Dead-End Pressure Loss and a Closed Valve
A distribution zone serving 1,850 connections is fed by a booster station through a 12-inch main. SCADA shows station discharge pressure steady at 58 psi, but a logger at a dead-end 2.4 miles away reads 22 psi at 03:00 and 16 psi at 07:00; normal is 45–50 psi. Chlorine residual at the dead-end is 0.15 mg/L, while the station shows 0.95 mg/L. Three customers reported discolored water and one reported no water. The zone has three pressure-reducing valves, two valves closed during a 2019 main replacement, and a new irrigation customer on a 4-inch meter. A hospital dialysis clinic is 0.8 miles from the dead-end. The state requires a boil water notice if pressure drops below 20 psi. The zone also has a 500,000-gallon elevated tank that normally floats on the system but has shown no level change for two days.
1. How would you use step testing and SCADA data to distinguish a closed valve from a leak or unusual demand?
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Use the pressure and flow pattern to localize the problem before digging.
- Compare station flow with customer demand. At 03:00, legitimate demand is low. If the booster is pumping near its normal daytime rate while the dead-end pressure is 22 psi, water is leaving the system through a leak or a stuck-open blowoff. If station flow is also low, the problem is downstream-a closed valve or an obstruction.
- Step test. Close valves in the zone in small sections, beginning at the source, and watch the dead-end logger and station flow after each step. A section that restores pressure when isolated contains the restriction. A section that changes nothing is not the cause.
- Check the obvious first. Verify the 2019 valves are open, exercise the PRVs, and read the irrigation meter and the elevated tank level. A tank that has not moved in two days may be isolated or the level transmitter may be stuck.
- Use acoustic leak detection on the suspect section at night, when background noise is low.
Step testing must be planned so you do not isolate the dialysis clinic or create a pressure drop below 20 psi.
2. What immediate customer-protection actions would you take for the dialysis clinic and the dead-end area?
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Protect the sensitive customer first because dialysis patients are at risk from both low pressure and water-quality changes.
- Call the dialysis clinic. Tell them pressure in the area is below normal and ask them to use their emergency water plan or suspend treatment until pressure and chlorine are restored. Do not wait for a boil water notice.
- Issue the boil water notice if pressure has been below 20 psi. The state threshold is the trigger; discolored water and 0.15 mg/L chlorine support the decision. Deliver notices door-to-door at the dead-end and to the clinic.
- Flush the dead-end to restore chlorine and remove sediment, but coordinate with the clinic. Sample for total coliform and E. coli, and measure chlorine at representative points.
- Increase disinfectant residual cautiously at the booster, within the system's target, and watch for disinfection byproduct effects.
Document the pressure history, notices, and samples. If the cause turns out to be a closed valve and pressure never fell below 20 psi at customers, the notice may be unnecessary-but the safe default is to protect the clinic.
3. Step testing shows a closed 12-inch gate valve from the 2019 project. What now?
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Reopening the valve is a controlled operation, not a quick fix.
- Confirm the valve is actually closed by exercising it and verifying stem position; a broken stem can look closed.
- Open it slowly-a quarter turn at a time-while watching pressure at the dead-end and station flow. A sudden full opening can surge the main, stir sediment, and cause discolored water at the clinic.
- Flush downstream at a controlled rate until turbidity clears and chlorine residual is stable.
- Sample for coliform and chlorine after flushing. If pressure stayed below 20 psi, keep the boil water notice until two consecutive negative sets 24 hours apart.
- Investigate why the valve was closed and update the valve map and tag. The 2019 project records should have shown it.
If pressure recovers and samples are clean, the notice can be lifted under the state's criteria. Record the step-test results and the valve exercise.