LOSSES AT VALVES 



249 



formation of a vena contracta and the subsequent re-enlargement of the 

 stream to fill the pipe. This reduces the energy available for producing 

 motion along the pipe and gives a 

 reduced flow with the same total 

 head. 



(2) Frictional Losses in the Pipe. 

 These have been considered in 

 pp. 195210. 



(3) Losses at Valves, etc. The loss 

 of energy caused by the presence of 

 a valve or sluice in a pipe line may 

 be looked upon as being due to the 

 sinuous motion set up by the expan- 

 sion of the stream to fill the pipe, 



after its contraction in passing the valve and, by an application of 

 the formulae deduced on pp. 8394, this loss can, except for irregularities 

 in design which themselves cause sinuous motion, and which may, even 

 when the valve is wide open, cause a loss of head amounting to as much 

 as from one to ten times v* -=- 2 </, be approximately determined. 



Loss caused by Sluice in Rectangular Pipe (Fig. Ilia). Depth of pipe D. 

 Depth of sluice opening d. 1 



<D 



FIG. ill. 



Here contraction is prevented in three directions, and for small 

 openings the result of this is marked by the reduction in the value 

 of F. 



Gate Valve in Circular Pipe (Fig. Ill b). Let h = ratio of height of 

 opening to diameter of pipe. 



Loss = F * -v- 2 g feet. Results due to Weisbach. 



