Inverted Siphon Hydraulics Help 2

[Tibbs01]

I am evaluating the feasibility of using an inverted siphon to convey sanitary flows below a creek and the hydraulic calculations suggest it will work, although I have pulled out all the stops to get it to work. I'd feel much more comfortable if there was a bit more cushion in the design so I have a question regarding free flow discharge from the siphon.

The siphon will discharge to a pump station wet well and I have considered that the highest "pump on" level (i.e. water surface) in the wet well would need to be at the invert of the siphon discharge. If the pumps were running and pumping out the wet well at the same rate as the influent sewers were entering the wet well, would this not be considered "free flow discharge" even if the water surface elevation was above the invert of the siphon? If so, I could lower the siphon discharge invert and have a more comfortable factor of safety in the design. Any thoughts would be sincerely appreciated.

Thanks, Steve

 

[655321]

I’m no expert, but I think that it would be reasonable to say it would be free flow discharge, especially if the switch was the highest one and triggered the secondary pump (and the lift station was designed and working properly), the “water” would only be that high briefly and at the worst peak flows. But, if it really bothers you, couldn’t you just lower the switch a few inches below the invert?

 

[bimr]

Not exactly sure what your concerns are.

You need to make sure that your siphon is always full of water. If you allow air to enter the inverted siphon, you have what is called two phase flow. The trapped air will then cause reduced flow in the pipes.

You can do this by placing a wall in your lift station where the flow has to go over the wall as it enters the lift station. Or you can put a manhole outside the lift station.

 

[cvg]

it seems that if you are to install a pump station on the downstream side of the river at the end of the siphon, wouldn't it be easier to eliminate the siphon and construct the pump station on the upstream side? Then run a force main under the river. What am I missing?

 

[rconner]

There is information relating to perhaps somewhat unique siphon design and maintenance issues in ASCE MOP No. 60/(former) WPCF MOP No. FD-5, "Gravity Sanitary Sewer Design and Construction", as well as "Recommended Standards for Wastewater Facilities" ("Ten States Standards"). There have also been many case studies on such structures, including reference to such design and maintenance issues, etc. e.g. presented in WEF and ASCE events, so Publications of these entities might be good resources as well.

 

[Tibbs01]

Thanks for all your responses to my question. Let me clear a few things up.

1) The pump station will be a retrofit of an existing pump station and the pumps will be operated by VFDs. During high flow conditions, the VFDs will attempt to maintain a wet well level such that they match influent flow. I can't afford to lower the pump on setpoint below the invert at all, and my goal is to lower the siphon invert to allow a better safety cushion w/ regard to teh siphon hydraulics.

2)The siphon will alwys be full and it will discharge to a stilling basin outside of an existing wet well.

3)I am not installing a pump station on the upstream side of the creek b/c there is already a pump station on the downstream side that would need to remain. I am hoping to make modifications to the existing station so we don't need 2 pump stations (although that was the original plan)

 

[bimr]

Going back to your question, it is not a free flow discharge condition in the siphon.

It is full-pipe flow condition (i.e., pressurized flows).

After the discharge of the siphon, it will be a free flow.

 

[655321]

I think the question then is how far above the invert of the siphon outlet would the water level rise in these peak flow situations. If the pumps hold the water level at about the invert elevation, I think it would be appropriate to model the siphon as free flow discharge. It's really the maximum height of the head on the discharge end of the siphon that matters, the flow rate out of the wet well isn't the issue.