What causes water hammer in a distribution system?
- A sudden change in the velocity of the water
- Air dissolving in the water at high pressure
- A slow rise in demand during the morning peak
- Corrosion deposits narrowing an old main
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A. A sudden change in the velocity of the water
Moving water has momentum. Stopping or starting it suddenly, by slamming a valve or hydrant shut or by a pump tripping, sends a pressure wave through the pipe that can break mains. Operating valves and hydrants slowly prevents it.
Reference: Hydraulics: water hammer (pressure surge)
A pressure-reducing valve (PRV) installed between two pressure zones is designed to:
- Hold the upstream pressure at a set higher value
- Stop all flow whenever demand drops to zero
- Hold the downstream pressure at a set lower value
- Boost the pressure for customers at high elevations
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C. Hold the downstream pressure at a set lower value
A PRV throttles automatically to keep its outlet pressure at the set point, protecting a lower zone from the higher pressure above it. A valve that maintains upstream pressure is a pressure-sustaining valve; raising pressure requires a booster pump.
Reference: Distribution system valves: pressure-reducing valve
Where on a transmission main should air release valves be installed?
- At the high points
- At the low points
- At the dead ends
- At each hydrant
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A. At the high points
Air collects at high points in a pipeline, where it restricts flow and adds head loss. Air release valves there vent it automatically. Low points are where blowoffs are placed to drain or flush the line.
Reference: Distribution system valves: air release valves