Water flow speed vs level

Discussion in 'Pumps and Waterflow' started by lIghty, 23 Dec 2007.

  1. lIghty

    lIghty

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    Hi

    With regards to water flow speed over a DSB, could one not control the speed at which the water flows over a DSB by increasing or decreasing the hight of the water over the sand?

    any advise?:whistling:
     
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  3. Kanga

    Kanga Retired Moderator MASA Contributor

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    lIghty, IMO it would make zero difference in terms of litres per hour going over it.

    I think i get the thinking behind the question (i hope :)) But here is the thing.

    Your return pump is a set volume per hour, lets say 6000l/ph for the sake of the argument.

    Now take 1000L drum and 6000L drum.

    The 1000L drum will empty in 10 minutes, the 6000L drum in 1 hours, so the flow is slower, right?

    Now when the drum starts overflowing, which in your sump would be the return chamber, it will overflow at 6000L (the volume of the return pump)per hour, regardless of the size(depth) of your sump.

    HTH
     
  4. viper357

    viper357 Admin MASA Contributor

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    Logically I don't think the water height will make a difference. Whether it is 5cm or 50cm high above the DSB, the flow will still solely depend on your return pump.
     
  5. dendrosa

    dendrosa Member

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    The speed over the DSB is determined by the surface area of the dsb not the ht of the water flowing over dsb
     
  6. viper357

    viper357 Admin MASA Contributor

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    Quite right, but at a fixed width and length, a variation in height will not make a difference.
     
  7. lIghty

    lIghty Thread Starter

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    My thinking is, Like you say the return pump is 6000l/ph (a constant) and the water level was only 1cm above the sand it would flow very fast over the top but if you increase the level to say 15cm, the flow RATE would still be 6000l/ph but the SPEED would be alot slower.
     
  8. viper357

    viper357 Admin MASA Contributor

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    Ok, I see what you mean. In that case then yes, the speed of the water at the top (water surface) will be a lot faster than at the bottom (directly on top of the DSB).
     
  9. lIghty

    lIghty Thread Starter

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    If i'm correct, the Speed will be proportional to the cross sectional cut surface area, in other words, if you have 100lph flowing thru a 2" pipe the speed will be slower than 100lph flowing thru a 1/2" pipe.
     
  10. Kanga

    Kanga Retired Moderator MASA Contributor

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    correct, almost like narrowing your DSB, however i dont believe it will improve denitification.

    However I could be wrong here;)
     
  11. sunburst

    sunburst

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    Correct simply put...the volume will be the same but the velocity will be different....deeper = slower

    Kanga would the speed not affect a sand beds efficiency.
     
  12. Kanga

    Kanga Retired Moderator MASA Contributor

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    Mhh would have to ask someone like Mille or Hennie.

    I would imagine that if your flow rate is 6000L an hour it would be the same.

    Sure is something to think about.
     
  13. lIghty

    lIghty Thread Starter

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    Well put, this was my reason, to experiment with different velocities:thumbup:

    </IMG>
     
  14. dendrosa

    dendrosa Member

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    I believe what is being overlooked here is pressure. If you have a fixed volume sump with a certain fixed volume of water entering one end and exiting the other such that the system is in equilibrium, ie the level of water stays constant, the level of the water will be a certain height above the exit pipe. This height represents the additional pressure required to overcome the frictional forces in the outlet pipe. If you now restrict the outlet pipe slightly, the level in the sump will rise to increase the pressure in order to overcome the additional frictional force. Once this new hight is reached the same voume of water is still flowing through the system (if you discount the reduced inflow into system due to increased backpressure). The speed of the water flowwing across the dsb is unaffected. The ht of the water only affects pressure not speed.
     
  15. sunburst

    sunburst

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    Agree with you Dendrosa....you could compare it to putting a hole at the top of a water tower and one at the bottom....you will easily plug the hole with your thumb at the top; but will get very wet trying to plug the one at the bottom:)

    I have often wondered how to best make use of the limited space of say a small remote sand bed. Somehow flow must play a part by affecting the depth at which denitrification takes place. If we were to put consecutive ridges like corrugated iron against the direction of flow; would this not:

    1)Increase the surface area of the bed
    2)The alternating high low areas would create alternating high/low pressures . Using fluid or aerodynamics or osmosis, movement would be from high to low. Would this not broaden the nitrification/denitrification zone, thereby improving efficiencies.
    3)If we were to increase water velocity even further, would this not broaden it further.
    4)If the bed was the shallowest at the entry and thickened towards the overflow would this not have an exponential effect on those pressures or depth/thickness of the denitrification zone. Like a ram jet

    :1confused::confused::rolleyes::chatterbox:
     
  16. lIghty

    lIghty Thread Starter

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    Maybe this sketch pic will help, remember you not affecting the down pipe as water would flow down it (even if it is submerged) just as if you where lifting a straw from your coldrink. The only thing that you would need to do is the adjust the height of the second pane of glass in the chamber, causing the water level to rise above the sand surface, therefore increasing the volume of water and decreasing the velocity of the water. Like a river runs fast into a dam, slows down and out again fast.

    Sorry don't know how to upload jpg
     
  17. dendrosa

    dendrosa Member

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    IMO the effectivness of a dsb is a function of its density of critter population - copopods, amfhipods, brisleworms etc, rather than the speed at which the water flows over it. Also, for my money, a dsb is only really effective within the main tank where critters can process wastes and draw them into dsb and down to anaerobic bacteria. Often I see guys putting the dsb in the sump, after the protein skimmer. What can critters feed on there?
     
  18. dendrosa

    dendrosa Member

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    The velocity of the water passing over the second pane would be the same no matter how high it was ( within reason). You would just have a deeper area of "still" water below it. What would you hope to achieve by that? Waste would just need to fall further to reach the dsb
     
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