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RE: Top Fill Tank and Mixing

Ref: http://communities.bentley.com/products/hydraulics___hydrology/f/5925/t/63081

Mal can you expand on this a little more? I have recently been awarded a project where the client is wanting a very large tank for detention time in case of emergency and fire flow and I am sketching out/flow diagram of what I would do. I am concerned about age and the quality of the water due to the age considering the setup they have is as fresh as it will ever be. I am thinking that my new tank will have a separate inlet that enters from the bottom of the tank to nearly the top of the tank and leave via a second pipe near the bottom of the tank. My assumption in this case would be to go toward FIFO but at the same time I almost consider it to be complete mix. Thoughts?

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  • Could you tell us a bit more about your tank? FIFO typically would make sense for a top-filling tank because the parcels of water that first enter at the top have to drain down vertically before they leave the bottom.

    Is there a particular reason why you feel that "completely mixed" would be more appropriate?

    Does your tank have a large diameter?

    Have you tried a sensitivity analysis to see if mixing in your tank is significant in the results you're interested in? (run two water quality analysis scenarios with the different mixing method and compare results)


    Regards,

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

  • Hi guys, sorry for the late reply.

    Nothing has been modeled so far yet because I am in the very beginning stages of this design but I am trying to identify all the variables associated

    Typically water tanks, especially if there is a possibility of chlorination at some point, needs to have a separate inlet and outlet and needs to be at opposite ends to prevent short circuiting (much like what Utility Services Corp indicated in attached pdf). The reason why I am leaning towards complete mix or FIFO is I can see both but the Caldwell tank pdf kind of made me rethink that it does. Right now the idea is to go more fat versus height which might be counter productive against mixing (unless I use multiple inlets to agitate the tank). At this point I am trying to think through this and get ideas in my head how I want to approach this.

    So in looking in the help file, I can do constituent and water age analysis which is great. However I am assuming there is nothing in the software to allow me to analyze for dead spots in the tank (i.e. CFD) or is there that capability?

    Thank you, I think, for those two pdfs as they gave me more things to think about. ;-)
  • There are two very different types of tanks and they have different goals.

    1. Tanks before the first customer are intended to provide contact time to meet Ct requirements. You want to maximize average contact time and this is done using baffles to approach plug flow. You would model this with a FIFO approach with a separate inlet and outlet.

    2. Tanks in the distribution system can cause a deterioration in water quality. You want to minimize average detention time and this is done by trying to achieve complete mixing. You can get good mixing by maximizing inflow velocity with a jet inflow in the bottom and the same pipe for inlet and outlet. Avoid very tall skinny tanks as these can stratify in warm weather. You need to have a lot of turnover. An excellent report on this is Grayman W. et al. (2000) "Water Quality Modeling of Distribution System Storage Facilities,' AWWARF. It give some good guidance on the amount of turnover you need to get complete mixing.

    There are a number of manufacturers of in tank mixers if you can't get sufficient mixing from the inlet jet.

    Tom
  • Thanks Tom for the insight!

    There area few issues with this project that I am concerned with. Currently my client doesn't do disinfetion unless loss of pressure or some other event that would necessitate its use. Fire supression is used and storage is deficient in this case by our state standards and the current potable storage is at least half the size it should have been several years ago. However, everything works, water tastes great and generally speaking it works for their purposes except when the well had to be replaced last year. My client's thought was to get way more storage than necessary, maybe for unannounced plans, but I am concerned about water quality and water age. That is why I brought this up because in general this tank will ultimately provide both 1 and 2 of your tank categories.
  • If you are dealing with ground level tanks, you might try a two tank solution. You install one tank now to meet the needs for the foreseeable future but have space at the site for a second tank at the site. As the system grows, you can install the second tank. In this way you can take one out of service for painting or cleaning without seriously impacting operations.

    It doesn't work for elevated tanks where two tanks next to each other would look silly. In that case, you would look to install one tank now at one far corner of the system and a second at the opposite corner in the future.
  • Tom -

    That is definitely the ideal setup and for this application we sort of will have that unfortunately it will incorporate an altitude valve and some other things. Come to find out there are other issues so I get the fun task of trying to change one's mind on type and location. :-)

    As far as CFD, I recently read an article on the use of OpenFOAM where it was used to model and perform dead zone analysis of a storage tank. I am looking into it now but appears to only run off of Linux. Any other potential CFDs that does not break the bank or does WaterGEMS have some sort of solver for tanks/reservoirs?
  • Bentley doesn't have a water CFD product.

    I can share some information off line with you. Send me you email address to tom.walski@bentley.com.
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