Design of overflow weir

[oscar6]

I have a toothless overflow I am making. The material thickness is approximately 0.25 - 0.375". I wish to reduce the potential for noise as water cascades over the overflow. My question is would it be beneficial to replace the flat portion of the top of the weir and replace it with a slanted portion on the inside of the weir. I believe the benefit of this would be to reduce the horizontal velocity of the water overflowing, thereby making the transition from horizontal flow to vertical flow smoother so water doesn't have a tendency to come away from the inside edge as it is flowing over.

Am I just overthinking this? I will have about 1200gph flowing over 12 linear inches of toothless overflow.

See attachment.

 

[Magwitch]

I think making a sharper edge would just move the small air pocket up to the slant instead of over the edge of the square top. What if you put a line/bump of silicon or something where the air pocket is in the first picture?

 

[BeanAnimal]

12" is pretty narrow.

1) split a piece of airline tubing or similar round stock to create a rounded profile. The larger the profile the more water it can handle.

2) Increase the level of the water in the overflow box to reduce the vertical fall.

 

[oscar6]

I am aware of #2 and I have used that extensively in the past. A toothless overflow can handle much more water than one with teeth since the teeth aren't obstructing the flow. I like the ideas about the rounded profile. I think using the silicone would not work very well since it may be difficult to get at the even and all portions. My two concerns about using something like air line tubing or larger tubing is the possible color difference between the tubing and the black overflow and the fact that the overflow is a semicircle not a standard flat shape. I guess I could just get some black tubing and give that a try.

 

[oscar6]

I suppose I could just notch the inside lip of the tubing so it properly accomodates the inside bend of the overflow...

 

[BeanAnimal]

A toothless overflow can handle much more water than one with teeth since the teeth aren't obstructing the flow.

Yes, the teeth make the 12" overflow more like a 6" overflow. However, 12" is still VERY restrictive for 1200 GPH. A wider weir creates a thinner sheet of water and provides better surface skimming and less noise.

I like the ideas about the rounded profile. I think using the silicone would not work very well since it may be difficult to get at the even and all portions. My two concerns about using something like air line tubing or larger tubing is the possible color difference between the tubing and the black overflow and the fact that the overflow is a semicircle not a standard flat shape. I guess I could just get some black tubing and give that a try.

You have a few things working against you. the semicircle is going to create a lot of turbulance due to interaction of the outflow. Finding something large enough in diameter AND flexible enough to create a smooth curve is going to be a challenge.

 

[oscar6]

1200gph per 12" overflow x2 overflows.
I may drop down to 400-500gph per 12" overflow x2 overflows. I'll just have to add a few more circulation pumps to the 180gal and have a slower flow through the sump as well. I actually planned on this originally but I had a larger pump laying around and thought it could be worth a try.

I never thought about the convergence factor of the semicircular overflow making more turbulance. Good point. Also the thinness of water overflowing at the surface escaped me as well and I was going for more of the "Tim Allen" approach.

With this new lower flow, should I still attemp a rounded tube on the top edge?

 

[BeanAnimal]

1200gph per 12" overflow x2 overflows.
I may drop down to 400-500gph per 12" overflow x2 overflows. I'll just have to add a few more circulation pumps to the 180gal and have a slower flow through the sump as well. I actually planned on this originally but I had a larger pump laying around and thought it could be worth a try.

I never thought about the convergence factor of the semicircular overflow making more turbulance. Good point. Also the thinness of water overflowing at the surface escaped me as well and I was going for more of the "Tim Allen" approach.

With this new lower flow, should I still attemp a rounded tube on the top edge?

Ignoring the surface skimming and other issues, how do you plan to keep the standpipes quiet?

 

[oscar6]

One "main flow standpipe" with a valve closed just enough so water just reaches high enough to very slowly drain any extra water into the "water level setting standpipe". It works fantastic. I have used it on several other tanks.

 

[BeanAnimal]

You are creating a rather complex system that is going to be hard to keep quiet and balanced.

