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Sewer Cad Result!

Dear All,

What is the meaning of these results?? (51058 "Model Run" "Manhole" "3600" "MH-484_098-C52" 0.000 "Manhole HGL is greater than pipe crown at pressure to gravity transition." Hydraulic Results)

The piping system is already existing.

Thanks & Regards,

June

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  • Hi June,

    In SewerCAD the calculations for the pressure pipes are handled separately to the calculations for the gravity pipes. At the transition between pressure to gravity we assume that the HGL is equal to the pipe crown.

    But if the actual computed HGL is greater than the pipe crown you get the error above. It's a way to indicate that the pressure network flows may be overestimated.

    In a system like this you may want to consider using SewerGEMS instead, since SewerGEMS can do a 'dynamic wave' analysis that solves for pressure and gravity pipes simultaneously, and can therefore do a more realistic job of modeling the transition from pressure pipes to gravity pipes when the gravity pipes are surcharged.

    Regards,

    Mal

          

  • Hi Mal,

    In this case, what are the possible problem may occur? and what are the possible solutions? Is it really the flow is the over estimated?or the head?

    Thank you very much!

    Best Regards,

    June

  • I think the easiest way to explain this is with a picture - see below.

    You can see that in SewerCAD there is a HGL discontinuity because the HGL in the gravity pipe is higher than what was assumed at the end of the pressure network (i.e. pipe crown). As a result, the pumps don't have to pump against as much head - so the computed flow is a little bit higher than what we'd actually see in the field. 

    In SewerGEMS (using the SWMM solver in this case), you can see that the HGL is continuous at the pressure-gravity transition. 

    Regards,

    Mal

          

    Answer Verified By: june_0809@yahoo.com 

  • Mal,

    Thank you very much! What do you mean "It's a way to indicate that the pressure network flows may be overestimated"? How to fix this? Please note that I am studying an existing system.

    Thank you very much again for your time.

    Best Regards,

    June

  • See how in my screenshots the flow in the SewerCAD profile is slightly larger than the flow in the SewerGEMS screenshot? That's because SewerCAD assumes the HGL at the downstream end of the pressure network is 172.2m (i.e. equal to the pipe crown), when it should actually be more like 173.8m  (based on the SewerGEMS results).

    So, because the downstream HGL is slightly lower in  SewerCAD, the flow through the pump is slightly higher - in other words the pressure network flows are overestimated.

    There is no real 'fix' for this in SewerCAD - it's just an assumption that you have to accept when dealing with surcharged gravity pipes. It often doesn't have a big impact on results (less than 5% in my example), and it's a conservative assumption anyway, so it's often not a concern for users - though you should satisfy yourself that you are ok with this assumption.

    Alternatively you can switch to SewerGEMS to perform the calculation. If you don't own it you can talk to your account manager about 'SELECT open access' where you can use any Bentley product you like and be charged quarterly for it, rather than have to buy it upfront.

    Regards,

    Mal

          

    Answer Verified By: june_0809@yahoo.com 

  • Wery well said Mal. Thank you very much. I am looking forward to have this SewerGems.

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