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too much discharge in SewerCAD

Hi again,

In SewerCAD V8i, all necessary data (invert and ground elevations, manhole discharges) were entered but total discharge comes out higher than the entered one and system has six pumps two of which notify negative pressure.

What can be the reason of that?

Best regards,

Seylan

Parents
  • I once had trouble with getting excess flow out of lift stations due to the pump control settings. If the pump controls are not set well then sewercad may get 'confused' when it runs the calculations leading to more effluent than influent. I would verify that the pump controls are set to cycle the pumps on/off at realistic elevations within the wet wells.

     I too struggle with some negative pressure results... that seems to be a new result in v8 that wasn't there in previous versions.

Reply
  • I once had trouble with getting excess flow out of lift stations due to the pump control settings. If the pump controls are not set well then sewercad may get 'confused' when it runs the calculations leading to more effluent than influent. I would verify that the pump controls are set to cycle the pumps on/off at realistic elevations within the wet wells.

     I too struggle with some negative pressure results... that seems to be a new result in v8 that wasn't there in previous versions.

Children
  • Negative Pressure warnings at pumps are new as of V8i.  The negative pressures occurred all along, and would be apparent if pressures on the upstream side of the pump were viewed, but the User Notification regarding this condition is new.

     Negative pressures at pumps occur when the HGL on the upstream side of the pump is at or below the elevation of the pump itself.  A common cause of this is setting up pump controls that only turn the pump off when the wet well from which it pulls has gone completely dry.  In the real world, the pump will stop with a foot or two of water remaining in the well, in order to maintain Net Positive Suction Head, so the same thing needs to happen in the model. 

     In order to troubleshoot the problem, examine the upstream conditions, particularly invert elevations of any elements that feed the pump, and the elevation of the pump itself.  Then examine HGL at the upstream locations, and compare it to the pump's control settings.

  • Sometimes this problem is due to using the ground elevation as the elevation of the pump instead of the elevation of the pump impeller. Check the pump elevation

    Pumps can run with a slightly negative suction pressure as long as the head exceeds the NPSH.

    What do you mean by "total discharge higehr than entered one"?
  • Since I don't have shape file, I cannot use Load Builder and I have calculated discharges by determining population which each manhole serves. Then, I have input this population values to sanitary load control center as unit load count and I have used unit sanitary load capita based. My unit load is 0.01 L/s ad population equivalent is 1. With these values, total discharge of the pipe that has the lowest elevation and goes to treatment plant should be 11.8 L/. However, SewerCAD calculates this discharge between 33 and 45 L/s.

    Is this because of the pump properties or is there any another reason?

    (Note: The system has 6 pump stations.)

     

    Best regards,

    Seylan 

  • Can you include some more information about the system?  Do your loads vary over time (with a pattern?) or are they constant throughout the day?  Either your SewerCAD file, or a screen shot of your setup and some information about the operating controls for your pumps would be very helpful.  It's possible that all your pumps are cycling on at the same time, leading to high flows at the outlet of the system simply because everything is "hitting" at once.
  • The behavior of the pumps is controlled by the calculation option "Use pumped flows?". If this is set to True (default), then the pumps will discharge accordign to their pump curves regardless of the loading on the station.

    See the help for this setting.