Inverted Siphon Hydraulics Help
Inverted Siphon Hydraulics Help
(OP)
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
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





RE: Inverted Siphon Hydraulics Help
RE: Inverted Siphon Hydraulics Help
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.
RE: Inverted Siphon Hydraulics Help
RE: Inverted Siphon Hydraulics Help
RE: Inverted Siphon Hydraulics Help
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)
RE: Inverted Siphon Hydraulics Help
It is full-pipe flow condition (i.e., pressurized flows).
After the discharge of the siphon, it will be a free flow.
RE: Inverted Siphon Hydraulics Help