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Negative Pressures - WaterCAD

I just upgraded my model to the latest version of WaterCAD. I'm running a steady state model and I'm getting negative pressures on the suction side of some of my pumps whether or not the pumps are on. I also have some FCV that are closed that have negative pressure. This doesn't make sense to me. What are some possible things I can be checking to fix this (assuming that it shouldn't be this way)?

  • Mikalea,

    If your reservoir water level is lower than your pumps in a real system, keep an eye on the net positive suction head available (NPSHA) versus net positive suction head required (NPSHR) for your pumps. As you may know you need to make sure the NPSHA is big enough, otherwise you get cavitation (formation of bubbles of water vapor) at the pump, and that is not good because it causes the pumps to wear out quickly.

    We have an add-in for WaterGEMS which will do the NPSHA calculations for you. Let me know if you would like that.

     

    Regards,

     

    Mal Sharkey

    Product Manager
    Bentley

          

  • That was helpful. The reservoirs are indeed lower than the pumps. I just didn't have enough experience with water modelling to decide if that was okay and I've nobody here to ask.
  • Mikaela,

    A negative pressure means that the calculated hydraulic grade is below the physical elevation of the element. This is the way it has been for pretty much every version of WaterCAD. You should closely examine your node elevations and boundary conditions (reservoir/tank hydraulic grades) to make sure they are correct.

    It is common for this to occur on the suction side of the pump, due to the elevation that you entered for the nearby reservoir. If the pump is off, then my best guess is that the reservoir elevation is set to be lower than the pump elevation. Creating a profile of the area in question (physical elevations and hydraulic grades) should give you a good visual idea of what is going on.

    Note that negative pressures will not prevent the model from computing - the messages that you get about these are just informational. WaterCAD still assumes that the pipes are pressurized and no vapor pockets will form. It basically just computes the hydraulic grade based on the system conditions and other data input, and then reports the pressure as the difference in head between that and the physical elevation. If the negative pressures occur at high points in the system, you probably would have an air valve at that point in reality.

    Some of the other modelers in this forum may be able to contribute more, based on their own experience. If you still need help and want someone to take a look at your model to explain why the negative pressures are occuring at specific places in your particular model, I would recommend sending it to technical support. In this case, please zip and send the model files to Support@bentley.com, along with your company information and exact version number, found under Help>About. 

    Regards,

    Jesse Dringoli

    Technical Support Group


    Regards,

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