Hilltop Zone Low Pressure During Peak Demand
A hilltop pressure zone is fed from a 300,000-gal elevated tank through a 12-in main and a PRV set to deliver 65 psi. The zone serves 180 homes and a school. In July, operators receive low-pressure complaints from the last 2,000 ft of a cul-de-sac. A hydrant gauge there reads 38 psi at 6 a.m. and 29 psi at 6 p.m.; the PRV outlet gauge reads 63 psi. The tank falls from 80% to 52% most days and the refill pump runs about 10 hours. A fire-flow test at a nearby hydrant yields 900 gpm with 12 psi residual. The utility has no current hydraulic model and has not exercised valves in the area for years.
1. What do the pressure and tank data tell you, and what are the most likely causes? List the additional data you would collect before changing any settings.
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The PRV outlet is holding near its setpoint (63 psi vs. 65 psi), but the problem area is 34 psi lower at 6 p.m. That points to head loss or restriction downstream of the PRV, not simply a low PRV setting. The fire-flow result (900 gpm at only 12 psi residual) also suggests the piping path, a valve, or demand is limiting delivery. The tank drawdown from 80% to 52% is about 84,000 gal, and the pump runs 10 hours, so the zone is using stored water heavily during the day. Likely causes include an undersized or tuberculated main, a partially closed or failing valve, high simultaneous irrigation demand, or a PRV that is not responding properly under flow. Before changing settings, collect 24-hour pressure logs at the PRV inlet and outlet and at the problem hydrant, tank level and pump runtime records, valve position checks, main sizes and elevations, leak history, and customer demand patterns.
2. What immediate operational actions would you take, and why is raising the PRV setpoint not a complete solution?
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Immediate actions: verify the PRV is actually open and responding, log pressures over a full day, exercise and confirm the position of isolation valves between the PRV and the cul-de-sac, check for leaks, and manage tank levels and irrigation demand. Notify affected customers, especially the school, and keep pressures above the utility's minimum target. Raising the PRV setpoint may improve the symptom temporarily, but the downstream pressure drop is caused by head loss or restriction; increasing upstream pressure can overpressure lower-elevation customers, increase leakage, and still not deliver adequate fire flow. It may also exceed the PRV's safe downstream limit. The fire-flow result shows a capacity problem that a setpoint change cannot fix.
3. What information or projects would you recommend for a permanent fix?
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Build or update a hydraulic model of the zone, including elevations, main sizes, PRV settings, tank levels, and fire-flow demands. Use it to test looping the cul-de-sac, replacing or lining the undersized main, adding a booster or local storage, or adjusting pressure zones. Start a valve-exercise and main-cleaning program, because an unexercised valve may be partly closed. Evaluate irrigation restrictions or remote pressure monitoring. Any permanent change should be checked against minimum normal pressure, fire-flow requirements, and maximum pressure at low-elevation customers.