Gravity Sewer Collection System Design ‐ Design Criteria ‐ FLOW • Usually we design for Q/Qfull to be not more than 91% or d/dfull to be not more than 75%
In normal practice, the ground slope is used as a first trial for selecting the slope of the pipe. However, there are number of exceptions: A) if the ground is flat, select the minimum slope to achieve the minimum velocity with the sewer flowing pipe B) If the ground slope yields a velocity greater than the maximum velocity (2.5 m/sec), the select a lower slope and drop manholes could be used. C) Because LS & PS are expensive to build, operate and maintain, avoid them as much as possible.
Sewer Layout
Flow
Select Ø and slope that will carry flow while flowing full
Is pipe Ø<200mm?
1
YES
Sewer Design Flow Chart for selecting pipe size Select 200mm
Select pipe Slope = Ground Slope
Is slope
Select Min. Slope
Check velocity for design flow Is Velocity> Min. Velocity
YES STOP
NO Increase Slope
YES
Is increase in slope reasonable?
YES NO
Is pipe Ø>200mm? NO
STOP Increase Pipe size
1
Example 1 Determine the velocity of 0.0081 m3/sec flow in a 200 mm diameter sewer at a slope of 0.004 m/m. The pipe is VCP. Solution n for VCP = 0.013 For full flowing pipe; R = D/4 Vfull = 1/n R2/3 S0.5 = 1/(0.013) * (D/4)2/3 (0.004)0.5 = 0.66m/sec A full = πD2/4 = π* (0.2)2/4 Qfull = A * Vfull = 0.0208 m3/sec Q/ Qfull = 0.0081/0.0208 = 0.39 From chart V/Vfull = 0.9 V = 0.9 * 0.66 = 0.6 m/sec
Example2: For population = 50000 persons and sewer flow Rate = 160 L/cap/d Design the dia. of the sewer pipe when dmax /D = 0.67 . Assume Peak Factor = 2 S = 0.004 Solution • Qav = 160 * 50 000 = 8000 000 L/d = 0.093 m3 / sec . • Qmax = PF * Qav = 2 * 0.093 = 0.186 m3 / sec . • d max / D= 0.67 from chart Qmax / Qfull = 0.78 & Vmax / Vfull= 1.11 • Q full = Qmax / 0.78 = 0.186 / 0.78 = 0.238 m3/ sec • S = 0.004 • Qfull = A/n R2/3 S0.5 = A/(0.013) * (D/4)2/3 (0.004)0.5 • D = 499 mm = 500mm
Example3: Design a VCP sewer to connect 2 businesses to an existing sewer main at MH No.48 at an invert elevation of 177.98m. The distance between manholes is marked on the below plan. The peak flow rates for Koffee Au Noir and Elsie’s Kreamer are 20.2 l/sec and 14.5 l/sec respectively.
