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  1. #1
    Senior Member Rank = Jussai Alburglar's Avatar
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    Any Practical Anecdotal Evidence for gravity flow rates through 2" pipe?

    Long story short:
    Has anyone ever run a QT or grow on setup and used a 2" gravity drain? Or even better, a 2" gravity return? What size pump worked? Did you try others - what did it max out at?
    5000lph? 10,000lph? 20,000lph?

    Long story long:

    I'm sure I'm not understanding the online calculators for gravity flow as they are churning out bollocks.

    I'm trying to figure out the maximum flow my four x 2" returns can gravity return back to my pond in order to figure out what my airlifts are capable of.

    I'm happy with what I'm inputting into the calculators. I've taken an average from two of my four returns. These two are very similar, and I've used that as my example. As for the others, one is much longer with the same 75cm drop and one is almost direct into the pond, with almost zero drop. But I haven't even bothered with those yet.

    With an average of 2m of 5.53cm (internal dimension of 2" pipe) with a roughness coefficient being 150 for PVC pipe. Dropping 75cm from airlift collector to return inlet in the pond. The online calculation says it will gravity flow 12.14 litres per second. 3600 seconds in an hour equals 43,704 litres per hour for one return only. I haven't bothered accounting for 90 bends etc as this is clearly ridiculously high to be realistic. Therefore, I'm interested in real world experience



    Last edited by Alburglar; 07-12-2023 at 09:15 PM.
    2660 Gallons. 4" Bottom Drain and Skimmer. Draco Solum 16 Drum. Anoxic Filtration. Air lift returns.

  2. #2
    Is that drop correct? 750mm over 2000mm length? Seems a lot?

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  3. #3
    Senior Member Rank = Jussai Alburglar's Avatar
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    Perhaps that's my issue?
    I took the drop to mean the distance , in height, from the collector outlet, at one end of the pipe to the pond inlet, at the other end of the pipe. That measurement is 750mm for my two bottom returns.
    2660 Gallons. 4" Bottom Drain and Skimmer. Draco Solum 16 Drum. Anoxic Filtration. Air lift returns.

  4. #4
    Quote Originally Posted by Alburglar View Post
    Perhaps that's my issue?
    I took the drop to mean the distance , in height, from the collector outlet, at one end of the pipe to the pond inlet, at the other end of the pipe. That measurement is 750mm for my two bottom returns.
    The drop will be from the point of water discharge to where it exits the pipe. I.e. top of air lift system to where it come out of the pipe to return to pond

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  5. #5
    Senior Member Rank = Jussai Alburglar's Avatar
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    I've just read something that suggest's that drop means the height of water level to the exit of an open ended pipe (open to the atmosphere). But the return isn't an open ended pipe, it's the pond which is full of water. So in this case, does the drop means the distance in height between the level in the collector and the pond which is probably 100mm lower than the collector?

    If I zero the drop this happens , which can't be right either:
    Last edited by Alburglar; 07-12-2023 at 08:51 PM.
    2660 Gallons. 4" Bottom Drain and Skimmer. Draco Solum 16 Drum. Anoxic Filtration. Air lift returns.

  6. #6
    Senior Member Rank = Jussai Alburglar's Avatar
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    Quote Originally Posted by Hovis View Post
    The drop will be from the point of water discharge to where it exits the pipe. I.e. top of air lift system to where it come out of the pipe to return to pond

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    That would be 750mm then.
    2660 Gallons. 4" Bottom Drain and Skimmer. Draco Solum 16 Drum. Anoxic Filtration. Air lift returns.

  7. #7
    Senior Member Rank = Jussai Alburglar's Avatar
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    At 100mm drop
    4.09 litres per second x 3600 = 14,724 litres per hour, which still seems unrealistically high. I definitely do not understand what I'm doing with any confidence. I would value any Practical experience over my idiotic maths.

    Last edited by Alburglar; 07-12-2023 at 08:43 PM.
    2660 Gallons. 4" Bottom Drain and Skimmer. Draco Solum 16 Drum. Anoxic Filtration. Air lift returns.

  8. #8
    Quote Originally Posted by Alburglar View Post
    I've just read something that suggest's that drop means the height of water level to the exit of an open ended pipe (open to the atmosphere). But the return isn't an open ended pipe, it's the pond which is full of water. So in this case, does the drop means the distance in height between the level in the collector and the pond which is probably 100mm lower than the collector.

    If I zero the drop this happens , which can't be right either:
    If the drop is 0 that's flat so there will be no gravity flow. On this occasion the zeros you are seeing are correct.
    Have I understood you correctly and you are saying you have a gravity discharge entering the pond below water level?

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  9. #9
    Senior Member Rank = Jussai Alburglar's Avatar
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    Yes. Two of them enter about 750mm down (I'd need to measure properly, but that's about right). Two are maybe 250mm below the water level in the collector, but I'm ignoring those in the maths for now. They all 'appear' to flow a similar amount to the eye, the difference is certainly not obvious at this stage.

    14,724 litres per hour for 100mm drop, certainly looks like a more realistic starting point than 43,794 litres per hour.

    If there's an equation to add in 90 bends (one has 6 bends!) Maybe that would reduce 14,724 litres per hour, down to something realistic.
    Last edited by Alburglar; 07-12-2023 at 08:56 PM.
    2660 Gallons. 4" Bottom Drain and Skimmer. Draco Solum 16 Drum. Anoxic Filtration. Air lift returns.

