Question

: During a heavy rainstorm, water from a parking lot completely fills an 18-in.- diameter, smooth, concrete storm sewer. If the flowrate is 10 ft3/s, determine the pressure drop in a 100-ft. horizontal section of the pipe. Repeat the problem if there is a 2-ft. change in elevation of the pipe per 100 ft. of its length.

Answer 1

diameter of parking lot = 18 in

flowrate = 10 ft³/s

pressure drop = 100 ft

using general equation

`V = (Q)/(A) = (10)/((\pi)/(4)((18)/(12))^2) = 5.66\ ft/s`

`\Delta P = \gamma (Z_2-Z_1) +(fLV^2)/(2\rho D)`

taking f = 0.0185

at Z₁ = Z₂

`\Delta P = (0.0185 * 100* 1.94* 5.66^2)/(2(18)/(12) (2))`

ΔP = 0.266 psi

b) when flow is uphill z₂-z₁ = 2

`\Delta P =62.4* 2 * (1)/(144) +0.266`

`\Delta P= 1.13\ psi`

c) When flow is downhill z₂-z₁ = -2

`\Delta P =62.4* 2 * (1)/(144) +0.266`

`\Delta P=-0.601\ psi`