Return pipe sizing

[mlb75]

I've always been a fan of using oversized piping for all plumbing whenever possible but is there a point of diminishing or negative returns?

As an example my existing return pump output is 1" and I immediately take it to 1.5" up to a T and then 1.5" across the bottom of the tank until it necks down to the factory bulkheads on the tank which are I'm pretty sure 1" at each end of the tank. This appears to provide VERY little head pressure to the pump but is it necessary or would the same setup in 1" or 3" do the same?

I'm getting ready to replace my return pump from a Blueline 70 to a Red Dragon 150 with 1.25" output so should I up the plumbing to 2" up and across or is the existing 1.5" good enough? I've got the space and the cost to increase the pipe size is negligible but is it worth doing to reduce the power the pump requires or won't it matter?

 

[Doclight]

I would think the number of elbow's and 90 degree bends would have a bigger effect then pipe diameter. I use a 3/4" return on my 75 gallon, but am a big fan of 1". A LOT of water can go through.

So the difference from 1.25 to 2" shouldn't be that big a deal. But having multiple bends can be.

Just my 2c worth..

 

[Terence]

Actually the pipe diameter even without any fittings can make a huge difference and it is not linear. At low psi such as the way most DC return pumps push water, 100 feet 3/4” pipe is equivalent to over 60 vertical feet if you try and push 1200gph through it. Every fitting has a straight pipe equivalent.

Going from 3/4” to 1.5” has a Dramatic effect. The pump can “breathe” as there very little resistance compared to the amount of water that can flow through the pipe.

Think about straws. Try and drink your coke through the coffee stirrer and then through a McDonald’s straw. You could possibly get the same flow rate, but how much sucking force would it take on the stirrer vs the big diameter straw?

 

[Reef Monkie]

I would think the number of elbow's and 90 degree bends would have a bigger effect then pipe diameter. I use a 3/4" return on my 75 gallon, but am a big fan of 1". A LOT of water can go through.

So the difference from 1.25 to 2" shouldn't be that big a deal. But having multiple bends can be.

Just my 2c worth..

Doubling the size of the pipe gives you 4x the capacity so it is quite a big deal.

 

[mlb75]

What's the point of diminishing returns I wonder? Using another calc for instance with a mag 5 the flow difference between 1" and 3" is 9GPH with 4' of vertical and 2GPH from 2-3". I imagine there's a point where the diameter either doesn't matter or the volume of water in the pipe becomes more head pressure than the friction loss benefit. I know that's probably out of the realm of necessary concern for our hobby but with some pumps not liking head pressure is it a thought or concern? The more I think about it I wonder what the sweet spots are for the smaller pumps, non pressure rated pumps, or DC pumps. IE I'm guessing a 4' section of 3" up pipe coming from a small mag drive might not even perform as well as a 2" in the real world but maybe I'm wrong.

Just a thought I was curious about.

What got me thinking about it is I'm getting ready to redo my return pump and got thinking about what size pipe to use. I'm going to use 2" for a Red Dragon 150 but was curious if there was really a benefit vs 1.5 then got to thinking about what would happen with 3" and at what point it just becomes bulky overkill.