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Appendix K
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Flow In
Sloping Drains
2009 National Standard Plumbing Code
Copyright 2009 Plumbing-Heating-Cooling Contractors--National Association
293
Flow in Sloping Drains
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Tables K-1 and K-2 list flow rates in gallons per minute and velocities in feet per second for various size
drains at various slopes. Table K-l is based on fairly rough pipe. Table K-2 is based on smooth pipe.
The minimum flow velocity to achieve scouring in horizontal sanitary drain lines is two (2) feet per second.
For this reason, based on Table K-2 for smooth pipe, drains that are 2 inches and smaller must be sloped at
not less than 1/4 inch per foot. Drains that are 3" size and larger can be sloped at 1/8 inch per foot.
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Even at 1/4" per foot slope, the uniform velocity in drains that are 1-1/4", 1-1/2", and 2" size is less than 2
feet per second. Either the slope should be increased or the length of such drains should be kept to a minimum so that the entrance velocity will provide scouring for the short distance involved.
or
D
Tables K-1 and K-2 are based on the Manning Formula for 1/2 full pipe. For full flow, multiply the flow by
2.00 and the velocity by 1.00. For 1/4 full flow, multiply the flow by 0.274 and the velocity by 0.701. For 3/4
full flow, multiply the flow by 1.82 and the velocity by 1.13.
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In Table K-l, which is based on fairly rough pipe with "n" = 0.015, for smoother pipe, multiply the flow and
velocity by 0.015 and divide by the "n" value of the smoother pipe.
No
Horizontal sanitary drain and waste pipes are sized to be one-half (1/2) full under design loads. Horizontal
storm drains are sized to run full under design loads.
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Table 11.5.1A Building Drains and Sewers and the horizontal piping in Table 11.5.1B "Horizontal Fixture
Branches and Stacks" is based on Table K-2 for smooth pipe.
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Table 13.6.2 "Size of Horizontal Storm Drains" is based on Table K-1 for fairly rough pipe, adjusted for full
flow.
294
2009 National Standard Plumbing Code
Copyright 2009 Plumbing-Heating-Cooling Contractors--National Association
1/16 in./ft. slope
Pipe Size
(inches)
Flow
gpm
Vel.
fps
1.250
2.40
1.25
1.375
3.10
1.35
1.500
3.90
1.625
4.83
2
8.41
Vel.
fps
1.60
26.7
1.36
37.7
1.92
5
48.4
1.58
68.4
2.23
6
78.7
1.79
111
2.53
8
169
2.17
240
10
307
2.51
435
12
500
2.84
707
15
906
3.29
1281
24.8
Vel.
fps
3.40
1.77
4.38
1.91
1.41
5.52
1.99
1.50
6.83
2.12
1.73
11.9
2.44
2.26
35.0
3.20
53.4
2.72
75.5
3.84
96.8
3.16
137
4.47
157
3.57
223
5.05
3.06
339
4.33
479
6.13
3.55
615
5.02
869
7.10
4.01
999
5.67
1413
8.02
4.66
1812
6.59
2563
9.32
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17.5
4
Flow
gpm
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Flow
gpm
1/2 in./ft. slope
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Vel.
fps
1/4 in./ft. slope
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Flow
gpm
1/8 in./ft. slope
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Table K-1
APPROXIMATE FLOW RATES AND VELOCITIES IN SLOPING DRAINS
(FOR FAIRLY ROUGH PIPE n = 0.015)
Flowing Half Full
2009 National Standard Plumbing Code
Copyright 2009 Plumbing-Heating-Cooling Contractors--National Association
295
Table K-2
APPROXIMATE FLOW RATES AND VELOCITIES IN SLOPING DRAINS
(FOR SMOOTH PIPE n = 0.011)
Flowing Half Full
Flow
gpm
Vel.
fps
1.250
3.27
1.71
1.375
4.22
1.84
1.500
5.32
1.625
6.86
2
11.5
3
Flow
gpm
Vel.
fps
23.9
2.18
33.8
1/2 in./ft. slope
Flow
gpm
Vel.
fps
4.63
2.42
5.97
2.60
1.92
7.53
2.72
2.04
9.32
2.89
2.35
16.2
3.33
3.08
47.8
4.36
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Vel.
fps
1/4 in./ft. slope
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Flow
gpm
1/8 in./ft. slope
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1/16 in./ft. slope
Pipe Size
(inches)
36.4
1.85
51.5
2.62
72.8
3.71
103
5.24
5
66.0
2.15
93.3
3.05
132
4.31
187
6.09
6
107
2.44
152
3.44
215
4.87
303
6.89
8
231
2.95
327
4.18
462
5.91
654
8.36
10
419
3.42
593
4.84
838
6.84
1185
9.68
12
681
3.87
964
5.47
1363
7.73
1927
10.94
15
1236
4.49
1747
2471
8.99
3495
12.71
No
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6.35
296
2009 National Standard Plumbing Code
Copyright 2009 Plumbing-Heating-Cooling Contractors--National Association