How to Guide

42
Plastics Technical Manual
DESIGN & ENGINEERING DATA
Cast Iron .011 - .015
Finished Concrete .011 - .015
 6OmOJTIFE$PODSFUF 
 $PSSVHBUFE.FUBM 
Glass .009 - .013
Clay .011 - .017
Fluid Flow Properties
Gravity Flow
Manning Roughness Factor (“N” Value)
'MVJEWFMPDJUZQJQFTJ[FBOEIZESBVMJDTMPQFGPSHSBWJUZESBJOBHF
DBOCFEFUFSNJOFEVTJOHUIF.BOOJOHi/wWBMVF5IJTDPFGmDJFOU
relates to the interior wall smoothness of pipe and is used for
liquids with a steady flow, at a constant depth, in a prismatic
PQFODIBOOFM5IF.BOOJOHTFRVBUJPOJTTIPXOCFMPX
V 3

S

  /
8IFSF
V 7FMPDJUZPGnPXGUTFDPOE
N .BOOJOHTWBMVF
r IZESBVMJD SBEJVT GU PCUBJOFE CZ EJWJEJOH UIF DSPTT
sectional area of flow by the wetted perimeter of the pipe
in contact with the flow. R is a special case for v with
QJQFTFJUIFSGVMMPSGVMM
  3*OTJEFEJBNFUFSJOGFFU
S 6QTUSFBNFMFWBUJPO%PXOTUSFBNFMFWBUJPOGUGU
pipe length
Example 1:
wEJBNFUFSTDIFEVMF17$
flowing full 30 foot pipe run, 7.5 inch drop
4 wwwGUGU
30 ft.
3 wwGU
4
7 3

S

  /
.BOOJOHTi/wWBMVFJTHFOFSBMMZBDDFQUFEBTGPS
Designing gravity sewer systems
7 

(0.0208)

0.009
7 GUTFDPOE
Example 2:
wEJBNFUFSTDIFEVMF"#4nPXJOHGVMM
10 foot pipe run, 1.5 inch drop
4 wwwGUGU
10 ft.
3 wwGU
4
"TTVNFi/wUPCF
7 

(0.0125)

0.010
7 GUTFDPOE
Fluid Flow Rate
Calculation of Volume Flow Rate:
QB7
8IFSF
a $SPTTTFDUJPOBMBSFBPGnPXGU
2
V 'MPX7FMPDJUZGUTFD
Q 7PMVNFnPXSBUFGU
3
TFD
Example 1:
w4DIFEVMF17$
8IFSF
EJJOTJEFEJBNFUFSPGQJQFJOJODIFT
B di
2
 (2.06712)
2
GU
2
4 4
7 GUTFD
2 YGU
3
TFD
2 GU
3
YHBMYTFDHBMT
sec ft
3
min min
Example 2:
w4DIFEVMF17$
B (di
2
)  (4.02612)
2
GU
2
4 4
7 GUTFD
2 YGU
3
TFD
2 GU
3
YHBMYTFDHBMT
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.
-BCPSBUPSZUFTUTIBWFTIPXOUIBUUIFi/wWBMVFGPS"#4BOE
PVC pipe ranges from .008 to .012. The table below shows
i/wWBMVFTGPSPUIFSQJQJOHNBUFSJBMT
“N” Values For Typical Piping Materials
Piping Material “N” Values