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Most cities get their water supply from local sources (springs, rivers, lakes) so the pressure in the system is never more than what is produced by the pumps.

But some cities get their water from elsewhere, typically from a much higher elevation.

Portland Oregon for instance gets its water from Bull Run Watershed, which is more than a kilometre higher than the city.

Portland's Water System | The City of Portland, Oregon

With that elevation difference, if there is no significant flow, the static water pressure in the city pipes would be over 100 atmospheres, which clearly isn't possible.

How is water pressure prevented from becoming catastrophically high?

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  • $\begingroup$ it's just pressure control valves. If the valve is shut, downstream won't pressurize. And municipal systems will never have zero flow. $\endgroup$
    – Tiger Guy
    Feb 9 at 21:52
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Where I live, the pressure is created due to reservoirs / lakes being up on mountains close to glaciers...

One thing that is done is the use of pressure relief valves that control the pressure and some are designed to drive samll turbines to produce electricity - there are papers on this topic.

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Note, the pressure in the pipe is rarely, if in any chance, equal to the natural head differential between the water source and the local. The water is conveyed down, through pipes or tunnels, to the water treatment plant. Depending on the elevation of the plant, the treated water can be send to the residential water pipes directly, or the water will be pumped to a reservoir, or water tower, to maintain proper pressure.

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