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    Thread: Preventing Air Bubbles in Gravity Fed Underwater Returns

    1. #21
      BWG is offline Senior Member
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      Go to this site and use the gravity flow calculator. http://www.calctool.org/CALC/eng/civil/hazen-williams_g

      Unless your system is small 1 1/4 and 1 1/2 pipe are a waste of time. 2 inch is cheap.

      Look up fittings used on the equivalent length chart and add to pipe length used.
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      Last edited by BWG; 12-15-2018 at 10:39 AM.

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    2. #22
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      And if you plug 1.25 inches into the same calculator it works out to just about 16 gpm (960 gph). With each
      fitting on the return pipe, it will increase the head and reduce the amount it'll flow without overflowing. It's
      less about the angle of the gravity return and more about the size of the pipe. The one on the video starts out 3"
      and drops down to 2".
      If you drop the height of the filter but have it as far away from the pond as the existing barrel filters, the size
      of the pipe will be even more important. 3" is not that much more than the others and would give you the ability
      to flow as much as the filter can handle.
      --Steve



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    3. #23
      Zac Penn is offline Supporting Member
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      BWG,
      Isn't that calculator assuming the discharge of the pipe is not submerged, and that the entire discharge pipe is above the water level in the tank it is pouring into?
      IE: Place a 55 gallon barrel on the top edge of the pond and then release the water from a bulkhead at the top of the barrel and then calculate the slope of the discharge pipe and radius.

      If the barrel is mostly submerged and the discharge pipe is submerged then you will not achieve any gravitational acceleration from a sloped pipe VS a horizontal then vertical pipe. You would simply get friction head loss from the pipe itself and then use the difference in water levels to determine the flow rate capable through the discharge pipe.
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    4. #24
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      The drop referenced in the calculation directly correlates to gravity head pressure. So the difference in gravity head pressure (water elevation change) from point A to point B. The wording on the calculator is such to relate to pipe slope and gravity flows but the calculation (Hazen Williams) is the same for pipe connected to a vessel and head pressure.

      Yes water flowing from an orifice under water has a little higher discharge coefficient than the same orface in free air. Any type of fitting, device or orface on or in the pipe creates resistance and will affect the number. Bulkheads, bottom drains, skimmers, all fitting connections, etc..... Best to estimate an equivalent pipe length for all such items in a system and add to the total pipe length. Plus best also to look at the number as an estimate and over engineer by at least 25%.

      Please excuse my shaky hands. In the attached example all have 2 feet of drop or head pressure and 10 feet of pipe. All will have approximately equal flow. The middle one discharging under water will be slightly less do to slightly increased resistance at the discharge orface.
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      Last edited by BWG; 12-16-2018 at 05:58 PM.

    5. #25
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      Okay here is how I can explain your drawings....
      To start with we have a ball valve at the discharge end of each 10' long pipe
      All of the tanks are filled completely with water and all discharge pipes have no air in them

      At this point all three examples have exactly the same static head pressure so that each ball valve is experiencing 2' of H2O pressure + atmospheric pressure

      Drawing one has the upper end of the 10' pipe substantially submerged below the surface of the upper holding tank water level (for fun lets just say that it is 8" below water level) and the lower end of the pipe above the water level in the lower holding tank.
      Drawing two has both ends of the 10' pipe substantially underwater.
      Drawing three has the upper under just barely submerged under water and the lower end is also just barely under water at the bottom holding tank
      (You may or may not have intended to draw the last tank this way but it does help me explain things better)

      When we open the ball valve on drawing one we instantly have a static force from the 2' elevation difference between the two ends of the 10' pipe. As the water flows into the lower tank the water is gaining inertia from gravitational acceleration and since the inlet to the pipe is substantially underwater it can create a siphon and continue to pull more and more water out of the upper tank as it accelerates, until it gets to terminal velocity due to the friction forces inside the pipe. So this drawing is experiencing two forces that are pushing the water through the pipe and one force that is slowing it down (friction).

      When we open the ball valve on drawing two we instantly have a static force from the 2' elevation difference between the two ends of the 10' pipe. The outlet of the pipe is also going to have a back pressure associated with it due to the water in the lower tank being stationary. The incoming water has to push against the stationary water to move it out of the way. As the water flows into the lower tank the water is gaining inertia but only from the static head pressure. The water will accelerate until it gets to terminal velocity due to the friction forces inside the pipe and the back pressure from the static water in the lower pond. This drawing is only experiencing one force that is pushing the water through the pipe and two forces that are slowing it down (friction and water back pressure).

      When we open the ball valve on drawing three we instantly have a static force from the 2' elevation difference between the two ends of the 10' pipe. As the water flows into the lower tank the water is gaining inertia from gravitational acceleration but since the inlet pipe is just barely underwater it can't create a constant siphon because air keeps getting sucking into the pipe breaking the siphon force. This example has a constantly variable water flow due to the starting and breaking of the siphon on the 10' pipe. The outlet of the 10' pipe is also submerged below water level so it is experiencing a back pressure from the static water, but this will be less than in drawing two because it only has to move water directly in front of and below the pipe, whereas drawing two had to move water above, in front of, and below the outlet pipe. This drawing is only experiencing one constant force (but a variable second force) that is pushing the water through the pipe and two forces that are slowing it down (friction and water back pressure).

