Low Pressure at the End of the Line After a New Subdivision Ties In
A 12-inch transmission main fed by a 500 gpm booster station runs 2.1 miles to a pressure zone boundary. A new 140-lot subdivision was connected 900 feet upstream of the zone's dead-end, adding an estimated peak-hour demand of 210 gpm. Since startup, customers at the dead-end report low pressure at upstairs fixtures in the early evening; a logging gauge on the dead-end hydrant shows static 62 psi and a residual that sags to 38 psi during the 6-8 pm peak. The booster discharge pressure is unchanged at 78 psi. A 6-inch cast iron loop that once served the area was abandoned in place years ago and is not connected. Records show the transmission main is 30-year-old cement-lined steel with a C-factor assumed at 130 in the model, but no field test has been run since installation.
1. Identify the most likely hydraulic causes of the sagging residual and describe how you would confirm each before recommending capital work.
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The sag from 62 to 38 psi during peak is a friction-loss symptom, not a source or booster problem, because the booster discharge is steady at 78 psi. Two candidates dominate.
- Understated pipe roughness. The model uses C=130, a new-pipe value. Thirty-year-old lined steel is more likely C=100-110. Confirming: run a field C-factor (head-loss) test on a measured reach using two pressure loggers and a known flow (hydrant or inline meter), or a step test. Recompute head loss with Hazen-Williams: for the same Q, dropping C from 130 to 105 raises loss by roughly (130/105)^1.85 ≈ 1.5x.
- Added demand concentrated near the dead-end. 210 gpm at peak on a long, single-path main compounds velocity and loss. Confirm with a 24-hour logging program at the dead-end hydrant plus a system-wide demand review, and compare measured flow to model-assigned demand.
Also verify the abandoned 6-inch loop is truly isolated (it is not a path, so it cannot be relieving the dead-end) and check for a partly closed valve or a tuberculated fitting between the tie-in and the dead-end. Only after the field C-test and demand reconciliation should you size relief options (looping, upsizing the last reach, or a local booster).
2. The utility wants a quick fix: raise booster discharge pressure by 10 psi. Evaluate this against the confirmed causes and state what you would recommend instead.
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Raising the booster 10 psi raises static pressure everywhere downstream, but the problem is loss during flow, and loss scales with Q^1.85 / C^1.85. A 10 psi bump buys roughly 10 psi at the dead-end at peak - enough to mask the symptom today - but it:
- increases pressure on the whole zone, raising stress on joints and service lines (check the MAOP and the low-lying areas, which could exceed safe working pressure);
- does nothing about the real loss driver (roughness and the long single path);
- leaves no margin as the subdivision builds out.
Recommendation: first complete the field C-test and demand reconciliation. Then, in order of cost-effectiveness, evaluate (a) restoring/looping the abandoned 6-inch main to create a second path and cut velocity in the last reach, (b) upsizing or replacing the worst reach if the C-test confirms high loss, and (c) only as a stopgap, a modest pressure increase documented against MAOP. Any pressure increase must be justified by the model re-run with the measured C-factor, not the assumed 130.