How to Guide

42
Plastics Technical Manual
DESIGN & ENGINEERING DATA
Cast Iron .011 - .015
Finished Concrete .011 - .015
 Unnished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coefcient
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ow,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,owing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ow,ft.
2
V =FlowVelocity,ft/sec
Q =Volumeow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 =1/2
(
πdi
2
)
=π (4.02612)
2
=0.0442ft
2
4 2 x 4
V =3.2ft/sec
Q =0.0442x3.2=0.141ft
3
/sec
Q =0.141ft
3
x7.48galx60sec=63.5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