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Old May 15th 2017, 08:59 PM   #1
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Fluid Mechanics Question

Looking for some expertise to help solve a problem. Total noob at this.

I have a water main that is being pumped at around 100 GPM. I want to tap off a parallel branch that will run at a lower flow rate < 1GPM for about 3 feet, that branch would then rejoin the main line.

How exactly would you achieve this? I need to maintain the branch at < 1GPM. I found a flow meter that will do just that. But I have questions and concerns.

Question 1: Does it matter where I put the flow meter on the branch in order to maintain a flow rate of <1GPM? I assume I only need one to regulate the flow through a secondary branch.

Question 2: Will the entire length of the branch be maintained at that flow rate? I assume yes.

Concern: If The main is flowing at a higher rate, will water even flow through that branch at a reduced rate with the meter in place? Will back pressure prevent flow through the meter/branch?

I'd like to learn more about the mechanics and physics of what I described above, but I'm not sure where to start.

Any help is appreciated.
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Old May 16th 2017, 05:50 AM   #2
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First order rough estimate: if you make the cross-sectional area of the side branch equal to 1/100th the cross section of the main line you will get flow rate of about 1/100th the main line, or about 1 GPM. However, flow through a small diameter pipe experiences higher friction due to the effects of the pipe's inner surface, so I would expect the flow rate through the side branch to be a bit less than that. Given that you are introducing a flow meter, which will add some back pressure, the flow rate will be reduced even more - by how much I couldn't say. So, your best approach may be to make the branch a bit larger in cross-section area than 1/100th the main line, and install a valve that you can use to adjust the flow down if need be.

To answer your specific questions: conservation of mass flow says that the flow rate through the entire length of the side branch is constant. The velocity of flow through the side branch (in feet/second) will likely be a bit slower than in the main line, again due to the friction effects.
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Old May 16th 2017, 09:12 AM   #3
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Originally Posted by JakeDa View Post
Looking for some expertise to help solve a problem. Total noob at this.

I have a water main that is being pumped at around 100 GPM. I want to tap off a parallel branch that will run at a lower flow rate < 1GPM for about 3 feet, that branch would then rejoin the main line.

I'd like to learn more about the mechanics and physics of what I described above, but I'm not sure where to start.
First Question who does the water main belong to?

I assume you have some use for the water flowing in the proposed takeoff line?

It would normally be unsound practice to return the takeoff line to the mainline, even if the pressure issues could be overcome, and I can't imagine any self respecting Water Authority allowing it for public health reasons.

It would also be sensible, but not mandatory, to install isolating valves at the takeoff point to permit future maintenance.

Last edited by studiot; May 16th 2017 at 09:34 AM.
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