210 HYDRAULICS AND ITS APPLICATIONS 



of the pipe vitally affect the resistance, and since the determination of the 

 relative physical condition of two surfaces, even though of the same 

 material, is a matter of the greatest difficulty, no pipe flow formula is to 

 be relied upon as giving the discharge corresponding to a given loss of 

 head, with any great degree of accuracy in a proposed pipe line. The 

 difficulty in exactly reproducing the conditions of the experiments from 

 which the formulae have been deduced prevents this, and the engineer who 

 prognosticates to within 10 per cent, the discharge from a pipe under 

 given pressure conditions has every reason to be satisfied with his choice 

 of constants. 



Velocities Adopted in Practice. In water supply systems the mean 

 velocity of pipe flow usually ranges from 3 feet to 6 feet per second. In the 

 pipe lines forming the supply and discharge mains of power installations 

 such velocities are often exceeded, velocities up to 10 feet per second being 

 common where such pipe lines are comparatively short, and velocities up 

 to 15 feet or even 20 feet per second being adopted in exceptional circum- 

 stances where the available head is great and the length of pipe line 

 comparatively small. 



ART. 66. FRICTION WITH FLUIDS OTHER THAN WATER. 



Very little is known as to the loss of head accompanying the flow of 

 liquids other than water. With crude oils the loss of head is very heavy 

 owing to the great viscosity. Attempts have been made to reduce this 

 loss of head by the addition of some 10 per cent of water. This reduces 

 the loss considerably but the discharge takes the form of an emulsion 

 from which the water is removed with difficulty. By using a pipe having 

 rifled grooves along its walls, centrifugal action keeps the heavier water 

 in contact with the walls, and reduces both emulsification and loss of 

 head. Tests made by the Southern Pacific Railroad Co., 1 give the 

 following values of C in the formula v = C \/mii 



8-in. plain pipe conveying oil C = 5*46 



8 90 % oil and 10 % water C = I'll 



8 rifled 90 % oil and 10 % C = 65 



8 pipe water only C= 113 



8 plain oil C= 3'76 



8 rifled 90 % oil and 10 % water C = 79 



8 pipe water only C = 100 



Water Charged with Sand. When a stream of water flows over a mass 



1 Engineering News, New York, June 7, 1906. Engineering Record, May 23, 1908. 