      1st Drawing will have a good bit more total water flow than Drawing 2
      3nd Drawing will have a good bit more total water flow than Drawing 2, but have less water flow than Drawing 1
      2nd Drawing will have the least flow rate.
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    6. #26
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      All examples were gravity turbulent flow with zero air in the lines. These are examples on how to determine drop or head elevation on various systems and use the Hazen Williams gravity flow calculation and equivalent length method.

    7. #27
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      Quote Originally Posted by BWG View Post
      All examples were gravity turbulent flow with zero air in the lines. These are examples on how to determine drop or head elevation on various systems and use the Hazen Williams gravity flow calculation and equivalent length method.
      Okay that is nice but did you read what I wrote?
      I used the Zachary Penn talking out of my butt method, to explain why I thought those three examples would flow different amounts of water even though they all have 2' of static head pressure.
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    8. #28
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      I'm curious what your guess "a good bit more" is equal to in percentage of flow?
      --Steve



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    9. #29
      BWG is offline Senior Member
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      I totally enjoyed the comparison between all three. The examples were purposely exaggerated to show what actually matters in a gravity flow pipe system is the actual elevation change in the water from point A to B. I'm trying to find the discharge coefficient of a pipe underwater and to see if the value can be converted to equivalent pipe length. Typically it is ignored.

      Not much manual calculations go on for most commercial firms nowadays. They plug the plumbing system into a CAD package with flow calculation software.
      Last edited by BWG; 12-17-2018 at 08:56 PM.

    10. #30
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      You folks are sure nice for sharing your thoughts on this. Now I'm going to study it all so it sinks in.
      Big hugs! Big grin!
      Last edited by Koigarden; 12-17-2018 at 09:31 PM.

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    11. #31
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      BWG, A side note..... I just had an 'ah ha!" moment. This explains what was going on between the 2 barrels last summer and I didn't figure out a fix, dreadful slurping noise but it did add air to the 2nd barrel . "When we open the ball valve on drawing three we instantly have a static force from the 2' elevation difference between the two ends of the 10' pipe. As the water flows into the lower tank the water is gaining inertia from gravitational acceleration but since the inlet pipe is just barely underwater it can't create a constant siphon because air keeps getting sucking into the pipe breaking the siphon force." By golly I get it now! I'll see if I can find a pic of the 2 barrels from last summer. This was a terrible design, the dirt was giving way with the rains and shifting the barrels like leaning towers of pisa, no valves, the kaldnes was growing algae from all the sunshine...try not to laugh too hard.[ATTACH=CONFIG]588352
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    12. #32
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      Quote Originally Posted by icu2 View Post
      I'm curious what your guess "a good bit more" is equal to in percentage of flow?
      Well aren't you asking for me to stick my neck out pretty far on that one? It is hard for me to put an actual % guess on this but we can gain some information from my previous video about the siphon effect on water dropping inside an enclosed pipe...


      with a 45" long siphon tube we were gaining 500+ gph more flow than when that down tube was removed, so that is roughly 10% increase in water flow due to the gravitational acceleration of the water inside the tube creating a siphon and pulling more water than it normally would.


      Quote Originally Posted by BWG View Post
      I totally enjoyed the comparison between all three. The examples were purposely exaggerated to show what actually matters in a gravity flow pipe system is the actual elevation change in the water from point A to B. I'm trying to find the discharge coefficient of a pipe underwater and to see if the value can be converted to equivalent pipe length. Typically it is ignored.

      Not much manual calculations go on for most commercial firms nowadays. They plug the plumbing system into a CAD package with flow calculation software.
      Knowing what the restriction from the static pond water on an underwater discharge pipe is valuable information so please try and find something like that, but do you not agree with my analysis about the siphon effect increasing the water flow in the 1st and 3rd drawings?
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    13. #33
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      A very interesting test Zac. I hadn't seen it, so thanks for re-posting.

    14. #34
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      No - These are 100% gravity flows. All with two feet of constant head pressure. The pipe length does not vary. Negating discharge resistance in the middle one the flow rate in feet per second will be equal and any what you are labeling as siphon effect will be equal.

      In your example you created a gravity flow section starting at the very top. Increase the down pipe length in the setup and you increase the gravity flow portion and increase the drop or gravity head pressue. More gravity head pressure more flow that interacts with the pump pressure flow.

      The Hazen Williams calculation is an emperical formula derived from real world testing and data collection on pipe gravity flowing water. It already includes any effect you are labeling as siphon.

      In the next example all gravity head pressures are equal and the equivalent length of all pipe (including fittings) are equal. All flows will be equal.
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      Last edited by BWG; 12-18-2018 at 11:45 AM.

    15. #35
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      Okay I went back and watched the video again and paid attention to the discharge side pressure gauge...
      Started at 10.8' of head pressure
      I removed the 45" long piece of vertical pipe
      Head pressure increased to 14.25' of head pressure (roughly 43" of head pressure change)
      So I guess you may be correct on this one.

      However, it just blows my mind that there isn't a second siphon force acting on these vertical drops to increase the total flow rate. The static head pressure force is obviously the same in all three examples, but I just figured the dynamic pressures would be different as the water gained inertia and started (what I call) a siphon at the inlet to the pipe.
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