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  • 1.  Hydraulic Modeling

    Posted 04-10-2017 04:27 PM

    I'm looking for some guidance on when the cost of performing physical hydraulic modeling of pump stations is considered justified, versus computer models or hand calculations.  Obviously, this is dependent on the judgement of the Engineer and Owner, but I'd like to hear about some different experiences on this.

    John Weiland P.E., M.ASCE
    Project Manager
    Wade Trim
    Eureka MO

  • 2.  RE: Hydraulic Modeling

    Posted 04-11-2017 10:44 AM
    We've always relied on computer modeling when it came to capacity and head.  But there are instances where vortexing, air entrainment and vibration become compelling issues that might not be adequately dealt with (or addressed with at all) in computer models, for situations dealing with large pump stations that are not entirely closed systems.

    We designed a large (130MGD) river intake structure for a WTP required extensive corner fillets and baffling to assure smooth flow to the pump intakes.  The model itself was quite large (20% physical scale) of one cell (of the three in the prototype). It was able to revealed significant flow inefficiencies and poor flow characteristics in the intake cells not anticipated although it was designed solely by dimensions from hydraulic manuals. With the model in place, the baffling and filleting were added one-by-one until the flow became completely laminar and smooth, as demonstrated visually by dye injected in the flow.

    For the same intake structure, but a different issue, another model at 5% scale of the entire intake structure and 800' of the riverbank were modeled to insure that shoaling of river sediment would not occur and block the intake "windows".  The model resulted in addition and positioning of sheetpile vanes to insure a permanent shallow scour trench at the outside face of the structure.

    Gene Rovak P.E., F.ASCE
    Sr. Consultant
    Saint Louis MO

  • 3.  RE: Hydraulic Modeling

    Posted 04-11-2017 03:44 PM
    ANSI/HI provides a great checklist for physical modeling, and section 9.8.7 provides guidance for using CFD. As of 2012 (that's the version I have on my desk), the standard states that CFD can be used for understanding general approach flow to the sump, but more simulating complex phenomena at the intake (free and submerged vortices and pre-swirl) was still not to a point that it could be standardized.

    CFD has evolved a lot in the last few years, so a more recent version of the HI may have more refined guidance on using CFD for detailed intake hydraulics. 

    Without listing each bullet in the HI here:
    - On the upper end, if your total station flow exceeds 144 mgd, or flow-per-pump exceeds 57 mgd, HI requires a physical modeling study
    - On the lower end (for more specific sump arrangements), if flow-per-pump exceeds 7 mgd, HI requires a physical modeling study
    - There is a great exhaustive list for specific requirements in HI

    This does not address your question about cost, but the standard should help you answer the question about what degree of modeling is required for your size of project.

    Brandon Billing P.E., M.ASCE
    Water Resources Engineer
    Coeur D Alene ID

  • 4.  RE: Hydraulic Modeling

    Posted 04-11-2017 05:06 PM
    When modeling a pump intake only physical modeling is recognized by the Hydraulic Institute to show compliance with the acceptance criteria. (The Hydraulic Institute Standards are recognized by all American pump manufactures and most world manufacturers.)  The HI Standrad further states that Computational Fluid Dynamic (CFD) models cannot be used to predict the flow approaching the pumps. CFD modeling can be used to assess the general approach flow to the sump or pump suction piping and for comparing designs. If the pump station design is an HI design and falls below the minimum criteria for physical modeling then desktop calculations are acceptable.The criteria can be found in ANSI/HI 9.8 -2012 for Pump Intake Design and ANSI/HI 9.6.6-2016 for Pump Piping.

    Thomas Demlow P.E., M.ASCE
    Northwest Hydraulic Cons
    Seattle WA