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I’m in a situation where a multi unit structure under construction will receive its water from local municipality via three metered 5/8” lines.

We’d like to combine all of the lines into a 3” horizontal main. The question is, can we assume that all three lines would have equal pressure, since they originate from the same water main? The concerb is that our volume would be reduced if any of the lines have lower pressure.

Any suggestions are appreciated.

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  • $\begingroup$ What is the advantage, assuming you're going to split the water into unit-by-unit feeds at the far end? And how are you going to share the cost against the 3 meters currently in place? $\endgroup$ – Carl Witthoft Jun 19 '18 at 17:37
  • $\begingroup$ This seems very strange. In the UK the minimum supply pipe diameter for a single house is 25mm, and for long or convoluted pipe runs that may be increased up to 63mm. The 3 5/8" lines combined are only equivalent to a single 1" or 25mm line. $\endgroup$ – alephzero Jun 19 '18 at 19:14
  • $\begingroup$ you need to talk to a licensed plumber $\endgroup$ – Mohammad Athar Jun 22 '18 at 15:36
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The pressure drop over feet from a main water line is near zero.

A main line is over powered to maintain pressure. They have to deliver pressure to the last person on the line.

3" is way bigger than 3 x 5/8". It is closer to a 1" line.

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  • $\begingroup$ OTOH the 3" shouldn't hurt, and if it appears the three lines are insufficient, it will be valuable when building a wider replacement. $\endgroup$ – SF. Sep 20 '18 at 12:19
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If there is no flow then pressure will be equal.

If there is flow and the lines are different lengths or have extra bends or other loss causing changes then the delivery pressures will be different but in some countries you must not allow reverse flow back into the supply so you may have to fit one-way valves to prevent that.

But feeding a 3” pipe from three 5/8 supplies seems odd... I suppose you have what you have.

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  • $\begingroup$ I can imagine a scenario where that happens: the plans foresee three standalone houses on three adjacent plots of land and provide the plots with endpoints at the time the water line is built. Then an investor buys the three plots and wants to build a block of flats or such, utilizing the current three connections to create a single mains supply to the building, instead of building a new one (which might involve damaging the street which has been built since.) $\endgroup$ – SF. Sep 20 '18 at 12:17
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As mentioned in other answers, hydrostatic pressure with no flow remains the same. Now, to estimate changes in pressure head with flow, you can use the Darcy-Weisbach-equation:

\begin{equation} (1)\qquad\Delta z_E=\lambda\frac{L}{D}\frac{v^2}{2g} \end{equation} $L$ is the length of the pipe, $D$ its diameter, $v$ the flow velocity within the pipe, $g$ the gravitational acceleration and $\lambda$ the friction coefficient. Note, that $v$ is needed to solve this equation, thus this usually is an iterative process, where you calculate the velocity without any losses, then calculate the losses with Darcy-Weisbach and so forth...

There is an empiric formula for $\lambda$, but its quite complicated and on an iterative basis as well. I think it will suffice to analyse the equations qualitatively. As you can see in equation (1), losses in pressure head $\Delta z_E$ due to friction are reduced, with an increasing pipe diameter. The same is generally true for the friction coefficient $\lambda$ as well.

In general, switching from multiple pipes to one with the same total cross-sectional area is beneficial, as there are fewer losses.

In your case, the initial total cross-sectional area is $$ A_0=3\cdot\left(\frac{5}{8}\mbox{in}\right)^2\frac{\pi}{4}=0.92 \mbox{sq in} $$

Afterwards it will be:

$$ A_1=(3\mbox{in})^2\frac{\pi}{4}=7.1\mbox{sq in} $$ Which is more than seven times the initial cross-sectional area, resulting in a decreased velocity for the same flow, thus fewer losses.

Therefore it should be safe to say that there will be no drop in pressure compared to the system you had before.

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