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Relationship between hydraulic grade and head

I'm modeling a pressurized wastewater system in SewerGems v8i.  I am treating the design-phase pump station that begins the pressure section simply as a Pressure Node.  The Maximum Hydraulic Grade at this start node (132 feet above msl) remains constant when I change only the elevation of the node and re-compute (GVF complex).  The high pipe elevation of the system is midway and 70 feet above msl.  Can this unchanging Max Hydraulic Grade be understood in light of the the fact that the static head is less with a proposed higher start node elevation (i.e. 70-10 = 60) and more with a proposed lower start node elevation (70-5 = 65)? It in an oversimplification to say Head Required = Maximum Hydraulic Grade - System Start Elevation?

  • Hello Philip,

    We may need some additional information on what you are modeling. First, pressure subnetworks typically start with a wet well. If you don't have one, one will likely be needed. One of the SewerGEMS sample files will include a setup that includes a wet well. 

    The maximum hydraulic grade is simply the highest hydraulic grade value reported during the calculation. There may be a lot of factors to take into consideration for how this may change, but it will be related to the pump head and the elevation of the wet well on the suction side of the pump. 

    I'm not exactly sure what the "head required" you mention is in reference to. Is it related to system head curves?

    Regards,

    Scott

  • Hello Philip,

    What is upstream of this "pressure node"? How do you get an HGL over here? Is there a pump upstream?

    The hydraulic grade at a point is the sum of pressure and datum head. The hydraulic grade won't change if you change the elevation in a pressurized system because your boundary (maybe an upstream pump) is not changing. If the pump upstream is supplying at a constant head the HGL at downstream points won't change much.

    Can this unchanging Max Hydraulic Grade be understood in light of the the fact that the static head is less with a proposed higher start node elevation (i.e. 70-10 = 60) and more with a proposed lower start node elevation (70-5 = 65)?

    The static head is usually the difference of elevation from your point of delivery (pump discharge, tank outlet) and the elevation of your discharge point. The statement you have proposed is obviously true but I am not getting the point of it.

    It in an oversimplification to say Head Required = Maximum Hydraulic Grade - System Start Elevation?

    Yes. Because head required depends on what head are you computing. Is it a system head as Scott suggested? Is it pump head that you are determining for an upstream pump? Also, head requirement does not depend only on static head. It depends also on frictional headloss and minor losses. See the below article for details on determining pump curve for a system;

    Estimating a pump curve for a model

    If you can elaborate more on what you are trying to model we can help you better.


    Regards,

    Yashodhan Joshi

  • The system includes two distinct pressure subnetworks, "in line" with each other.  Pump Station #1 is at a landfill, conveying leachate a distance to a free outfall at what *is currently* a waste water treatment plant, but is going to be decommissioned/rebuilt as a PS.  Call this PS #2.  My query concerns the subnetwork from PS #2 (wet well elev +1.0) to a free outfall (elev -3.0) > 5 miles away, that includes one tie-in (pressure node) where sanitary loads will be added and air valves throughout the subnetwork.  The high pipe invert elevation of this subnetwork is 70 feet above msl.  The modeling is in support of pump size recommendations for PS #2.  Thanks

  • Hi Philip, I am not quite clear on some of the things you mentioned. For example when you say that you have a "high pipe invert elevation" 70 feet above sea level, do you mean that you have a local high point that the pump needs to overcome? This should be automatically accounted for in SewerGEMS and the pump should add enough head to overcome it, or you can use an air valve with the GVF-Convex (SewerCAD) solver as mentioned here.

    However, if you are saying at the high point is at 70 feet and the calculated HGL is 132 feet, then it suggests that the pumps are adding enough head to keep this high point pressurized, in which case changing the physical elevation of the pressure junction would indeed not impact the hydraulic grade, only the pressure (as long as the node elevation is kept below the HGL), as the pump operating point will be based on the system head characteristics based on boundary conditions (upstream wetwell elevation and downstream gravity discharge point). See more in the system head curve articles previously provided.

    If this does not help, please provide a copy of the model for review, and specific steps for us to follow with that model open, so we can see what you are looking at and better understand your concern.

    As a side note, I noticed you mentioned that you are using SewerGEMS V8i. We recommend upgrading to the CONNECT Edition which also uses updated licensing, as Support for V8i will be discontinued (and we've made many enhancements that you can take advantage of in the CONNECT Edition.) See this and this.


    Regards,

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

  • The beginning point of the system is a submersible pump in a wet well with an average water surface of elevation 1.0 feet.

    After traveling thousands of feet the water reaches elevation 71 feet which is the high point in the system. From here the piping system extends another 30,000 feet with several intermediate high points, but none above elevation 71 feet, to a free discharge. 71 is the pipe centerline elevation but for the problem we list below whether it is the pipe invert or pipe centerline elevation we think that is just terminology and not relevant.

    The problem with the model is if we raise or lower the wet well water level the hydraulic gradient elevation at 0 flow does not change. This is not correct for proper pump selection. If we make the wet well 10’ deeper the pump needs to lift the water an additional 10’ and theoretically the hydraulic gradient should increase by 10 feet. We were taking the hydraulic gradient elevation in the software as being the submersible pump discharge head. Is that a correct assumption?

    Is there a “wet well” option that we should activate that will result in the hydraulic gradient changing with varying wet well levels?