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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.

  • 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.

Reply
  • 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.

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