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SewerGems - Pressure pipe friction loss very wrong

Samuel Bush
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I'm working with an odd pipe configuration (an inverted siphon which discharges into a basin, controlled by an overflow weir).

Elsewhere in my model, I have some channels that I'd like to calculate at a high resolution (0.2 feet, ideally, but I can coarsen it some).

The inverted siphon friction losses are significantly incorrect and the error seems inversely linearly proportional to the calculation distance. The system is something like this (and I have attached an example):

[Manhole with loads] ---------[Pressure pipe] ---------------[Outfall with stage-discharge relationship]

To reproduce the error, change the calculation distance. On my machine, as I drop the calculation distance from the default 50' to my target (0.2') I see the headloss in my pipe rise from 0.8 ft (should be 0.5 from hand calcs) to about 8.00 ft. 

What gives? It seems maybe additional headloss is being artifically generated in each 'calculation section' of the pressure pipe - how do I get the solver to not do that? 

Best,

Sam

Error In This Pipe.stswpkg.zip

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  • 0 Offline
    It’s a clarifier - basically a pond with about 800 small v-notch weirs around it. Water enters via a vertical pipe that has four rectangular orifices cut in its sides. So the relevant headloss-generating elements of the structure include the weirs (Hl = kQ^0.4) and the orifice (hl = kQ^2 plus some other terms), with some discontinuity. You can see the form of the resulting curve in the bc of that model I sent earlier.

    We tried using a pipe with an outlet and inlet control (a weir and an orifice) and a pipe with a large (fictitious) diameter but that led to problems. 

    The specified head/flow curve works fine except that the friction model breaks :(

    We looked at ponds - maybe we should’ve looked closer. I think we gave up on them b/c you can’t route flow downstream of a pond but obviously now that we’ve gone to the BC approach, maybe a pond is a better fit

    Sent from my iPhone
  • 0 Offline in reply to Samuel Bush

    Ok, thanks for the information. You could try a pond with a single composite outlet structure that uses a stage-discharge curve based on all the outlet components. When using ponds and control structures (both pond or conduit), the SWMM solver tends to work better as well. Based on the example model and your descriptions thus far, I would recommend the SWMM solver + simplified pond approach.


    Regards,

    Jesse Dringoli
    Technical Support Manager, OpenFlows
    Bentley Communities Site Administrator
    Bentley Systems, Inc.

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  • 0 Offline in reply to Samuel Bush

    Ok, thanks for the information. You could try a pond with a single composite outlet structure that uses a stage-discharge curve based on all the outlet components. When using ponds and control structures (both pond or conduit), the SWMM solver tends to work better as well. Based on the example model and your descriptions thus far, I would recommend the SWMM solver + simplified pond approach.


    Regards,

    Jesse Dringoli
    Technical Support Manager, OpenFlows
    Bentley Communities Site Administrator
    Bentley Systems, Inc.

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