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WaterCAD - Negative pressures due to Submersible Well Pump Not delivering enough head with storage tank at low elevation

Hi All,

In waterCAD, I am modeling submersible well pumps which will pump into a tank about 12,000+ ft away. The wells are located at a higher elevation compared to the storage tank.

My main question is on how to model accordingly to have no negative pressures in the pipeline shown in the profile views? Currently, the head is still below the the physical pipeline even after including combinations of PSV and air valves. Some junctions near the well are still showing negative pressures.

I believe we have the correct size pump since the pump should be able to achieve the head required to pump out of the deep well and into the pipeline near the surface grade. With the appropriate pump and well at such high elevation, I don't expect to see the HGL to fall below the physical elevation as the water travels down to the tank.

Pump: Pump size and curve should be ok. well water level to the surface elevation is about 120' + 16' headloss. Eventually, I will model with the well's drawdown water level resulting in total required head of 170' + 16' headloss. Currently only modeling with 1 pump, but the pump is not performing at the anticipated head required. Eventually, I will be adding in more wells and submersible pumps to contribute to the pipeline.

PSV/Air Valve: I added these components into the model to add in "boundaries" to allow the model to realize that the pump needs to pump to higher elevation since the pumps are deep in the well. Added a PSV upstream of tank to allow pressure to buildup. Without the PSV at the tank, the tank would be treated as an open surface boundary in the model, which resulted the HGL being calculated to be lower than the physical elevation.

PRV: if pressures get too high along the pipeline (since the pipeline starts at a high elevation), we can look into adding PRVs.

attached image and zip of the model.

Thanks

Raw Water Line.wtg.zip

Parents
  • It looks as if you are trying to siphon the water over the high point. This usually doesn't work well in practice, especially if the high point is more than 32 ft above the HGL.

    You need to size the pump to get the HGL to the high point.  One solution is to put an air release valve at the high point. This will give you 0 pressure at the high point an increasing pressure as you go downhill.

    The slope of the HGL looks very flat. You may be able to downsize the pipe a bit.

  • Tom, the model is showing a -45 psi pressure for the highest elevation portion of the system.  Is this accurate?  It would seem that the negative pressure could not exceed -14.7 psi (Atmospheric pressure).  Can you provide some additional explanation of whether the pressures below -14.7 psi in the model are accurate or whether we should consider all values below -14.7 psi as simply -14.7 psi.  Thanks.

  • om, the model is showing a -45 psi pressure for the highest elevation portion of the system.  Is this accurate?  It would seem that the negative pressure could not exceed -14.7 psi (Atmospheric pressure).  Can you provide some additional explanation of whether the pressures below -14.7 psi in the model are accurate or whether we should consider all values below -14.7 psi as simply -14.7 psi.  Thanks.

    WaterCAD will not impose any special handling of sub-vapor pressure conditions. The negative pressure indicates a problem that needs to be addressed. See this: Troubleshooting negative pressures

    If an upstream pump would actually "lift" the water above the high point, make sure you use an air valve node element at the high point, with "treat as junction?" set to "False". Negative pressures downstream of the air valve indicate part full flow. See the article mentioned in my previous reply.


    Regards,

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

Reply
  • om, the model is showing a -45 psi pressure for the highest elevation portion of the system.  Is this accurate?  It would seem that the negative pressure could not exceed -14.7 psi (Atmospheric pressure).  Can you provide some additional explanation of whether the pressures below -14.7 psi in the model are accurate or whether we should consider all values below -14.7 psi as simply -14.7 psi.  Thanks.

    WaterCAD will not impose any special handling of sub-vapor pressure conditions. The negative pressure indicates a problem that needs to be addressed. See this: Troubleshooting negative pressures

    If an upstream pump would actually "lift" the water above the high point, make sure you use an air valve node element at the high point, with "treat as junction?" set to "False". Negative pressures downstream of the air valve indicate part full flow. See the article mentioned in my previous reply.


    Regards,

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

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