Low Pressure at the Top of the Zone After a Main Replacement
A 12-inch cast iron main on Elm Street was replaced with 12-inch cement-lined ductile iron over a 1,400-foot stretch. The old main had a measured Hazen-Williams C of 78; the new main is assumed at C = 140. Two weeks after the tie-in, customers on a dead-end loop fed through this main report low pressure at the second-floor fixtures during the 7-8 a.m. peak. The system's elevated tank floats at 212 ft. The Elm Street main carries 1,850 gpm at peak. A pressure logger at the loop's far end read 44 psi at 7:30 a.m., down from 52 psi before the project. No main breaks, no new meters, and no pump or valve changes occurred. The crew recalls that the old 8-inch loop main on the dead end was abandoned in place but not cut and plugged at both ends.
1. Compute the friction loss through the Elm Street main before and after replacement and determine whether the main replacement itself explains the pressure drop.
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Use Hazen-Williams in the form hL = 4.727 × L × Q^1.852 / (C^1.852 × d^4.871), with Q in cfs and d in ft.
Q = 1,850 gpm = 4.122 cfs; L = 1,400 ft; d = 1.0 ft.
Old (C = 78): C^1.852 ≈ 78^1.852 ≈ 2,860. hL = 4.727 × 1400 × 4.122^1.852 / (2,860 × 1) . 4.122^1.852 ≈ 13.6. Numerator ≈ 4.727 × 1400 × 13.6 ≈ 90,000. hL ≈ 90,000 / 2,860 ≈ 31.5 ft ≈ 13.6 psi.
New (C = 140): 140^1.852 ≈ 8,600. hL ≈ 90,000 / 8,600 ≈ 10.5 ft ≈ 4.5 psi.
So the replacement should have reduced loss by about 9 psi at the far end, not raised it. The observed 8 psi drop is opposite in sign and cannot be attributed to the new main's roughness. Something else - likely a closed or partially closed valve, an air pocket, or the abandoned 8-inch loop now acting as a dead leg with a closed plug creating a hydraulic bottleneck - is causing the loss. Investigate before assuming the pipe is at fault.
2. The abandoned 8-inch loop was left in place without plugs. Explain how this could degrade pressure and water quality at the dead end, and what you would do.
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An abandoned main left connected at one end becomes a stagnant dead leg. It does not directly add friction to the through-path, but it creates three problems: (1) stagnant water sits for days, losing residual chlorine and accumulating sediment and biofilm, which shows up as taste/odor and coliform risk at the dead-end taps; (2) the dead leg can trap air that migrates into the active main and causes air-binding or erratic pressure at high flow; (3) if the old loop is still connected at both ends but valved off mid-run, a partially closed valve can throttle the only path.
Actions: isolate and plug or cut-and-cap the abandoned main at both ends per the utility's abandonment standard; flush and disinfect the dead end; then re-log pressure at 7:30 a.m. to confirm the drop was hydraulic, not roughness. If pressure remains low, do a valve survey (open/closed status of every valve between the tank and the loop) and check for a stuck check valve or a partially open gate.
3. If the true cause turns out to be increased demand on the loop (new connections), what hydraulic remedy would you evaluate first, and what data would you need?
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First remedy to evaluate: loop the dead end (connect it to a second feed) so flow splits and velocity drops - this reduces friction loss far more than upsizing a single feed. Second: raise the tank float level or add a booster if the elevation head is the binding constraint.
Data needed: peak-hour demand at each service on the loop (meter records), the loop's actual C after cleaning, valve status, the elevation of the lowest-pressure fixture, and a calibrated hydraulic model run at peak hour. Without those, any fix is a guess.