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August 2017
3-5
City of Morgan Hill
Sewer System Master Plan
Minimum Pipe Sizes and Design Velocities
In order to minimize the settlement of sewage solids, it is standard practice in the design of gravity
sewers to specify that a minimum velocity of 2 feet per second (fps) be maintained when the
pipeline is half-full. At this velocity, the sewer flow will typically result with self-cleaning of the pipe.
Due to the hydraulics of a circular conduit, velocity of half-full flows approaches the velocity of
nearly full flows.
Table 3.1
lists the minimum slopes, varying by pipe size, in accordance with the
City’s design standards. The design standards also specify minimum pipe sizes, depending on
the peak dry weather flows, as shown on
Table 3.1
.
Changes in Pipe Size
When a smaller gravity sewer pipe joins a larger pipe, the invert of the larger pipe is generally to
maintain the same energy gradient. One of the methods used to approximate this condition
includes placing the 80 percent depth point (d/D at 0.8) from both sewers at the same elevation.
For master planning purposes, and in the absence of known field data, sewer crowns were
matched at the manholes.
3.1.2
Force Mains and Lift Stations
The Hazen-Williams formula is commonly used for the design of force mains as follows:
x
Hazen Williams Velocity Equation:
V = 1.32 C R
0.63
S
0.54
Where:
V = mean velocity, fps
C = roughness coefficient
R = hydraulic radius, ft
S = slope of the energy grade line, ft/ft
The value of the Hazen-
Williams ‘C’ varies and depends on the pipe mat
erial and is also
influenced by the type of construction and pipe age. A ‘C’ value of 11
0 was used in this analysis.
The minimum recommended velocity in force mains is at 2 feet per second. The economical
pumping velocity in force mains ranges between 3 and 5 fps. A maximum desired velocity is
typically around 7 fps and a maximum not-to-exceed velocity is at 10 fps.
The capacities of pump stations are evaluated and designed to meet the peak wet weather flows
with one standby pump having a capacity equal to the largest operating unit. The standby pump
provides a safety factor in case the duty pump malfunctions during operations and allows for
maintenance.
3.2 DRY WEATHER FLOW CRITERIA
Sewer unit flow factors are coefficients commonly used in planning level analysis to estimate
future average daily sewer flows for areas with predetermined land uses. The unit factors are
multiplied by the number of dwelling units or gross acreages for residential categories, and by the
gross acreages for non-residential categories, to yield the average daily sewer flow projections.