Pressure Change: Understanding the Physics Behind Closing a Valve

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Discussion Overview

The discussion revolves around the physics of pressure changes observed when closing a valve in a residential water system. Participants explore the implications of pressure readings, potential causes for pressure fluctuations, and the role of various components in the plumbing system.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant reports measuring 120 psi at a hose bib and notes that pressure decreases when the valve is closed, despite no flow downstream or upstream.
  • Another participant questions the initial setup, asking if the gauge was attached with the valve off and suggests that 120 psi is unusually high for residential service, indicating a possible leaky regulator.
  • Some participants express skepticism about the pressure drop occurring without flow, suggesting that the situation does not align with expected behavior.
  • A participant mentions that their house is on a loop without a pressure regulator and discusses the altitude difference from the storage tank, indicating that this may contribute to the high pressure.
  • One participant proposes the possibility of a failing pressure reducing valve within the house.
  • Another suggests that closing the valve may increase the volume of the chamber downstream, potentially leading to a drop in trapped pressure.

Areas of Agreement / Disagreement

Participants generally express uncertainty regarding the observed pressure changes, with multiple competing views on the causes and implications of the high pressure and fluctuations. No consensus is reached on the underlying physics or the specific mechanisms at play.

Contextual Notes

Participants note the high pressure reading and its implications, but there are unresolved questions about the system's configuration and the behavior of pressure in the absence of flow. The discussion highlights the complexity of plumbing systems and the potential for various factors to influence pressure readings.

bbrady
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I was measuring the water pressure on my house using a pressure gauge with a hose bib fitting. So I screwed it on and the meter read 120psi. Then I started turning the valve off and the pressure decreased the more I closed the valve. There was no flow downstream of the valve (or anywhere upstream). I don't understand the physics of this. Why would the pressure decrease just by closing the valve?
 
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bbrady said:
I was measuring the water pressure on my house using a pressure gauge with a hose bib fitting. So I screwed it on and the meter read 120psi. Then I started turning the valve off and the pressure decreased the more I closed the valve. There was no flow downstream of the valve (or anywhere upstream). I don't understand the physics of this. Why would the pressure decrease just by closing the valve?
I don't understand: did you screw the gauge on with the valve off, then open the valve and then close it again?

120psi is very high for residential service, and implies to me a leaky regulator and a pressure that may vary with flows elsewhere.
 
bbrady said:
Then I started turning the valve off and the pressure decreased the more I closed the valve. There was no flow downstream of the valve (or anywhere upstream).
Yeah that doesn't make sense if all is as it should be.

russ_watters said:
120psi is very high for residential service, and implies to me a leaky regulator and a pressure that may vary with flows elsewhere.
I agree, the houses I've had showed more like 35 to 45 psig at the hose bibs.
 
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The 120psi is accurate. I spoke with the water board and mine and a few others are on a loop that does not have a pressure regulator on it. The house is about 280' less altitude than the storage tank. Houses above the tank have a booster pump in the loop and ones lower than mine have pressure reducers. I will likely put a pressure reducer on the house. 120psi is a bit much.

I still don't understand the fluctuations in pressure when I turn the hose bib off or on
 
Can you post some pictures? Use the "attach file" feature.

Thanks. :smile:
 
Is there perhaps a pressure reducing valve in your house that's failing?
 
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It's possible that the action of closing the valve is increasing the volume of the chamber on the downstream section of the valve itself, leading to a drop in the trapped pressure.
 
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