Use and Care Guide

21
Piping Material “N” Values
“N” Values For Typical Piping Materials
Cast Iron .011 - .015
Finished Concrete .011 - .015
Unfinished Concrete .013 - .017
Corrugated Metal .021 - .027
Glass .009 - .013
Clay .011 - .017
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
DESIGN AND ENGINEERING DATA
Self-cleansing Velocities
Table 1 on the following page indicates the slopes
required to obtain self-cleansing or scouring velocities
at various rates of discharge. A self-cleansing velocity,
or one sufficient to carry sewage solids along the pipe,
permits the system to operate efficiently and reduces the
likelihood of stoppages. A minimum velocity of 2 feet
per second is the generally prescribed norm consistent
with the removal of sewage solids, but a velocity of 2.5
feet per second can be used in cases where an additional
degree of flow is desired. In addition to designing self-
cleansing velocities into sanitary sewers, it is considered
good practice to impose an upper-velocity limit of 10
feet per second in both sewers and drains. This restricts
the abrasive action of sand and grit that may be carried
through the system. However, because cast iron soil pipe
is highly resistant to abrasion, it is most suitable for use
where high-velocity operation cannot be avoided.
Design of Sewer and Drains
Table 2 on the following pages provides data to help
determine the adequate size of the cast iron soil pipe to
accommodate the expected peak flow at a designed, self-
cleansing velocity. The peak flow that governs design is
that projected to occur in the future during the service
life of the particular system.
The factors affecting peak flow vary with the type of
system to be installed. In a sanitary sewer for domestic
waste, the maximum quantity of sewage depends
primarily upon the density and distribution of the
population and its per capita use of water. In a sewer
for commercial and industrial waste, it depends on the
number and type of businesses to be serviced by the
system. The peak load in a storm sewer, on the other
hand, is determined by the duration and intensity of
rainfall and the extent, condition and slope of streets
and other areas requiring drainage.
For a particular hydraulic system, the factors affecting
peak flow are analyzed by means of procedures in design
handbooks. Unfortunately, this analysis is generally
imperfect from the standpoint of system design. In most
cases, current peak flow can be accurately quantified,
but only a rough approximation can be made of future
peak flow, which is usually based on population trends
and area development over a period of fifty or so years.
This requires that provisions be made for any unforeseen
increase in runoff, and therefore, cast iron soil pipe
hydraulic systems are most frequently designed for half-
full operation at probable future peak flow. Greater or
less than half-full operation can be employed, depending
on design requirements and the relative accuracy with
which future flow can be forecast.