Static pressure in a water pipe system

  • #1
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Main Question or Discussion Point

I have a problem where I want to find out the static pressure over a city in a water pipe system. As I know the elevations over the city I was first thinking of just using p = rho * g * h, but this should only apply if the pipe system were under water (if I use the density of water at least).
So my second thought were to use the different pressure manometers that exists in the system. These are piezometers which should give me the static pressure at that point. And this leads to my confusion, I believe that the static pressure should be constant, and these manometers should be giving me the value I need as It's not dependent on fluid velocity. But these values varies quite a lot, which indicates that the dynamic pressure has something to do with these values.
After this I started to calculate on a extended Bernoulli whereas I can get the flow velocity at some points as well, but it feels like I'm really making this a lot harder than what it should be.

Can someone explain why the manometers which is of piezo type varies so much in value?
And how can how can I utilize the manometers at the different locations to get the static pressure?

Thanks in advance!
 

Answers and Replies

  • #2
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I have a problem where I want to find out the static pressure over a city in a water pipe system.
How is that useful? When does the city have zero water flow everywhere?
 
  • #3
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The goal is to increase the pressure in the entire system, and as the system is only classified for 16 bar pressure I can't exceed it. The elevation difference makes the static pressure to be higher than the dynamic pressure even if it's quite far away. What I'm trying to do first is calculating the static pressure at the location where the elevations is the lowest. Maybe I'm just going at this the wrong way?
 
  • #4
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Do you know about water hammer? The sum of static and dynamic pressures can burst pipes.
https://en.wikipedia.org/wiki/Water_hammer

If your approach is to just calculate with advice from the Internet, then yes it is wrong. There are engineering codes, plumbing codes, and building codes that tell you how to design things that might affect human health and safety. The authors of the codes already considered issues such as static and dynamic pressures. Sometimes, the codes specify methods, and sometimes the codes specify the answers.

The approach of a city engineer should be to first get copies of the applicable codes and to consult them.
 
  • #5
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Yes, I know of it.

What I wrote earlier made it look like I'm doing something dangerous without correct knowledge, and I like the way you handled it. But it's only for my personal curiosity where I set up problems during downtime at work. I'll try some more on my own.
 
  • #6
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Can you provide a rough schematic?
 
  • #7
russ_watters
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I have a problem where I want to find out the static pressure over a city in a water pipe system.
This sentence is not grammatically correct, which is making the purpose of this thread confusing. You can find the static pressure *of* a system or you can find the static pressure loss over a system, but "static pressure over a system" is gibberish.

Please re-state your question and provide some more detail.
 
  • #8
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I guess I mean static pressure loss, although in this case it's a gain.
I did a rough sketch of the problem:
(https://gyazo.com/0a9f9f01221aa906388cefb7be09e3d4)
Where there's only one pipe, with the hot water towards the customer.
The pressure at point A is lower than at point B, although the water has to travel 4 km towards one of the most distant customer. This is due to the height difference and static pressure gain making up for the dynamic pressure loss.

Something to note is that there is a pump on the return pipe back to the boilers.
The pressure indicators is of type piezometer as I wrote earlier.
 
  • #9
CWatters
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I was first thinking of just using p = rho * g * h, but this should only apply if the pipe system were under water (if I use the density of water at least).
It works for water in pipes as well. Just needs modifying because the pressure at the highest point in the town isn't zero.
 
  • #10
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From my understanding, that formula origin is from the force exerted from the mass of the fluid above the point where you want to get your pressure. How does it work for a pretty much horisontal water pipe? The force from the gravity shouldn't work then, or am I missing something?

And the modifying part, do you mean adding on the atmospheric pressure? Or to use the pressure after the pump?
 
  • #11
CWatters
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It works just the same. The vertical height difference is what matters, the horizontal distance has no effect.

For example the shape of a ocean doesn't effect the pressure, only the depth does.
 
  • #12
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Researched some more and I finally get it.
Thanks for all the help!
 

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