We are modeling a raw water transmission line from a reservoir (lake) to a water treatment plant (discharge to clearwell open to atmosphere). The line will siphon over the reservoir dam (model shows vacuum condition as expected). Further down the hill we have serveral local high points where The overall gradient is downhill, but we have several local highpoints where combination air valves will be installed to break vacuum conditions and allow for gravity flow. Currently for the segment of pipe downstream of the combination air valves the model shows high negative pressure, however this doesn't match our expectations that this flow would be partially full (gravity flow) with 0 pressure (atmospheric). We have corrected the attached PDF with bold blue and red lines to show what we think the HGL and pressures should be in the "partial flow" sections of pipe. Is this correct? Is there a way to better model this so the WaterCAD model shows zero pressure for these sections?
75104.00 Upper Wrangell Raw Water Pipeline 2020.09.30.wtgPDF
75104.00 Upper Wrangell Raw Water Pipeline 2020.09.30.wtg
How are you going to fill the siphon? You need to get all of the air out of the siphon but the water will be running away from the siphon. Are you going to use a pump to fill the siphon?
We plan to have priming and air purging lines connected to a vault at the dam to be able to purge the air and fill the pipeline with a pump to initiate the process.
This is a pretty tricky design. I would tend to run WaterCAD down to station 2600 ft. From that point down, I would use a gravity flow model like SewerGEMS to study the gravity section. The stretch around station 2000 to 1800 tt is really an inverted siphon which I would model as manholes with bolted covers.(or very high rims).
Thanks Tom, we have SewerCAD, so we'll look at modeling the section indicated with that to get a better handle on it.
Below wiki will help you with design of siphon in SewerCAD.
Modeling an Inverted Siphon
Regards,
Sushma Choure
Bentley Technical Suppport