Collection Systems, Pumping, and Flow Measurement
Collection, pumping, and flow measurement
Gravity sewers must be self-cleansing. A minimum velocity of about 2 ft/s at design flow prevents deposition; velocities above 10-15 ft/s cause scour and hydrogen sulfide release. Manning equation links slope, roughness (n about 0.013 for new concrete, higher for old pipe), and hydraulic radius. Infiltration and inflow inflate flows during storms and can cause sanitary sewer overflows; smoke testing, dye testing, and flow monitoring locate the source.
Pump stations convert a pump curve and a system curve into an operating point. Best efficiency point sits near the middle of the curve. Affinity laws: flow varies with speed, head with speed squared, power with speed cubed - a 10% speed reduction cuts power about 27%. NPSH available must exceed NPSH required or the pump cavitates; symptoms are noise, vibration, and reduced flow.
Flow measurement: Parshall flumes and weirs in open channels, magnetic and ultrasonic meters in pipes. A submerged flume needs a correction; a weir needs a stilling well and free fall.
Traps: sizing a wet well without checking cycles per hour; using average flow instead of peak; assuming a pump runs at its rated point; and misreading weir or flume tables.
Worked example: a wet well 8 ft by 10 ft with 3 ft drawdown holds 240 ft3, about 1,795 gal. A 500 gpm pump with 250 gpm inflow empties in 1,795 / (500 - 250) = 7.2 min and refills in 7.2 min, a 14.4-min cycle - about 4 starts per hour, acceptable for most motors.