Sewage Force-Main Scour Velocity
V = 0.4085 Q / d^2 (ft/s, gpm, in), vs the ~2 ft/s minimum scour to keep solids suspended. 50 GPM in a 2 in main -> 5.11 ft/s (scours); the largest ID still holding 2 ft/s is 3.20 in, so a 4 in main (1.28 ft/s) lets solids settle. Ten States Standards; the state criteria and pump curve govern.
Formula and source
V = 0.4085 Q / d^2 (ft/s, Q gpm, d in); d_max = sqrt(0.4085 Q / 2); scours when V >= 2 ft/s.
The force-main scour-velocity criterion (about 2 ft/s minimum) from the Ten States Standards (Recommended Standards for Wastewater Facilities), by name.
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