 



1 Trans. Amer. Soc. Civ. Engin., 74 (1911), p. 435. 