  10. #10
    Quote Originally Posted by Alburglar View Post
    Yes. Two of them enter about 750mm down (I'd need to measure properly, but that's about right). Two are maybe 250mm below the water level in the collector, but I'm ignoring those in the maths for now. They all 'appear' to flow a similar amount to the eye, the difference is certainly not obvious at this stage.
    Well they can't be gravity then. The water pressure would just mean they back up? They must be under pressure?

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  11. #11
    Senior Member Rank = Jussai Alburglar's Avatar
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    The air lifts fill the collector and raise the water height inside, which in turn, escapes via gravity into the pond. So yes there is some minimal head pressure there. Which I reckon is negligible
    and does not influence, what I'm trying to achieve. The gravity flow of a 2" pipe is all I'm interested in.
    I can summarize the rest from that. The calculations based on those inputs alone are clearly bananas, so pointless to worry about it at this stage.



    https://youtube.com/shorts/69PZAAd1D...PJhWWyJuKllK7R

    - I know of absolutely zero other systems of this design for comparison. As far as I know, it's the first, which is not a boast as it's most definitely NOT the best. In hindsight, The more common way of surface returns inside an enclosed pipe seems much more fool proof and has the added vantage of momentum it would seem.

    How much flow are the returns capable of gravity returning would get me somewhere in the ball park of what the collector could handle.

    Once I max out the box I know what the airlift returns are capable of course, but I'm trying to preempt that.

    I could easily add two lifts to each side of the collector, but no point adding four more airlifts if the returns can't handle it.

    I know the drum triggers more often than when running a 5000lph pump, so I know it's more than that, as a baseline, but that's it.

    I suppose I could just pump varying amounts into the collector until it maxes out and over flows, but I don't have a vari-pump. I also am under no illusions. I'm sure if I pumped my 16'000lph pump into the collector it would hit the box and splash straight back out, because it is very small, so that's not an option.
    Last edited by Alburglar; 08-12-2023 at 11:39 PM.
    2660 Gallons. 4" Bottom Drain and Skimmer. Draco Solum 16 Drum. Anoxic Filtration. Air lift returns.

  12. #12
    Senior Member Rank = Jussai Djstiles999's Avatar
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    I wonder if the piece missing from your maths is the resistance of the water in the pond/pipe that the returning water has to overcome when it reaches the ponds natural water level in the pipe, if that makes sense?
    13,243 gallons, Filtreau HF30ís K1 capacity of 1,400l, Bakki Shower, BHM and understanding wife

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  14. #13
    Senior Member Rank = Jussai Alburglar's Avatar
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    That does make sense yes. But I reckon until two water heights are the same, they would 'flow' from one to other, until they've equalised (of course). But probably at a decreasing rate, until they're level. No idea how you'd figure that out. There must be a lot missing from those calculations, which is why, "short story even shorter"
    I'm not interested in the calculations. I've elaborated only to show I only understand them enough to realise they don't add up and I don't understand the subject further than that. For example, What I can fathom is:

    Will an open ended 2m length of 2' pipe dropping 1m in height flow 43,000 litres an hour? I reckon you'd be hard pressed to pump 43'000 litres an hour through it. Let alone gravity.
    If it's open ended, and the drop is 0, would a 2" hole in the bottom of a water container flow zero water. - No it would empty pretty much at the same rate as the 2m pipe scenario.
    I just don't get it.

    So any Practical experience would be golden at this stage, if we have any?
    Last edited by Alburglar; 08-12-2023 at 06:02 PM.
    2660 Gallons. 4" Bottom Drain and Skimmer. Draco Solum 16 Drum. Anoxic Filtration. Air lift returns.

  15. #14
    Senior Member Rank = Adult Champion Twhitenosugar's Avatar
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    I'm not sure I follow everything you've said, but could you do a bucket test on one of more of your returns to get an idea on flow. You'd probably need to put a bung in some of the returns to do that.



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    Last edited by Twhitenosugar; 09-12-2023 at 06:38 PM.
    13,000L fibreglassed raised pond with window

  16. #15
    Senior Member Rank = Jussai Alburglar's Avatar
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    ...I'm not sure I follow anything I've said. Lol
    2660 Gallons. 4" Bottom Drain and Skimmer. Draco Solum 16 Drum. Anoxic Filtration. Air lift returns.

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  18. #16
    No practical info sorry, but this is taken from Koi Organisation International, which is a reputable source. Max gravity flow through a 2" pipe is about 3500 l/hr. This is based on a real world pond scenario; but of course what you actually achieve on your system will be determined by your set up, complexity of the pipe run and your total frictional losses. But it's a good starting point.
    (It also assumes a 2.5cm drawdown - difference in water level between pond and filter, but let's not over complicate things).
    To optimise flow rate in a given system you need to minimise frictional losses - pipe runs as short as possible, minimum number of bends (swept bends are much better than 90s or 2x45s, minimum number of fittings.
    Bigger diameter pipes give much improved flow - yes they're bigger but frictional losses are relatively much smaller.

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  20. #17
    Senior Member Rank = Jussai Alburglar's Avatar
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    Ok that's great, thanks. Anything near 14,000lph for the four returns, I'd be over the moon with, so good to have that to aim at.
    2660 Gallons. 4" Bottom Drain and Skimmer. Draco Solum 16 Drum. Anoxic Filtration. Air lift returns.

 

 

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