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    Thread: Understanding the placement of a RDF

    1. #21
      danbo is offline Senior Member
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      As I mentioned in another post, gravity flow to a skimmer and thru bottom drains is very probably going to be 'laminar' & not 'turbulent'. According to Poiseuille's Law for laminar flow, the flow is a function of D^4. Not D^2 (D squared = area). So for 2x 4" drains and one 3" line from skimmer, the flow ratio will be 256:256:81 (assuming line lengths all equal). Or 512:81 (86%) to the bottom drains. It's why my skimmer didn't work. (I have 2x4" bottom drains & had 2" from skimmer, so 256:256:16 or 97% to the bottom drains). Check http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.html Go Mechanics, Fluids, Poiseuille's Law.

      Dan

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    2. #22
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      Ah, engineering... okay, then we must consider additional factors such as: pipe length, number of bends, angle of bends, amount of draw-down necessary to sink the floating ring or door in the skimmer, etc. Also, it's an over simplification to assume laminar flow. If it was true, we'd all be using cheap short compact 90-degree elbows, but we all know that they introduce flow restrictions and turbulence downstream, hence the large expensive long radius turns that are supposed to be used.

      Regardless, I already know that in MY case with the amount of flow I'm running, I simply cannot plumb the skimmer into the RDF as the system stands now. I should add that the reason I considered doing so was to eliminate the second pump in order to lower the energy costs. Currently, the second pump pushes skimmer water through two SG filters, so even if there was sufficient flow into the RDF to make the skimmer work, I'd still have to eliminate the SG filters to avoid the second pump. It's an entertaining problem.
      Last edited by kimini; 06-29-2020 at 08:56 AM.

    3. #23
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      Quote Originally Posted by kimini View Post
      Ah, engineering... okay, then we must consider additional factors such as: pipe length, number of bends, angle of bends, amount of draw-down necessary to sink the floating ring or door in the skimmer, etc. Also, it's an over simplification to assume laminar flow. If it was true, we'd all be using cheap short compact 90-degree elbows, but we all know that they introduce flow restrictions and turbulence downstream, hence the large expensive long radius turns that are supposed to be used.

      Regardless, I already know that in MY case with the amount of flow I'm running, I simply cannot plumb the skimmer into the RDF as the system stands now. I should add that the reason I considered doing so was to eliminate the second pump in order to lower the energy costs. Currently, the second pump pushes skimmer water through two SG filters, so even if there was sufficient flow into the RDF to make the skimmer work, I'd still have to eliminate the SG filters to avoid the second pump. It's an entertaining problem.
      Are you just overwhelming your RDF with the amount of flow you're trying to put through it by including the skimmer line? Is the answer as simple as (in theory anyway, not many people have this money to throw around) just getting an RDF that handles more flow? And a pump at the other end of it sized accordingly?

    4. #24
      DragonFireSG is offline Senior Member
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      Quote Originally Posted by kimini View Post
      Yes we are and we're talking about exactly this setup -
      If the skimmer is is of the floating ring type, it won't work unless there's really high flow through the (gravity-fed) RDF. I state this as fact because I've tried it and it fails—unless there's so much flow through the RDF (due to a strong pump downstream of the RDF) that it generates enough restriction in the bottom drains that it empties the input side of the RDF enough to generate sufficient flow to sink the skimmer float.
      Since the float skimmer should have a basket on it taking bulky stuff out of the water, a restriction on the line between RDF and skimmer can help in balancing the flow out. It'll definitely need a bunch of tweaking to get right, but should work like a charm once dialed in. That's pretty much my personal experience. I have my BD and skimmer feeding a common waste chamber that sits in front of the RDF. I use a standpipe to limit the hydraulic head on the skimmer line between pond and collection chamber, thus stopping the float from bottoming out when the pumps are turned up.

    5. #25
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      Quote Originally Posted by DragonFireSG View Post
      Since the float skimmer should have a basket on it taking bulky stuff out of the water, a restriction on the line between RDF and skimmer can help in balancing the flow out. It'll definitely need a bunch of tweaking to get right, but should work like a charm once dialed in. That's pretty much my personal experience. I have my BD and skimmer feeding a common waste chamber that sits in front of the RDF. I use a standpipe to limit the hydraulic head on the skimmer line between pond and collection chamber, thus stopping the float from bottoming out when the pumps are turned up.
      Thanks Dragon, have you got a pic of your standpipe set up?

      Cheers.

