Help with overflow

Discussion in 'Anything DIY Related' started by Forbes, 6 Jun 2014.

  1. Forbes

    Forbes

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    Hi Guys,
    I am planning a tank build between our lounge and family room where part of the tank will sit on the wall and visibility will be through front and back with skinny filters at both ends.
    The size of the tank will be 1650 x 700/750 x 600 high, approx 750 litres. As a starting point, if I use the Gravity-fed pipe flow calculations from the internet, and use a 500 drop with a pipe length of 750.
    The calculations indicate that a 1 ¼”” pipe will give a flow rate of 13200 l/hr, a 1 ½” pipe 25920 l/r and a 2” pipe 42690 l/hr.
    If I allow for 20x tank capacity for sump feed, I need a pipe that will accommodate 15000 litres split between two outlets or 7500 litre /hour, which effectively means I could use a 1 ½”“ tank connector and piping for the sump feed.
    For circulation I am planning a closed loop system with four nozzles (two each end) each delivering 2300 l/hr. I have used these figures because I am planning to use the ‘Spin Stream Nozzles’ which have a maximum throughput of 2300 l/hr. To accommodate a volume of 4600 l/hr I was planning to use a 1 ¼” outlet and two 1” delivery pipes back to the tank.
    Assuming that the above is workable, I now move on to the overflows. My logic tells me that if I use a 1 ½” outlet, the area of the pipe is 1134 mm² which if I match with a 10mm high slot for the overflow, I only need a 113 long slot, however I know that this is not going to work.

    Can anyone tell me how to calculate the depth (area) of water for a given length of overflow, and how to calculate the minimum length of overflow required for flow rates. As I mentioned above, I only have the width of each end to use so I want to make sure I get it right.

    Any help and suggestions will be appreciated.

    Regards,
     
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  3. RiaanP

    RiaanP Moderator

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    Q = 1.84(L – 0.2H)H3/2
    H to the power of (3/2)

    Same calculation to work out the flow rate across a weir or dam wall
     
  4. RiaanP

    RiaanP Moderator

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    for a water head of 20mm, you need a 800mm wide overflow. If not mistaken.
     
  5. Forbes

    Forbes Thread Starter

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    Thanks for the info. Can you just confirm the definitions, of Q= Flow Rate, L= Length of Overflow and H= Water Head. If I am correct, it looks like you based your calculation on a flow rate of 44000 l/hr. What rate did you use for your calculations.
     
  6. RiaanP

    RiaanP Moderator

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    Q is the water flow rate in m3/sec,
    L is the length of the weir in m
    H is the head over the weir in m

    So
    broken down
    800mm wide overflow, 20mm head

    H to the power of 3/2 = 0.00282843 (A)
    (L – 0.2H) = 0.796 (B)
    1.84(A)B =0.00414263

    Q
    0.00414 m3/sec
    4.14263 L per sec
    248.56 L per minute
    14 913 L/h

    So about 15000L/h
     
    Last edited: 6 Jun 2014
  7. Forbes

    Forbes Thread Starter

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    Riaan,
    Thanks, I will play around with the numbers, however there are two issues I am still unclear on, firstly on a 750 litre tank what flow rate should I work on to the sump as this will obviously dictate the size of the overflow, and secondly if I require an overflow width of 800 with a head of 20mm, it means that at any point in time the area of water is 800 x 20 = 16000 mm[FONT=&quot]²[/FONT]. My brain struggles to understand how firstly how a skinny overflow with the same width but say 12mm wide can take the same volume and even more so a 1 1/2" pipe with an area of 1134mm[FONT=&quot]².
    I am trying to make sure I get all this right before I start otherwise it will be a lot of time and money out the window.

    [/FONT]
     
  8. RiaanP

    RiaanP Moderator

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  9. Forbes

    Forbes Thread Starter

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    What turnover should I allow for in the tank
     
  10. RiaanP

    RiaanP Moderator

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    how wide is the sump, front to back?
    What filtration are you planning to use? DSB?

    Remember that the volume of water over the overflow is equal to the volume of water over the last divider in the sump. In sump you can have higher water heads, but you do not want 20mm water head in the display. All smaller fish will flush over, and snails will enter the overflow.

    In a DSB setup, you should aim to have a 6 to 10mm head. So on the divider after the DSB, the distance between the top of the glass and the actual water surface should be between 6 to 10mm.

    So if your sump is 450mm wide, that means a pump that delivers at least about 3240l/h back into the display. After head loss calculations. Every 150mm difference in sump width is about another 1000L/h.

    So lets say you got 450mm wide sump, and have 3240l/h delivered back into your display, that means your overflow will release the same volume as well. Going for less water head in the display, say max 5mm that means your overflow width must increase to 130cm

    45cm wide with 1cm head is 9 liter per second
    130cm wide with .5cm head is also 9 liters per second.

    9*60*60 = 3240L/h

    Less than 6mm head in the sump with a DSB, and you end up with lots of settlement on the sand.
    if you use NP-pelets or Orca cubes, the calculations will differ. But what is the point of having a filtration system if your waste does not get the time to flush over towards the sump? And you want descend surface skimming.
     
  11. Forbes

    Forbes Thread Starter

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    If I don't break the wall down between the two rooms, the maximum space I have for a sump is about 350 x 1500, if I break the wall down, I have about 600 x 1500. At this point in time I don't know what is best to use in the space available, but I the pellet option with a DSB has been suggested to me. Ideally I don't want to have to break the wall out, but if it makes the difference between a mediocre tank and a great tank I will break the wall out.
    If I take the two flow rates I need and try and fit them in a 700 wide tank, I can't get the volume I need without a big head, essentially, I have a total length of about 1200mm for both the filtration and circulation systems.
     
    Last edited: 6 Jun 2014
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