Low Pressure at the End of a Dead-End Main
A 12-inch ductile iron main runs 4,200 ft down a commercial corridor at a steady grade. At its terminus, a 6-inch dead-end lateral serves four customers, the last of which is a restaurant at 3,600 ft from the main. The restaurant reports intermittent low pressure during lunch service; a gauge at the meter reads 38 psi at 6 a.m. and 22 psi at noon. System pressure at the main tap is a steady 68 psi. The dead-end lateral has no blow-off, and the last 1,800 ft was installed in 1958. A hydrant flow test at the lateral's midpoint showed a static pressure of 60 psi and a residual of 41 psi at 1,100 gpm. The utility's minimum service pressure standard is 35 psi under peak demand.
1. Diagnose the most likely cause of the noon pressure drop and rank the candidate causes using the flow-test data.
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The flow test is the key discriminator. A static-to-residual drop of 60 to 41 psi at 1,100 gpm indicates a system that can deliver flow but with substantial head loss - consistent with a long, undersized, likely tuberculated lateral rather than a collapsed or fully blocked pipe (which would show near-zero flow).
Ranking:
- Friction loss in the 6-inch lateral under peak demand - the primary cause. The 6-inch line is long and old; internal tuberculation in 1958-era unlined iron can cut effective capacity sharply. The fact that pressure recovers overnight (low demand) and falls at lunch (high demand) tracks demand, not a static defect.
- Elevation - a candidate only if the restaurant sits materially above the tap; verify with survey before committing capital.
- Partially closed or failing valve - possible; check valve position and operating nut condition.
- Main tap or meter restriction - less likely given the healthy residual at the midpoint.
The 22 psi at noon is below the 35 psi standard, so this is a compliance issue, not just a complaint.
2. The utility can fund one project this year. Recommend the corrective action and justify it against the alternatives.
Read the worked answer
Recommend replacing or rehabilitating the 6-inch lateral with 8-inch (or upsizing the critical segment) and adding a blow-off at the terminus.
Justification:
- The flow test shows the source main and mid-lateral can move water; the bottleneck is downstream friction and the dead-end configuration. Upsizing reduces velocity and friction; a blow-off allows flushing of tuberculation and sediment and improves turnover.
- Alternative: pressure-reducing/booster station - a booster would mask the problem, add energy and maintenance burden, and could create unintended pressure zone interactions. Not preferred for a single dead-end.
- Alternative: loop the dead end - hydraulically superior but likely far more expensive and may not be feasible through private property.
- Alternative: only flush - cheap but temporary; tuberculation returns.
Before construction, confirm elevation difference and verify the tap and meter are not restricted, so the capital is spent on the true bottleneck. Document pre- and post-project pressures to demonstrate return to the 35 psi standard.