How to Guide

47
DESIGN & ENGINEERING DATA
Cast Iron .011 - .015
Finished Concrete .011 - .015
Unfinished Concrete .013 - .017
Corrugated Metal .021 - .027
Glass .009 - .013
Clay .011 - .017
Fluid Flow Properties
Gravity Flow
Manning Roughness Factor (“N” Value)
Fluid velocity, pipe size and hydraulic slope for gravity drainage
can be determined using the Manning “N” value. This coefficient
relates to the interior wall smoothness of pipe and is used for
liquids with a steady flow, at a constant depth, in a prismatic
open channel. The Manning’s equation is shown below:
V = 1.486 R
2/3
S
1/2
N
Where:
V = Velocity of flow, ft./second
N = Manning’s value
r = hydraulic radius, ft. obtained by dividing the cross
sectional area of flow by the wetted perimeter of the pipe
in contact with the flow. R is a special case for v with
pipes either 1/2 full or full:
R= Inside diameter / 4, in feet
S = Upstream elevation - Down stream elevation (ft./ft.)
pipe length
Example 1:
2” diameter Schedule 40 PVC,
flowing full 30 foot pipe run, 7.5 inch drop
S = 17.5”-10.0” / 12” = 0.0208 ft./ft.
30 ft.
R = 2.067” / 12” = 0.043 ft.
4
V = 1.486 R
2/3
S
1/2
N
Manning’s “N” value is generally accepted as 0.009 for
Designing gravity sewer systems
V = 1.486 (0.043)
2/3
(0.0208)
1/2
0.009
V = 2.9 ft./second
Example 2:
4” diameter Schedule 40 ABS, flowing 1/2 full
10 foot pipe run, 1.5 inch drop
S = 20”-18.5” / 12” = 0.0125 ft./ft.
10 ft.
R = 4.026” / 12” = 0.0839 ft.
4
Assume “N” to be 0.010
V = 1.486 (0.0839)
2/3
(0.0125)
1/2
0.010
V = 3.2 ft./second
Fluid Flow Rate
Calculation of Volume Flow Rate:
Q = aV
Where:
a = Cross sectional area of flow, ft.
2
V = Flow Velocity, ft/sec
Q = Volume flow rate, ft
3
/sec
Example 1:
2” Schedule 40 PVC
Where:
di=inside diameter of pipe in inches
a = πdi
2
= π (2.06712)
2
= 0.0233 ft
2
4 4
V = 2.9 ft/sec
Q = 0.0233 x 2.9 = 0.0676 ft
3
/sec
Q = 0.0676 ft
3
x 7.48 gal x 60 sec = 30.3 gals
sec ft
3
min min
Example 2:
4” Schedule 40 PVC
a = π(di
2
) = π (4.02612)
2
= .088 ft
2
4 4
V = 3.2 ft/sec
Q = .088 x 3.2 = 0.283 ft
3
/sec
Q = 0.283 ft
3
x 7.48 gal x 60 sec = 127 gals
sec ft
3
min min
It is widely recommended that the flow velocity in sanitary
sewer systems to be equal to or greater than 2.0 feet per second
for self cleaning drain lines.
Laboratory tests have shown that the “N” value for ABS and
PVC pipe ranges from .008 to .012. The table below shows
“N” values for other piping materials.
“N” Values For Typical Piping Materials
Piping Material “N” Values