1/2" drains verboten?

[Trever]

There seems to be some notion one should never use 1/2" drains on any size tank. Minimum being 3/4".

Anyone have any comments on this?

I have a 20 gallon long/15 gallon sump. Total actual water will be maximum 27 gallons.

Herbie. Internal/external overflow style.

 

[Righteous]

It basically comes down to how much flow you want. 1/2 drains are limited to about 200 GPH, which for a 20 gallon is probably enough. I had a 1/2 drain on a 10g without an issue.

 

[Trever]

@Righteous ok, I have heard something about snails/critters meaning you shouldn't mess with 1/2". Something about pressure issues as well to get a full siphon going.

The pressure might make sense but given the gate valve/tuning, not so sure about critters, who are going to hit the minimum diameter at some point (whether the pipe or the semi-closed valve).

I over plumb'd my first tank and it makes for some knocking. Smaller might be better...

Where is there a flow rate chart, re 200 GPH? I had seen one once... something that said how many gallons flow down the various diameters under gravity alone, may have had some tube length as well ("terminal velocity"). Can't find the dang thing.