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## Theoretical question about bottom drain efficiency

Seeking advice & thoughts from those more exoerienced and knowledgeable than I please.

Two theoretical questions about bottom drains - theoretical as I haven't decided the best way to implement yet.

Question 1) On my 2m deep tank, I have 3 bottom drains with 4 inch pipe. Assuming pipe lengths are similar length, If the other end of these pipes connected to the same filter chamber but at different heights - say 30cm difference between each - and this chamber then outflowed into a second chamber, would the flow be different on each bottom drain? As they all enter the same chamber, the water head height is equal...

Question 2) Same situation but now the pipe lengths are not equal. Would the flow rate on the bottom drain with longest pipe and most elbow joins be the lowest due to friction head? PVC pipe is reasonably smooth.

Question 3) What is a practical method to get equal amounts of pull from these 3 bottom drains? I'm able to set individual pipe entry heights into the filter chamber but I'm not sure how to calculate. Trial and error would be quire costly.

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What type of mechanical filtration do you have in mind ?

A top of the range rotating drum filter, which you would feed your 3 bottom drains into.

Attempt to keep the lengths as equal in length, as friction loss as possible.

Always bear in my mind it's deep down and costly to correct.

You can always increase the pump velocity a wee bit to speed up the water returns.

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1) If all pipes are the same length and the level of the water on either end is the same for all pipes, it wont matter that one pipe is lower than the other.
2) Yes, the pipe with the highest friction will have the lowest flow. This is affected by length, bends, and wall smoothness.
3) Difficult... Hard to measure and if you were to try to throttle flow to even them out, this introduces a place where debris could lodge and cause more blockage. May be a re-design, but if each bottom drain were going to its own pump (separate filter circuits for each drain) then they would be pretty close. Again, the circuit with the highest friction would be the lowest flow but the pump will dwarf this effect and they would all be running at pretty darn close to the pump capacity. Another re-design would be to lengthen your shortest run. Of course this opens up a whole other can of worms... Friction calculators based on pipe diameter and elbows are available but there are so many factors that can't be considered (these calculators are mostly for forced / pressurized piping and laminar / gravity flow is not well represented by them. Putting an extra bend in a short pipe to make its resistance the same as a longer pipe is to be avoided IMHO.

Unless you have a huge difference in pipe length or one run with LOTS of elbows, you may just want to let it ride.

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How about add a valve to control the flow for each drain line? This set up will allow you to do much more customizations.

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Hello, thank you for the replies. I don't mind if there is a differencebetween the 3 bottom drain flow rates but my tank has a glass front 10m long x 2m deep so I don't want one drain not performing adequately.

Yes, having a valve would allow me to control flows but on 4 inch pipe, that would be expensive. Perhaps a DIY internal pipe sleeve that could be rotated to align holes or air lift assistance if one bottom drain wasn't satisfactory. Overall, it might be easiest to have each drain pipe exit to individual chambers for more control.

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There would be more of a possibility of dirt hooking up on the valve blades.

Design the drains to be as close as possible to be the same length as possible while building and setting

it up. Cheaper and less hassle with the seizing of the valve blades and the guides the blades run in.

You would probably have to drain the pond to carry out any maintenance in the future.

Once the pond is running, the flow rates are hardly noticeably.

Unless of course there is a radical difference between the length of the runs and number of elbows.

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Mechanical filtration is also under consideration. It's very difficult to get rotating drum filters in Australia so parts and service also challenging. Also I'd like to flow through 30,000 - 40,000 LPH which would require 2 drums at least.

I'm looking at static K1 in upflow configuration using raintanks 2m deep x 80cm diameter. Not pressurised, media only 45cm deep. The water level would rise up as media clogs which could easily trigger an automatic siphon drain similar to aquaponic Bell siphons. The cost saving in relation to a drum filter would allow me to have individual filters per bottom drain (3) in parallel as the idea of cleaning without stopping water flow is very appealing. Especially if the flush is automatic without any moving parts. I use these elsewhere in fish rooms and other filters that benefit from a rise and fall surge .
Last edited by aquaholic; 06-07-2023 at 01:11 AM.

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Glass walled tank, 10 meters x 2 meters deep glass widow?

Aquarium?

A big aquarium, in the foyer of a hotel in Germany burst a while back.

They collected flapping fish in the street outside.
Last edited by coolwon; 06-07-2023 at 02:53 AM.

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Aquarium? Tank? Pond with a window? Pond with underwater cameras? Not much difference between these except lots of glass into clear water under lighting really shows up annoying dirt, especially if there are three drains to directly compare against.

Yes, public and professionally built aquariums fail all the time. Orient Shopping Center (Shanghai), Dubai Mall Aquarium (when it was the worlds largest), Mazatlan in Mexico etc come to mind. Some of these failures involved acrylic over a meter thick!

My tank is home built on a humble DIY budget, 100 tonnes of water and right against the house ... what could possibly go wrong?! ha ha. I have reduced my property value considerably by building this tank and I have house replacement insurance. The plan is to expect things not going to plan.

An example of something going to plan:

This photo below shows the steel frame from inside the tank before the glass went in. You can see how close the house is and one of the bottom drains under a circular disk cover.

The crane driver had to install the steel frame by lifting over the back concrete wall completely blind. We hand pushed into final place with slides as there was only 60mm total tolerance because I don't want fish hiding in blind spots. This process was repeated for glass panels too. I got custom glass built to my specification and self imported.

An example of something not going to plan:

The steel frame to hold the glass was delivered by a flat bed truck with a light hydraulic crane. The frame is only 4 tonne however this truck got stuck in the soft earth so he called in a 10T franna crane to get his truck out. Despite keeping a distance with a steel cable tow rope, the 10T crane got badly stuck too so that driver called in a 20T franna crane who used a chain tow rope... that needed heavy haulage recovery to get everyone out. My back yard was completely trashed with several weeks delay until the soil dried out but the steel frame did get installed.

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My tank's water level is 2m height. Unfortunately I didn't build in side filter chambers as I was going to use closed pressure filtration but I've been leaning towards open gravity filtration, ideally driven by several airlifts. It's been challenging trying to design filtration whilst maintaining water level at 2m off the ground which is much harder to service unless I build an elevated deck and walkway. It would be much simpler dropping the filtration water level to waist height but that would require water pump returns.

So I keep circling undecided as I haven't had the time to implement anyway. The tank is running successfully on back up filtration only the last 2 years but maintenance is high.

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WOW !!!!

Back to your original question, Core drilling walls for piping may be necessary... you may need three independent systems...All returns will be underwater? Plenty of height for airlift, but will they move enough water, what is your volume? In a aquarium you have a very high turn over rate...

Thanks for those photos and your story, this is A REAL POND PROJECT and given me a real lift this morning!!

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GRUMPY, was it a loud airlift on your first visit ?

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This must be a marine tank?

Koi are nothing to look at side on?

14. just

15. This is an epic build! Thank you for sharing it with us here on the forum.

I love seeing koi from the side - they are magnificent fish from every angle!

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They are, but judged from the top.

Marines should be referred to as the real living jewels.

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