Distribution System Pressure Complaint and a Closed Valve
A pressure zone serving 1,100 connections is fed by a 12-inch transmission main from a 0.5-MG ground storage tank with a booster pump station (two 1,200 gpm pumps, one lead/one lag). Normal zone pressure is 65–75 psi. Over two weeks, customers at the far end of the zone (about 9,000 feet from the tank) report low pressure - some as low as 28 psi at the tap during morning peak. The SCADA trend shows the booster station discharge pressure is steady at 80 psi, but the far-end pressure drops 30 psi below the station during peak. A new subdivision of 150 homes was connected six months ago at the far end. Fire flow tests at a hydrant near the new subdivision show only 600 gpm at 20 psi residual, versus a design requirement of 1,000 gpm. The state requires a minimum 20 psi under normal conditions and 20 psi residual during fire flow.
1. Calculate the approximate headloss between the booster station and the far end during peak, and list the likely causes of the pressure drop in order of probability.
Read the worked answer
Pressure drop: station discharge 80 psi, far end 28 psi → headloss ≈ 52 psi. Convert to feet: 52 psi × 2.31 ft/psi ≈ 120 feet of head over 9,000 feet of main.
That is an enormous loss for a 12-inch main at ordinary flows - a 12-inch main at, say, 2,000 gpm loses only a few psi per 1,000 feet. So 52 psi over 9,000 feet points to a restriction or a flow far above design.
Likely causes, ranked:
- A partially closed or broken valve in the transmission main between the station and the far end - the classic cause of a large localized pressure drop with steady station pressure.
- Unexpectedly high demand from the new subdivision (150 homes) plus peak morning use, exceeding the main's capacity.
- A closed valve isolating part of the zone, forcing all flow through a smaller main.
- Internal tuberculation or a partially blocked main (less likely in a newer 12-inch main).
- Booster pump not actually delivering - but station pressure is steady at 80 psi, so this is lower probability.
First action: valve survey and locate the restriction; check the new subdivision's connection valves and any isolation valves that may have been left closed.
2. Fire flow is only 600 gpm at 20 psi versus a 1,000 gpm requirement. What does that mean for the zone, and what would you do before the next fire season?
Read the worked answer
Meaning: the zone cannot deliver the required fire flow at the minimum residual. This is a public-safety deficiency and may violate the state's fire-flow or pressure requirements. The 600 gpm at 20 psi residual confirms a restriction or undersized path - consistent with the 52 psi peak headloss.
Actions:
- Find and open the restriction (valve survey). If a valve is closed or partially closed, opening it may restore both pressure and fire flow without capital work.
- Re-run the fire flow test after the fix to confirm ≥1,000 gpm at 20 psi residual.
- If the main is genuinely undersized for the new subdivision, evaluate looping the main or adding a second feed, or upsizing the segment.
- Notify the fire department of the current deficiency and coordinate on hydrant use until corrected.
- Document the test results and corrective actions; many states require reporting fire-flow deficiencies.
Do not rely on the booster station alone - steady station pressure with a big far-end drop means the problem is in the main, not the pumps.