City of Morgan Hill Sewer System Master Plan 2017

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.

August 2017

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City of Morgan Hill Sewer System Master Plan