Pressure Change: Understanding the Physics Behind Closing a Valve

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SUMMARY

The discussion centers on the physics of pressure changes when closing a valve, specifically in a residential water system with a pressure gauge reading 120 psi. Users noted that this pressure is unusually high for residential service, suggesting potential issues with a leaky regulator or the absence of a pressure regulator in the system. The pressure decrease observed when closing the valve is attributed to the dynamics of fluid mechanics, where closing the valve increases the volume in the downstream chamber, leading to a drop in trapped pressure. Recommendations include installing a pressure reducing valve to manage high pressure levels effectively.

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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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