52 



BULLETIN" 852, U. S. DEPARTMENT OF AGRICULTURE. 



approaches the pipe outlet at a velocit}- relatively high compared 

 with the velocity in the open water below the outlet chamber. This 

 total may be expressed by the formula 



H e =7h v + 7b e +7b f -'h , v (20) 



where H e has the significance shown in figure 1 and Ji v , 7i e , and Ti f , 

 and Ji v ' have the significance defined on page 51. The influence of 

 gentle curves was included in the data upon which the formulas 

 were based, so that an additional loss for slight bends or curves need 

 not be considered in the design of the usual pipe on irrigation systems,, 

 If sharp bends can not be avoided, then an additional loss of head 

 must be anticipated. The results of such tests as have been made 

 on bends in pipes are given in standard works on hydraulics. 



VELOCITY AND ENTRY LOSSES. 



In designing pipes of small diameter and great length, the losses 

 due to velocity and entry heads, Ji v and h e , are so small compared 

 with the friction loss that they may be neglected. Otherwise they 

 should be included in the allowance for total lost head. 



It is best to consider the water above any intake as at rest. From 

 this state of rest velocity must be created and increased to the mean 

 velocity, V, existing in the pipe. The head, Ji v , necessary to create a 

 given velocity is shown in column 2, Table 5. The entry loss will be 

 from 0.5 Ti v where the pipe of standard size begins at a headwall and 

 is without bell or taper mouth, to about 0.25 Ti v , for a rounded intake, 

 and 0.05 Ji v for a bell-mouth intake. In Table 5, column 3, is 

 shown the amount of entry loss when taken as half the velocity 

 head (column 2), and the sum of the entry and velocity losses is 

 shown in column 4. 



Table 5. — Mean velocity in pipe V, in feet per second, and head of elevation lost creating 

 this velocity and overcoming entrance conditions, h v and h e , in feet. 



1 



2 



3 



4 



1 



2 



3 



4 



.V 



hv 



he 



hv+he 



V 



hv 



he 



hv+he 



Ft. per sec. 



Feet. 



Feet. * 



Feet. 



Ft. per sec. 



Feet. 



Feet. 



Feet. 



1.0 



0.010 



0.008 



0.024 



5.0 



0.389 



0. 195 



0.584 



.2 



.022 



.011 



.033 



.2 



.420 



.210 



.630 



.4 



.030 



.015 



.045 



.4 



.453 



.227 



.680 



.6 



.040 



.020 



.000 



.6 



.488 



.244 



.732 



.8 



.050 



.025 



.075 



.8 



.523 



.262 



.785 



2.0 



.062 



.031 



.093 



6.0 



. 560 



.280 



.840 



.2 



.075 



.037 



.112 



.2 



.598 



.299 



.897 



.4 



.090 



.045 



.135 



.4 . 



.637 



.319 



.956 



.6 



.105 



.053 



.158 



.6 



. 077 



.339 



1.016 



.8 



.122 



.061 



.183 



.8 



.719 



.359 



1.078 



3.0 



.140 



.070 



.210 



7.0 



.762 



.381 



1.143 



.2 



.159 



.080 



.239 



.2 



.806 



.403 



1.209 



.4 



.180 



.090 



.270 



.4 



.851 



.420 



1.277 



.6 



.202 



.101 



.303 



.6 



.898 



.449 



1.347 



.8 



.224 



.112 



.336 



.8 



.946 



.473 



1.419 



4.0 



.249 



.125 



.374 



8.0 



.995 



.498 



1.493 



.2 



.274 



. 137 



.411 



.2 



1.045 



.523 



1.568 



.4 



.301 



.151 



.452 



.4 



1.097 



.549 



■ 1.646 



.6 



.329 



. 165 



.494 



.6 



1.150 



.575 



1.725 



.8 



.358 



.179 



.537 



.8 



1.204 



.602 



1.806 



