66 BULLETIN 376, U. S. DEPARTMENT OF AGRICULTURE. 



FACTORS OF SAFETY. 



A study of Plate VII shows that a general formula may be often 

 10 per cent, sometimes as much as 15 per cent, and in isolated cases 

 25 per cent, at variance with observed capacities for given losses of 

 head. Likewise a study of the conditions holding in various pipes 

 fails to disclose just when high or low relative carrying capacities are 

 to be expected. However, the following factors of safety appear to 

 be warranted: 



Five per cent when only a rough approximation to the actual needs 

 of the pipe is possible; when water enters the pipe from a settling 

 reservoir and velocities in the pipe are so high that a clean-scoured 

 condition will always be present inside the pipe; and also where con- 

 ditions of operation are such that no penalties are attached to a 

 shght insufficiency of carrying capacity. 



Ten per cent when the above conditions for a very clean pipe are 

 assured, but where penalties are attached to lack of capacity; or 

 where no direct penalties are attached but silted waters and low 

 velocities may permit deposits and growths of Spongilla or other 

 vegetable life. 



Fifteen per cent where rock ravelings may reduce the interior area 

 of the pipe, or when penalties are attached and silted water or vege- 

 table growth are likely to cause excess retardation of flow. 



The designer may safely assume that the capacity of wood pipe will 

 not change unless the pipe is subject to silting, ravehngs, or vegetable 

 growth. (See fig. 5, p. 58.) 



ESTIMATE DIAGRAMS AND TABLE; SOLUTIONS FOR TYPICAL PIPE 



PROBLEMS. 



1. An inverted siphon is required to convey 60 second-feet of 

 water a length of 2,800 feet with an allowable total loss of head of 

 1.8 feet. Water has settled in a reservoir before entering the canal. 

 No direct penalty has been attached for lack of capacity. Required, 

 diameter of the pipe. 



Allowing a 5 per cent overload as a factor of safety, the rated 



capacity will be 60 + 3 = 63 second-feet. Since the velocity is not 



known, the entry and velocity head combined can not be determined 



at present. For preliminary figures, 2,800 feet = 2.8 X 1,000 feet; 



1 8 

 therefore 0^ = 0.642 =H. Referring to Plate X,^ enter diagram at 



63 second-feet. Intersection of Q = 63 with H = 0.642 is about on 

 the diameter line for 58 inches and at a velocity point of about 3.6 

 feet per second. Referring to Table 5, opposite V = 3.6 the combined 



1 Plate X was prepared by the writer from the new formula (No. 14, p. 7) in a manner similar to that 

 first used by Schoder in Engineering Record, Sept. 3, 1904. 



