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 
slight 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, ravelings, 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 ^o = 0.642 = H. Referring to Plate X, 1 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 
i 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. 
