Overflow / circulation design for 1,200 gallon

[coreytrv]

Looking to start new tank project. This will take up an entire wall inside a home for custom reef / fish tank.

8' wide x 3' deep x 6'-8" tall. EST 1,200 gallons

Does anyone know how to properly size the overflow box?

Reversing into the design using PVC gravity flow rates:

2 x 2-1/2" drains = 6,400 GPH

Questions:

1. Is 5.3x turnover OK for overflow?
2. What size box is needed to accommodate 6,400 GPH flow rate?
3. Circulation via closed loop bulkheads on each side of the tank + gyre style pumps up top, enough points of circulation for tank of this size?

 

[Rtaylor]

The size of the drain pipe and the return flow rate are really what matters. As long as the drains can keep up, the overflow volume should be relatively small. Make sure to include an emergency drain as well as the main drain. 5.3X is plenty. I’d be a bit nervous about drains that large, you’ll need to take extra precautions that livestock and other large items can’t get into the drain. I’ve had more than 1 fish in my sump with a 1” drain.

 

[iReefer12]

On my 700 gallon I run 2” beananimal drains.

I actually ended up turning my return pumps down, so only running about 1200gph of return flow.. not because the drains can’t handle it, or the overflow box can’t handle it, but because of noise.. the noise not from the drains themselves, but from 1200 gallons + flowing into the actual overflow box.. and then over the dividers in the sump.

I generate flow inside the display with PH (Vortechs in my case) then can dial the return flow down to a bearable noise level.

The noise I’m referring too was something I’d totally overlooked in planning, so it’s worth pointing out.

 

[MartyK]

2. What size box is needed to accommodate 6,400 GPH flow rate?

You need to specify one more component for an accurate answer. The three components of the calculation are water flow rate, width of the weir, and water height above the weir (overflow box). This is the Francis formula for flow through a rectangular weir in imperial units, q = 3.33 (b - 0.2 h) h^(3/2), https://www.engineeringtoolbox.com/weirs-flow-rate-d_592.html.

There's a convenient chart on that page which shows flowrates in gpm for 3 different weir lengths and the resulting water heights and although they don't show anything under 2" of water height, you can extrapolate. At just over 100 gpm, you're going to want a long weir unless you have a bunch of 'freeboard'. Don't forget to factor weir teeth into your length as those chew up valuable space.