This discussion has been locked.
You can no longer post new replies to this discussion. If you have a question you can start a new discussion

Continuity Error Question

I am putting together a sewer system model for a semi-large city.  My model includes 1 outlet (wastewater treatment plant), about 10 pump stations spread out in the system, a number of force mains, and the rest gravity lines.  Right now I am only trying to deal with the main conduits in the system reducing my model to about 350 pipes.  I would like to start calibrating but when I run the model I keep coming up with a continuity error of over 25%, basically losing 25% of my input wastewater into thin air.  Has anyone had a similar problem or have any suggestions on how to reduce this percent error?  I have tried all the basic methods of making sure my data was accurate and so on but there is only so much tweaking I can do to the system.  Thanks for your help.
  • Hello katzi,

    I assume you are referring to Bentley SewerGEMS.

    Stabilizing your model to acheive a good continuity error is a typical task when working with a dynamic modeling solution like SewerGEMS. This is why the continuity error is reported in the calculation summary. Of course, there are numerous things that could cause it, so it's very important that you first examine all warnings in your user notifications, examine all your input data for errors and make appropriate corrections. Simplifications to the system should be done wherever possible, such as removal of upstream pipes that will run dry and combining pump stations with parallel pumps into a single pump collection.

    In systems that have numerous force mains, especially those with complex manifolds, fast cycling, etc, it is recommended that you use SewerGEMS Sanitary. SewerGEMS Sanitary is a separate program included with the latest version of SewerGEMS (avialable for download on our website if your company has a SELECT subscription) and is better suited for complex pumping situations. If you do not have the latest version of SewerGEMS or are required to use a dynamic calculation engine for your results, you may need to reduce the hydraulic timestep in the calculation options of your SewerGEMS model to help with continuity error related to pumping.

    The latest version of SewerGEMS also includes the Hydraulic Reviewer module (under the Tools menu) to pinpoint locations in the model where continuity error occurs (among other things). It also has a new virtual flow depth option, which can improve stability in near-dry pipes (if you must include them).

    SewerGEMS also exposes many other useful advanced calculation options (such as the NR Weighting Coefficient) which you can tweak to acheive better results in hydraulically challenging models. The help documentation has a chapter entitled "troubleshooting dynamic model results", which will explain in more detail.

    These are just a few basic things. The exact solution to acheiving good continuity is very dependent on the particular system being modeled. So, if this does not help and you're using the latest version, you should consider submitting your model to technical support via the online ticket manager .

    Regards,

    Jesse

     


    Regards,

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