The combination of high flow, multiple overflow boxes and siphons is going to be troublesome. You may want to consider a single large coast-to-coast (A.K.A. "calfo style") overflow with (3) standpipes instead of (4). You will get much better surface skimming and a more stable, silent system. You will not have to worry about balancing two overflow boxes, keeping the flow over the weir quiet or flooding.

http://beananimal.com/projects/silent-and-fail-safe-aquarium-overflow-system.aspx
http://www.reefcentral.com/forums/showthread.php?threadid=1310585

 

[oscar6]

Unfortunately I already drilled my tank. I think I will have the high flow drain from each box join together and with a valve after their union. That should fix any issues with imbalance. Should still be just as quiet as well. Maybe next time I'll go with a coast to coast.

 

[BeanAnimal]

You are not going to be able to control the amount of water that flows into each box

Are your holes drilled in the back or bottom of the tank?

 

[oscar6]

Drilled the back of the tank. If the two main drains share the same line, they will have the same water depth in each overflow, not figuring the "water level setting standpipe" in each box (they will each drain individually to the sump). There is no reason why this should not work. However, I do see the potential issues with this setup. On the plus side, I can switch the drains easily if needed.

 

[BeanAnimal]

Drilled the back of the tank. If the two main drains share the same line, they will have the same water depth in each overflow,

It is not a matter of the standpipes regulating the depth, it is a matter of the same amount of flow entering both boxes per given time to keep the siphon balances between the two input. The work around is to restrict the flow sufficiently via the common valve to FORCE the level adjusting standpipes to carry a significant portion of the flow. However, doing so removes the fail-safe nature of the system

I do see the potential issues with this setup. On the plus side, I can switch the drains easily if needed.

If the holes are drilled in the back, then why not go with a a coast-to-coast style overflow and utlize the holes in a more fail-safe configuration? You will get far better surface skimming AND silent, fail-safe operation.

 

[oscar6]

There are two holes arranged vertically on each end of the back of the tank. Since they are each handling such a low flow (400gph), I honestly don't think it will matter.

 

[BeanAnimal]

A few more thoughts:

If you try to push 1/4 of the flow through each of the two "level setting" standpipes, then they will gurgle and be noisy. They will also NOT be able to act as failsafes if the siphon standpipe becomes clogged (there is a restriction at the valve(s).

Why not encompass the two upper bulkheads in a coast-to-coast overflow and use the same system at a lower flow rate. You can then use the LOWER bulkhead holes for closed loops. You will get MUCH more efficient surface skimming and a more efficient system with a much smaller return pump. Closed loops have zero head (other than the friction of the pipe) and therefore are MUCH more efficient at providing the needed flow in the display. Lastly, you could drill 1 more hole and create a dead silent AND fail-safe system.

Just food for thought....

 

[oscar6]

The amount of flow traveling through the leveling standpipes will be essentially nill. The water level will primarily be set by the valved main lines. The leveling standpipes will not be restricted by any sort of valve so if water were to somehow build up, the leveling standpipe would easily accomodate it.

On second thought I will do two separate valved main lines. I have absolutely no doubt whatsoever it will work.

The only way I'll ever do a coast-to-coast is on an extremely wide tank or preferably with the overflow constructed externally. I just don't like how it creates a boxed area inside the length of the top back of the tank. If I still had my 8x4x2.5' tank, I'd consider it.

 

[uncleof6]

 

[oscar6]

:lolspin: the coast-to-coast is a good design... but not for me... at this time.

 

[BeanAnimal]

Am I just overthinking this? I will have about 1200gph flowing over 12 linear inches of toothless overflow.

1200gph per 12" overflow x2 overflows.

I may drop down to 400-500gph per 12" overflow x2 overflows. I'll just have to add a few more circulation pumps to the 180gal and have a slower flow through the sump as well.

There are two holes arranged vertically on each end of the back of the tank. Since they are each handling such a low flow (400gph), I honestly don't think it will matter.

The amount of flow traveling through the leveling standpipes will be essentially nill.

So are you going with 2500 GPH or 800 GPH? There is a HUGE difference. The difference will dictate how easily system balances and how quiet it is.

Again, because you are using (2) overflow boxes, you will not be able to control the amount of water that enters each "level adjusting" standpipe becuase you will not be able to dictate the amount of flow over each weir. The system balance will oscillate back and forth between the two boxes. This may or may not be audible, depending on many factors.

Use as many standpipes and valves as you like my friend... as long as you are happy with the end result, that is all that matters. I was simply offering informed advice based on the information you have given.