Water pressure question

[Scott2187]

Originally Posted by h_curtis

I am not arguing here, because I have not experienced this except for a faucet that needed replaced or adjusted, but why would a mineral deposit have the effect of the water pressure changing that much? If you turn on the faucet and you have pressure and it all of a sudden decreases would that just be lack of flow?

As far as my knowledge spans on this subject, I think you answered your initial question with the second question. The mineral deposits will form and restrict the inside of the pipe, resulting in a lack of flow. This would be noticed as stated earlier in the thread: high pressure in the beginning since the pressure is equalized through out the pipe, then the pressure would drop off anywhere after the restriction. I'm sure one of the Navier-Stokes theories for fluid mechanics would explain it mathematically.

Originally Posted by greg42

If you're throwing hot into the mix then this will no longer be clear. But if it's strictly with cold, then I would think as noted already that the regulator could be bad and maybe should be replaced.

I would disagree. The regulator should be positioned before the incoming pipe splits to the hot water tank. Both supplies of water go through the regulator, so both should see issues if the regulator was bad.

[gg]

Scott, all this being said the cheapest and first line of defense would be the regulator. Correct? I mean that would be one heck of a lot cheaper than dealing with the entire system.

[ML North]

Originally Posted by Scott2187

...then the pressure would drop off anywhere after the restriction. I'm sure one of the Navier-Stokes theories for fluid mechanics would explain it mathematically.

Navier-Stokes would do it, but it has lots of partial derivatives that aren't necessarily intuitive.

Basically you have a constant pressure at the outlet of your regulator, assuming it is functioning properly. Between the regulator and each faucet there are losses -- head loss due to elevation, elbows, reducers, friction, etc. Mineral deposits would effectively reduce the flow area of the pipe, causing the linear velocity to increase at the blockage.

The head loss (or pressure drop) due to friction can be calculated using the following equation:



Density, friction factor, and length are constant. Velocity, V, goes up, diameter, D, goes down, and the pressure drop, DP, increases.


All that aside, if the pressure starts off high and then slowly rolls off, that sounds like a regulator issue to me. They have little rubber diaphragms for actuation that wear out over time.

[gg]

Originally Posted by ML North

All that aside, if the pressure starts off high and then slowly rolls off, that sounds like a regulator issue to me. They have little rubber diaphragms for actuation that wear out over time.

This makes the most sense. Pressure should be pressure from start to finish unless something changes or more outlets are open.

Guess my thoughts were correct. Replace the regulator first.

[ditchdigger]

I am not arguing here, because I have not experienced this except for a faucet that needed replaced or adjusted, but why would a mineral deposit have the effect of the water pressure changing that much? If you turn on the faucet and you have pressure and it all of a sudden decreases would that just be lack of flow?

I'll take a shot at explaining it without the mathematics (which aren't my strong suit anyway.)

Go outside, to your garden hose. Take the hose nozzle off. Now, just crack the faucet open very slightly, so that only a trickle of water is coming out. Hold you thumb over the open end of the hose. You'll feel the pressure building up, and if you have good water pressure, it'll build to the point where your thumb can no longer hold it. When you let it go, lots of water will flow out at first, but soon, it'll reduce back to a trickle. Cover it with your thumb again, and the process will repeat.

That illustrates how pressure is one thing, but volume is a different thing, and how a restriction in the line (mineral deposits being the equivalent of a very slightly opened faucet), can make a lack of volume seem to be a lack of pressure.

[Scott2187]

Originally Posted by ML North

Navier-Stokes would do it, but it has lots of partial derivatives that aren't necessarily intuitive.

Basically you have a constant pressure at the outlet of your regulator, assuming it is functioning properly. Between the regulator and each faucet there are losses -- head loss due to elevation, elbows, reducers, friction, etc. Mineral deposits would effectively reduce the flow area of the pipe, causing the linear velocity to increase at the blockage.

The head loss (or pressure drop) due to friction can be calculated using the following equation:



Density, friction factor, and length are constant. Velocity, V, goes up, diameter, D, goes down, and the pressure drop, DP, increases.


All that aside, if the pressure starts off high and then slowly rolls off, that sounds like a regulator issue to me. They have little rubber diaphragms for actuation that wear out over time.

Thank you... I've been out of practice with the fluid dynamics for a while and I couldn't remember which one explained it best.

Originally Posted by ditchdigger

Go outside, to your garden hose. Take the hose nozzle off. Now, just crack the faucet open very slightly, so that only a trickle of water is coming out. Hold you thumb over the open end of the hose. You'll feel the pressure building up, and if you have good water pressure, it'll build to the point where your thumb can no longer hold it. When you let it go, lots of water will flow out at first, but soon, it'll reduce back to a trickle. Cover it with your thumb again, and the process will repeat.

That illustrates how pressure is one thing, but volume is a different thing, and how a restriction in the line (mineral deposits being the equivalent of a very slightly opened faucet), can make a lack of volume seem to be a lack of pressure.

I can't think of a better real-life, simple demonstration to show this. As for trouble-shooting the water system in a house, I would definately start with the regulator.