Backyard Aquaponics
A place to discuss aquaponics

"Ryan, if I put air stones in the ST would that address the issue?"
Not unless it is a very lightly stocked tank. The air needs to be in the culture tank.

For the design I am planning what would be my effective fall (Zf)?


Would it be from the bottom of the overflow pipe to the water level in the FT decided by the GB inlet?

I'm not sure which fall you are asking about.

If it is the fall from the FT to the GB then the effective fall (Zf) is the vertical distance from the water level in the FT to the level of the water in the pipe delievering water to the GB at the outfall into the GB.

If this pipe delivers water into the top of the GB then Zf remains constant but if it delivers water into the bottom of the GB then Zf will be larger when the GB is drained and at its smallest value when the GB is full.

Ok cool, so if we were going with a constant Zf because the FT water level and GB inlet are the same level, how would I effectively measure the size pipe and flow necessary for sufficient solid transfer to the GBs? I would think I would only be able to get a negative number from the above steps.

Could you also explain why the larger the pipe coming off the pump the better? I might have been missing something, but when I was calculating velocity between a 4" and 2" pipe, the 2" V was higher.

Thanks

Yep that is right. The higher the velocity the more friction. It is just like wind resistance. The faster you go the more the wind resistance builds exponentially. That is why it can be so much more economical to drive at 80km/hr instead of 100km/hr.

In pipe the faster the water is moving the more energy the water is losing to friction. This is why pipes should generally be big on the pump side (assuming you are pumping filtered water) and as big as they can be without causing settling in drain pipes.

I am working on design for a dual drain system in which ~20% of water should be removed via bottom drain with the rest flowing through skimmer per Cornell design, trying to lock in the proper drain pipe size. My total FT volume will be around 2000-3000 gal, would like to turnover that volume at a rate of once every hour or 2. That would come to 400-600gph through the bottom drain.

I work through these numbers and keep getting a number for drain pipe diameter that seems low. To make it worse, my numbers fall on the "overly forgiving" side when compared to the results of gravity fed pipe calculators like this one, and quick reference tables for "max flow through gravity fed pipe also show a 600gph flow through a 1" pipe with 9" fall, which is about the flow I would need to take 20% of a 3000gph flow through the bottom drain.

All of the commercially available bottom drains tend toward 3-4" (with a few outliers as low as 2"), and the recommendation for a koi pond (for instance) seems to be universally 3+ inches with a preference for 4"... but for even a 2" drain pipe it seems a 2000gph flow through the drain would be required to maintain flush speed, and if that is only 20% of total flow then the pump would need to deliver 10K gph to FT?

Is there something I am missing here? I can certainly put a gate valve at the end of a larger drain pipe to restrict the flow and keep drain flow at 20% of FT inflow, but that seems like it would allow settling in the pipe. Do I find a drain solution that will accommodate 1" drain pipe (and hope nothing gets clogged), or accept the low velocity flow of a larger drain pipe? Thanks for your input.

It's hard to you give a quick answer on the numbers because my head doesn't work in Imperial volumes but. ...

Many of the commercial drains are for very big tanks so the drains are pretty big. Especially since bottom drains often take less than 10%.

A bottom drain on a tank design I'm working on uses a 75mm bottom drain but it's a 50m3 tank. On small tanks the bottom drain is going to be small.

Solids won't get stuck in a small drain but leaves sticks or worse a fish might.

Wow, only 75mm for 50m3 eh?

Sorry for the imperials, should have converted to same units as used throughout the thread. My system will be 10-15m3 with a 60mm drain then? Thanks for the tips, really hoping to "do it right the first time" here, or a reasonable approximation.


As for bottom drain kits, I should have said "retail availability" too, not that it would make much difference. Most of what I am seeing in terms of BD for liner pools is geared to the koi pond market, and related guidance likely does not account for a Cornell dual drain design and assumes the BD must accommodate the full volume delivered to FT.

It makes sense that a drain designed to essentially max out the pipe's flow rate would be clog resistant even at small diameters so long as the inlet is adequately protected. Will just have to make sure to do the cover right.

I have seen this table on a couple sites, correlating pipe diameters up to 2" (120mm) with their maximum flow rate and the minimum fall required to achieve it:

size _GPM __GPH
1/2 = 2.50 = 150
3/4 = 5.63 = 337.5
1.0 = 10.0 = 600
1.25=15.6 = 937.5
1.5 = 22.5 = 1350
2.0 = 40.0 = 2400

Min. Vertical Length
1/2 = 2"
3/4 = 5"
1.0 = 9"
1.25= 14"
1.5 = 20"
2.0 = 36"

... metric conversion:

30mm max .75m3/hr @ 5cm fall
45mm max 1.7m3/hr @ 13cm
60mm max 3m3/hr @ 23cm
75mm max 4.7m3/hr @ 35cm
90mm max 6.75m3/hr @ 50cm
120mm max 12m3/hr @ 1m

... it seems like using that as a starting point would simplify drain sizing considerably, especially in the context of a dual drain setup. FT * (desired turnovers per hour) * .2 = rough estimate of maximum drain requirements (in gph or m3/h), select the next pipe size down and retool calculations with that pipe's max flow as the starting point.