You are trying to decide between using 21- or 24-in. pipes for a sewer that will be laid at a…
You are trying to decide between using 21- or 24-in. pipes for a sewer that will be laid at a slope of 0.004 with a Darcy-Weisbach roughness height of 0.0002 ft. Your design standards require that it be able to carry a peak flow of 14 ft3 /s, but the average flow will be about 5 ft3 /s and the minimum flow will be 2 ft3 /s. First, determine the full pipe capacity for each diameter. To do this, you will need to assume a flow, calculate the Reynolds number, determine f, and check if the hydraulic gradient slope is 0.004. You can use the Moody diagram, Colebrook-White equation, or Swamee-Jain equation. Iterate until you reach a solution. Use a kinematic viscosity of 1.0 × 10–5ft2 /s. (Alternatively, you can use a program such as Flow Master or Sewer CAD.)
From the answers in the previous table, you might think that a 21-in. pipe would not work. But remember that a pipe’s peak carrying capacity occurs when it is approximately 95 percent full. Using a hydraulic elements chart or a computer program, find the velocity and depth of flow at each of the flow rates given below. Assume that you can ignore any gradually varied flow effects (i.e., assume normal flow exists).
Given the points from the previous table (and the full pipe value), draw a graph with depth on the vertical axis and flow and velocity (for both pipe sizes) on the horizontal axis. At the lower flow rate, is there a significant difference in the velocity? Which size pipe would you recommend and why?
