Determine Sewer Pipe Size

The minimum size of underground sewer pipe, branch or drain hub, should be 4 in. The minimum size of the sewer main (collecting two or more 4-in sewer branches or drain hubs) should be 6 in. The minimum size of the sewer pipe in any curbed or diked area should be 8 in.

Sewer size depends on plot size, amount of rainfall, and quantity of process waste, fire water, and any other liquids requiring disposal. As previously stated, the sewer mains should be sized to include the estimated flow of future expansion.

Velocities used in sewer system design should have a minimum of 3 ft per sec and a maximum of 7 ft per sec. Flow capacities, velocities, and slopes (for sewers running f full) can be coordinated so that the curves shown in Figure 7.10.4 give the sewer line size required for the slope and velocity for the flow capacity in gallons per minute.

The flow capacities, velocities, and slopes of sewer lines are contingent on plant site grades available. The flat grades necessary at most plant sites are a determining factor in the slope and velocity of the sewer lines. Where possible, and if gradients permit, the maximum velocity should be used.

The following design example determines the sublateral size for a sewer system:

Given: Rainfall = 3 in/hour Process waste = 100 gpm Fire water = 250 gpm per catch basin Runoff in unpaved areas = 40% Computations: Storm water

Area involved: Paved, 2 areas, 3280 sq ft total.

Unpaved, 3 areas, 6920 sq ft total.

Process Waste + Fire Water = 100 + (4*250) = 1100 gpm

Using 1100 gpm and the sewer sizing chart (see Figure 7.10.4) shows that a 10-in line sloped .017 ft/ft with a flow velocity of 4.4 ft/sec can handle this quantity.

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