Sanitary Sewer Junction Chamber Dimensions

[chocobo1198]

Hello,
I need advice on a sanitary sewer junction chamber that is at its preliminary design stage. I'm looking for guidelines regarding dimensions. Please see attached PDF drawing with redline markups showing my questions. Feel free to comment on the drawing in general too.
Thanks

 

[gbam]

I looked briefly at your drawing and the specific questions would be structural. I layout similar structures for storm drain junctions all the time. I start with the geometry I need for the hydraulics and fit the walls then have our structural engineer review the geometry to see if it works with the structural requirements.

Hope this helps.

 

[chocobo1198]

Hello Gbam,
I should have clarified that the dimensions I'm inquiring about are related to hydraulics. You mentioned that you layout similar structures for storm drain junctions... so what are the guidelines and/or rules of thumb that I should follow? I know it should depend on flows too.

I already have a structural engineer that I will be talking to.
Thanks.

 

[bimr]

It is difficult to evaluate hydraulic losses at junctions and you have not provided any information on the flows or elevations outside the junction.

A general rule of thumb is to limit the angle of entry to 30[sup]o[/sup] or 45[sup]o[/sup] and the junction is sized so that the velocities of the merging streams are approximately equal at maximum flow.

You would want to size the channels so that most of the flow passes through the junction chamber without appreciable expansion of the flow.

Don't believe it is practical to use drop logs in this type of structure. The logs will be very heavy and difficult to remove. If one downstream sewer was blocked off, would not the junction overflow?

 

[chocobo1198]

Hey Bmir,

Thanks for the feedback.

Here I'll give more info and see if you can provide more feedback:

Inlet flows are as follows:
Angled pipe = 4,800 gallons/min or 300 l/s
Straight pipe = 37,000 gallons/min or 2,300 l/s

Combined outlet flow is just the sum of the above, but not knowing how much goes into each pipe.

Regarding elevations, the grade for all pipes is near 0%, so it is pretty flat.

Regarding the logs, they are for maintenance purposes so it's a rare occurrence that they are used. We do understand that these gate frames would likely become corroded to the point that the logs would no longer fit in, but this is something that our O&M has requested. There isn't enough ground cover to install a single gate and keep it in open position. The junction chamber design HGL is based on our safe operating head and we expect it to be pressurized a majority of its life-cycle. We have controlled spillways to control overflows.

So if you have any further advice... would be greatly appreciated.

Thanks,
Chocobo

 

[gbam]

Sorry for my delay in response. Take a look at the FHWA HEC22 manual chapter 7. It discusses head loss through access holes (manholes/junction chambers). I use as much benching/shaping through the chamber as feasible, this reduces head loss. The wall dimensions are based on fitment of the pipes in regard to the reinforcing steel in the walls. I hope this helps.

 

[fel3]

Take a look here:

http://eng2.lacity.org/techdocs/stormdr/haj.pdf

and here:

http://eng2.lacity.org/techdocs/sewer-ma/figures/f200/f260ao.pdf

These two documents from the City of Los Angeles go together. I have never calc'ed the hydraulics of a junction structure myself. The closest I came was structural design for three junctions on the same project. I just happened to remember that I had a paper copy of the second document in my archives, so I went searching on the internet and this is what I found.

The rest of the City of Los Angeles' sewer design manual can be found here:

http://eng2.lacity.org/techdocs/sewer-ma/index.htm

Their storm drainage design manual can be found here:

http://eng2.lacity.org/techdocs/stormdr/Index.htm

-- Fred

==========
"Is it the only lesson of history that mankind is unteachable?"
--Winston S. Churchill

 

[amlinerrichard]

You are mentioning "pressurized". Are these pipes connected to pumps upstream? Then you bring them to a structure that allows the flows to expand and mix? Or is the whole thing pressurized?

 

[chocobo1198]

Sorry I was away on vacation. The chamber is expected to be completely pressurized a majority of the time due to high flows. About a mile or two upstream is a pump station. The chamber should be designed for both gravity and pressure flows.