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  #1  
Old 06-04-2015, 12:48 PM
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Giving head

Now that I have your attention ....


I need a solution for solving the following problem.

What is the discharge (whatever units) of a certain diameter PVC pipe of a given length with variable head pressure?

I have a pond with a 12" PVC siphon. The emergency overflow is about 1 ft above the height of the siphon. I have a few input flow measurements (cfs by computing cross sectional areas on straight section of stream, using average trapezoid) taken over time from different rainfall events. Ultimately, I'd like to relate rainfall events to outflow and predict when the emergency spillway will operate. This will inform me concerning diversion construction of the outflow.

This I want to diy rather than hire an engineer. A mistake won't be threatening to anything except my fragile ego.

Thanks for your comments!

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  #2  
Old 06-04-2015, 01:07 PM
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Too many unknowns B but typically, good design for water you want to size the pipe to result in 10 to 12 ft/sec velocity for carbon steel. From what you have shared the Cameron hydraulic data book indicates 3500 to 4000 GPM gives you a range of 10.1 to 11.5 ft/sec in new 12"dia carbon steel pipe. I don't do much pvc pipe but you can probably run a little higher on it because I would think the ID would be smoother than CS. You can probably get by with flowing 6500 GPM with a velocity of 18.6 ft/sec.

Not sure this helped any.
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  #3  
Old 06-04-2015, 07:44 PM
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I looked at the friction constant for PVC @ 12 inches and it's just about as low as it goes. I screw that up with 4, 45 deg elbows, so it isn't laminar flow throughout. But that's how I get the siphon up the dam and down the other side. The flow is insufficient to initiate siphon by simply closing the air cock, I have to prime it by closing the outlet valve until the air is evacuated from the pipe.

That's generally interesting but irrelevant information.

The problem I face is the drainage basin is on sandy hills (like the hilly parts of Georgia's piney woods) and drains about 800 acres. The normal low water flow is about 1/2 cfs. I haven't calculated the high water flow (I need my Jesus shoes to do that). But I do know the pipe dia, height (head) at pond-full, pipe-full stage. I also know the dimensions of the emergency overflow. So If I can get the pond-full flow rate through the pipe I can figure-out the spillway flow rate.

For the drainage basin flow the NRCS has a bunch of modeling tools and spreadsheets.

Anyway, I'll keep playing with this stuff. Hell, I could take my tractor down there and dig a trench until I don't get overflow, but where's the fun in that?
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  #4  
Old 06-08-2015, 10:54 PM
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Think about global warming. Think about the 50 year flood. Think about what the next (new data) 20 year flood will be. Small watershed = fast response. Build a secure spillway as that removes all the guesswork.

Bot said watch my beer and hold this, to which I respond talk to your wife ya prevert!
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Old 06-09-2015, 03:24 AM
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So you will forgive me if I don't understand the details, but I will try to help.

The depth of your pond creates pressure at the entrance to your pipe. You can determine that pressure with p=rho*g*h. That pressure drives the fluid flow and overcomes all losses through your pipe system. You will have an entrance loss, major losses (pressure drop along straight lengths of pipe), and minor losses (pressure drop due to those 45s). Each loss is a function of fluid velocity. It sounds like you also have an altitude change while in the pipe, so that needs to be included as well. So you basically write out a conservation of energy equation and the only unknown in the equation is the fluid velocity.

The calculation is sometimes referred to as the bernoulli equation with head loss. This website explains it reasonably well and has a few examples which seems relevant to your situation.

The Bernoulli Equation

Calculating the actual head loss terms is not explained well on that site, but you can find it via google. The units can be a little tricky to get right, because everyone writes the bernoulli equation differently and so the appropriate units on the loss terms must match whatever units you need. Also, there will be a friction factor (f) to be determined. It is simply a function of the pipe type (smooth in your case) and the Reynolds number (Re= rho*V*D/mu). There are correlations to calculate directly, but if you only need a few values, using something called the Moody Diagram is probably the easiest way to estimate f.

It is turning out to be harder to explain than it would be to actually calculate it. If you tell me the water depth, all the pipe geometries, and the altitude change between the pipe entrance and exit, I can try to calculate it for you if you want.
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  #6  
Old 06-09-2015, 09:25 PM
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Quote:
Originally Posted by Shortsguy1 View Post
So you will forgive me if I don't understand the details, but I will try to help.

The depth of your pond creates pressure at the entrance to your pipe. You can determine that pressure with p=rho*g*h. That pressure drives the fluid flow and overcomes all losses through your pipe system. You will have an entrance loss, major losses (pressure drop along straight lengths of pipe), and minor losses (pressure drop due to those 45s). Each loss is a function of fluid velocity. It sounds like you also have an altitude change while in the pipe, so that needs to be included as well. So you basically write out a conservation of energy equation and the only unknown in the equation is the fluid velocity.

The calculation is sometimes referred to as the bernoulli equation with head loss. This website explains it reasonably well and has a few examples which seems relevant to your situation.

The Bernoulli Equation

Calculating the actual head loss terms is not explained well on that site, but you can find it via google. The units can be a little tricky to get right, because everyone writes the bernoulli equation differently and so the appropriate units on the loss terms must match whatever units you need. Also, there will be a friction factor (f) to be determined. It is simply a function of the pipe type (smooth in your case) and the Reynolds number (Re= rho*V*D/mu). There are correlations to calculate directly, but if you only need a few values, using something called the Moody Diagram is probably the easiest way to estimate f.

It is turning out to be harder to explain than it would be to actually calculate it. If you tell me the water depth, all the pipe geometries, and the altitude change between the pipe entrance and exit, I can try to calculate it for you if you want.
THAT'S IT!!

I knew I could do it, I just couldn't strain the extraneous information to get the right bits. Your explanation and links are exactly what I needed.

Truly grateful!
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no sense of responsibility at the other'
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  #7  
Old 06-10-2015, 01:13 AM
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Ya gotta love it when a bot giving head thread has a happy ending
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  #8  
Old 06-10-2015, 05:56 AM
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Based on the information given the maximum head available is 1' or 2.31 psi.

Quote:
The emergency overflow is about 1 ft above the height of the siphon.

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