    6. #26
      kimini is offline Senior Member
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      Quote Originally Posted by DragonFireSG View Post
      Since the float skimmer should have a basket on it taking bulky stuff out of the water, a restriction on the line between RDF and skimmer can help in balancing the flow out. It'll definitely need a bunch of tweaking to get right, but should work like a charm once dialed in. That's pretty much my personal experience. I have my BD and skimmer feeding a common waste chamber that sits in front of the RDF. I use a standpipe to limit the hydraulic head on the skimmer line between pond and collection chamber, thus stopping the float from bottoming out when the pumps are turned up.
      I respectfully disagree. Yes, my skimmer does have a basket, which adds to its restrictive nature. The total restriction of the float, basket, and 3" pipe is far higher than the 4" bottom drains. Because of this, the water takes the easy and less restrictive path through the 4" bottom drains. Worse, what little flow that wants to flow through the skimmer is unable to suck down the float ring, so there's zero flow through it. This is true for my setup, with its particular flow, pipe lengths, and number of bends. Other people's systems may work better.
      Last edited by kimini; 07-01-2020 at 10:20 PM.

    7. #27
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      Quote Originally Posted by DragonFireSG View Post
      ......... I have my BD and skimmer feeding a common waste chamber that sits in front of the RDF........
      Slightly off topic : Wouldn't this common waste chamber, commonly known as a Settling Chamber (SC) that collects all the crap, defeat the purpose of an RDF?
      ....."Never argue with stupid people, they will drag you down to their level and then beat you with experience.".....Mark Twain

    8. #28
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      It'll catch "some" amount of waste, so presumably the RDF will cycle less often. It is redundant, but doesn't hurt to have, other than adding more chances for leaks and needing its own maintenance.

    9. #29
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      Quote Originally Posted by kimini View Post
      It'll catch "some" amount of waste, so presumably the RDF will cycle less often. It is redundant, but doesn't hurt to have, other than adding more chances for leaks and needing its own maintenance.
      So it's a choice of the RDF cycling less or leaving "some" amount of waste in the water column?

      kimini; I believe we had this discussion before ....... should go back on topic
      ....."Never argue with stupid people, they will drag you down to their level and then beat you with experience.".....Mark Twain

    10. #30
      coolwon is offline Senior Member
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      Whats in suspension gets caught in the rdf.

      The heavies settle to the bottom of the settlement chamber and are drained periodically to waste through your hand operated ball valve which should be mounted at the

      lowest point of the settlement tank.

      Garfield
      Find more about Weather in Durban, ZA

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    11. #31
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      Quote Originally Posted by coolwon View Post
      Whats in suspension gets caught in the rdf.
      Garfield
      Not strictly true Garfield. I know several RDF users who long for better fines removal in their systems.

    12. #32
      coolwon is offline Senior Member
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      Quote Originally Posted by perniciousviper View Post
      Not strictly true Garfield. I know several RDF users who long for better fines removal in their systems.
      A fine,is a fine.

      Polish the water after the rdf with sand filter.

      Garfield
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    13. #33
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      That's what I advise (S&G)

    14. #34
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      My water is seriously super crystal clear......in thinking my current filter system is kicking ***
      Neil

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    15. #35
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      My ammonia nitrates and nitrites are also good........ maybe it has something to do with me......I mean the weather,
      Neil

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    16. #36
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      How long since you refilled after your refurb Neil? Are you also running a trickle in from a well or something? Can't remember...

    17. #37
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      Quote Originally Posted by perniciousviper View Post
      How long since you refilled after your refurb Neil? Are you also running a trickle in from a well or something? Can't remember...
      With both my pumps running I'm loosing about 2 to 300 gallon a day via small leaks so I'm just using this loss as a trickle

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    18. #38
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      Quote Originally Posted by perniciousviper View Post
      How long since you refilled after your refurb Neil? Are you also running a trickle in from a well or something? Can't remember...
      3 weeks

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    19. #39
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      Quote Originally Posted by coolwon View Post
      Whats in suspension gets caught in the rdf.

      The heavies settle to the bottom of the settlement chamber and are drained periodically to waste through your hand operated ball valve which should be mounted at the

      lowest point of the settlement tank.

      Garfield
      I believe RDF owners enjoy the low maintenance aspect (at my age I do) and the ability to leave the pond unattended for several days without the worry of filters getting dirty. Putting a SC in front of it just seems to defeat the RDF capabilities of removing "heavies" (sh*t) from the water column. How often is the "periodic draining" of this toilet done so that no "heavies" break down in the water column?
      ....."Never argue with stupid people, they will drag you down to their level and then beat you with experience.".....Mark Twain

    20. #40
      DragonFireSG is offline Senior Member
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      Sorry for the belated reply. It's a fairly small chamber that directly feeds the RDF. No chance for anything organic to settle out, save for really dense things like stray rocks and the odd golf ball tossed in by the neighbours kids.

      My RDF is a modified Blue Eco with a single 200mm intake.

      Here's an old video before more recent tweaks.

      https://www.youtube.com/watch?v=JEdd32KlbBs

      Drawing of the chamber as it is now. The water level in the pre-RDF chamber varies depending on how dirty the RDF screen is.
      The modifications had to be made as I disabled the waste tray valve and propped it open.

      Name:  setchamb.PNG
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      Last edited by DragonFireSG; 07-15-2020 at 02:02 PM.

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