What is the correct flow over your DSB?

Discussion in 'Biological/Natural Filtration and Deep Sand Beds' started by RiaanP, 19 Oct 2010.

  1. RiaanP

    RiaanP Moderator

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    What is the correct flow over your DSB?

    I posted this in another thread, but as not to loose this thought in another thread, let me direct my comment directly.

    Flow should be 3 to 5 times turnover over the DSB. That is what is normally advised standard given on this forum.

    Pulling this standard apart, what do we really want?

    We want a certain rate of water flow to prevent too mush settlement on the DSB, and slow enough for heavier particles to do sink down and be consumed.

    So in theory I need between 2250 and 3750 for my own 750L display system (ignoring head height).

    But now I think that this theory must be re-visited.

    First thing out of the way: The length of the DSB got nothing to do with the flow rate as measured with the flow direction over DSB chamber.

    The flow rate is directly related to the width of the DSB as measured from front to back or across the flow direction. Now that is an important aspect.

    We want the water to move at a certain water speed.

    Put this in another comparison.
    You move 100L in 5 minutes in a 100mm wide gutter over a certain point.
    Now you increase the gutter to 200mm. If you still move 100L in 5 minutes, your flow rate across the point will be slower. To have the same flow, you need to double up on the amount of water that you move.

    The result is that when you compare two different DSB's, the first at 300mm wide and the second at 600mm, then the old advice of 3 to 5 times your display turnover should now be changed to 6 to 10 times turnover. To be able to get the same water speed.

    As stated my display is about 750L. And I used a 3000L/h pump. Ignoring head height, that is 4 times turnover. Directly in the middle of the "standard" advise.

    But my sump is 600mm across.

    In reality, my flow always looked to slow, and I had a lot of settlement on the DSB. I got better results when I used 2 pumps of 3000L/h.

    Do we need to revisit the normally given comment of 3 to 5 times your display turnover. And rather use a water speed across the DSB. For example, test it by adding flakes to the inlet side of the DSB, it should be able to travel at least a certain distance before it settle on the sand. And the flakes should not travel over to the return pump.

    Or should we go more mathematical. Measure the width of the dsb. And we hopefully know what the flow speed should be. Then we should be able to calculate the speed of water inlet to DSB and that is equal to the Return pump rating, excluding head height. DSB's of different width should all have the same water speed or flow rate.

    So what will be the correct water speed?
     
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  3. butcherman

    butcherman Moderator MASA Contributor

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    I have also been wondering how much flow one needs over a DSB.

    How did we determine that we need 3 to 5 times turn over?

    I Agree that having a wider sump, means that the water moves much slower across the surface of the water.

    I recently put a bigger return pump in and as far as i can see it was a good thing as the tank is looking noticeably better.
     
  4. magman

    magman

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    from what I understood about DSB's is that the slower the better.

    I think you should consider the sump as a whole and not the dsb. The main part in the sump is the skimmer, and your flow should be that of what the skimmer is rated at, so that as the water comes into the sump, it all gets skimmed.

    And the sump should be built or then have enough flow so that you don't get any surface settlement.
     
  5. Travis1

    Travis1 (wilsontravy)

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    mods, i think you should make this thread sticky! good info Riaan.
     
  6. neo

    neo

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    Use the math to work out what is needed and then translate it to the food-flake test so one can do an easy test.
     
  7. RiaanP

    RiaanP Thread Starter Moderator

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    Yes Magman, valid point.

    BUT
    take the same skimmer, for example TS2 on 2 different sumps. One being 300mm wide and another being 600mm wide. Then obviously the flow across the DSB will be double that on the 300mm compared to the 600mm.

    And too slow flow is not good either. So that is where I ended up. Too much settlement.

    And skimmer should not take out everything. The DSB does need some food.

    Do not know, but that is what is normally mentioned.
    It is the easiest method I think that could be used to calculate your return pump size. Easy, how big is your display? hmmm 500L, ok need a 1500 to 2500L/h pump. Easy hey. But that exclude the actual physical proportions of the sump.

    But if we now say that the speed the water flow across the DSB should be 20cm a minute (that is a thumbsuck figure - do not quote me).
    And my DSB is X wide (being the only variable in the calculation)
    And water level above sand is Y deep.

    From that I should be able to calculate the volume of water needed to comply to that flowrate.
    And that should then indicate my return pump size.

    Still with me?
    Or did I lost it...
     
  8. RiaanP

    RiaanP Thread Starter Moderator

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    where's the formula?
     
  9. butcherman

    butcherman Moderator MASA Contributor

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    I'm with you Riaan :thumbup:
     
  10. leslie hempel

    leslie hempel Moderator MASA Contributor

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    another vauge area in this discussion is : how big must your DSB be in relation to your system to adequatley to perform its function of denitrification? remember not only a DSB performs this function, LR of the correct density does aswell and can remove the pressure on the aquarist to dedicate a massive area to a DSB.

    i dont run a DSB and i have never seen it to be critically necessary to my style of reefkeeping but i do agree we need to find a general standard..

    is there a calculator for DSB that tells you what ratio it should be in toyour system? i have always ever heard people say it should be as big as possible.

    all i can find is this: Sand bed calculator but its for working out how much sand is needed to fill the designated area, not working out the area in relation to what is needed.
     
  11. RiaanP

    RiaanP Thread Starter Moderator

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    butcherman likes this.
  12. butcherman

    butcherman Moderator MASA Contributor

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    Interesting link riaan
     
    Last edited: 20 Oct 2010
  13. sihaya

    sihaya

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    IMO, anything anyone comes up with regarding flow over a sand bed is just an educated guess. Obviously, in the natural world, the flow over any sand bed is immense and far greater than anything we'd be doing in an aquarium. However, there's really no comparison... and the natural reef doesn't have issues of "fly away" sand and other such complications of a closed system. That, and, the flow over real sand beds is just totally different (it's not constant one direction). Some people say the flow should be slow, some say it should be as much as possible... who knows? I'd just go by trial and error (i.e. try different rates of flow and see what results in the most life in the sand bed). Of course, if you can put a surge over the sand bed, that's probably even better.
     
    Last edited: 25 Oct 2010
  14. leslie hempel

    leslie hempel Moderator MASA Contributor

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    i think the biggest issue here is settlement which could clog the sand bed..
     
  15. RiaanP

    RiaanP Thread Starter Moderator

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    Thanks Sihaya
    I increased mine a lot. Changed my Cryptic tank to flow back to first chamber as well. And added pump to push water diretly from return to skimmer chamber. A lot more flow now, and so far after 2 days looks a bit better. Let me give it a week.
     
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