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RE: Top Fill Tank and Mixing

Offline nkg2 over 9 years ago

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?

Parents

Offline Jesse Dringoli Fri, Mar 27 2015 9:06 AM
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.
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Offline Mal Sharkey Fri, Mar 27 2015 10:22 AM in reply to Jesse Dringoli

I think traditionally FIFO mixing is used for things like chlorine contact tanks where you have separate inlets and outlets, and baffles to prevent mixing / short circuiting.

It's not so clear cut in a regular tank though. You will get some mixing, but it's hard to say how much.

As Jesse says, it would be worth running simulations to test out both the FIFO and 'fully mixed' options to see how the results compare. If they are similar then you don't need to worry which option you choose. If you still have some concerns then you can consider the next steps (i.e. more detailed CFD analysis of the tank, installation of a mixing system, installation of a chlorine booster, etc).

Here are a couple of papers I have seen on the subject:
www.caldwellwatertanks.com/.../Opflow-August-2011-TankMixing-FINAL.pdf
www.utilityservice.com/.../Best%20Practices%20Improve%20Storage%20Tank%20Performance%20and%20Reliability.pdf

Regards,

Mal
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Offline Mal Sharkey Fri, Mar 27 2015 10:22 AM in reply to Jesse Dringoli

I think traditionally FIFO mixing is used for things like chlorine contact tanks where you have separate inlets and outlets, and baffles to prevent mixing / short circuiting.

It's not so clear cut in a regular tank though. You will get some mixing, but it's hard to say how much.

As Jesse says, it would be worth running simulations to test out both the FIFO and 'fully mixed' options to see how the results compare. If they are similar then you don't need to worry which option you choose. If you still have some concerns then you can consider the next steps (i.e. more detailed CFD analysis of the tank, installation of a mixing system, installation of a chlorine booster, etc).

Here are a couple of papers I have seen on the subject:
www.caldwellwatertanks.com/.../Opflow-August-2011-TankMixing-FINAL.pdf
www.utilityservice.com/.../Best%20Practices%20Improve%20Storage%20Tank%20Performance%20and%20Reliability.pdf

Regards,

Mal
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