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Inverted Siphon - Basic Fluid Mechanics 2

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JoelTXCive

JoelTXCive

Civil/Environmental
Jul 24, 2016
920
We are providing a structural design fee estimate for an inverted siphon that will go under a roadway or stream. From the structural point of view, it's basically just a couple buried junction boxes.

We have been provided a go-by project to help us work up a fee. I have studied the go-by plans, and I'm curious about the fluid mechanics of the siphon.

See attached...

Questions......

[ul]
[li]What does the separate 'Air Bypass' line do? I've googled a bunch of inverted siphons and do not see this line? If you are concerned about the primary siphon lines getting vapor locked wouldn't you have direct vents or relief pipes connected directly to them? How does a secondary independent line help?[/li]

[li]The 'Air Bypass' line has a rope in it. My assumption is that this rope is there to pull a pig or other cleaning device through the line? If it's just an air line, then why would it get clogged? Maybe it is truly an overflow line?[/li]
[/ul]

What do you think? I've seen existing inverted siphons on roadway underpass projects; but I've never been involved in the initial design before.
 
 https://files.engineering.com/getfile.aspx?folder=ba82850a-054e-4fe0-a483-d6aaea9080d1&file=Inverted_Siphon_Go-By_Excerpt.pdf
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Is this sanitary sewer or storm water?

Your question piqued my interest and some of my googling indicated that the air bypass is to allow the flow of gasses that usually flow in the top of partially filled sewer pipes. Apparently odors can be problematic around inverted sewers.

air_jumper_d4hm6o.png
 
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I think you are on to something AzCat.

Yes, this is a Sanitary sewer line. And yes, I think odor control is definitely a concern. We had to provide a supplemental bid amount for an odor control unit's foundation.

I know the normal sewer line is a gravity line (partially filled pipe), but changes to a pressure flow while in the siphon.

If we assume the influent 'flow' is comprised of liquid AND gas arriving at the influent box, but only the liquid portion goes through primary siphon lines. Then all the smelly gas would escape into the atmosphere.

By providing an additional air-only line, then the majority of the bad air will continue to travel with the waste fluid versus being released into the atmosphere.

This makes sense.....The gas and fluid portions basically travel in their own pipes through the siphon.

That explains everything except the rope in the pipe, which must be for cleaning?
 
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Excerpt from ASCE Manuals No. 60
Gravity Sanitary Sewer Design and Construction


AIR_JUMPER_w18adj.jpg








Tim was so learned that he could name a
horse in nine languages: so ignorant that he bought a cow to ride on.
(BENJAMIN FRANKLIN )
 
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The HS20/HL93 load right in between the 2 manhole cover with full impact factor is going to be awesome :). I would put those 2 holes farther apart as far as possible.
 
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