Proper Size Return Line?

[madrefkepr]

I have a Mag 9.5 on the way for a return pump for my 65 gallon. Probably should have gone bigger, but when I set up a larger tank in the near (I hope :wink: ) future, I will be using it to drive a DIY skimmer.

I was using a head loss calculator, and got all confused. Here are the numbers I entered:

950 gph
4.5' vertical
(3) 90 degree elbows
0 horizontal run
And the friction factor for 3/4" pipe

Ended up with 4.5' vertical head loss and a little over 6' head loss due to friction.


If I use the same numbers, only change the friction factor from that of 3/4 to that of 1/2, I end up with 4.5' vertical head losss, 0 friction loss.

Question 1 : How can 1/2" pipe have no friction loss?

Question 2 : I am assuming a Mag 9.5 isn't going to push 950 gph through a 1/2" pipe. Is this correct? Can't seem to find anything about how much pipe diameter affects flow rate (other than changing velocity).

Question 3: I feel really dumb for not being able to figure this out, but what size should my return line be?

Thanks!

 

[liquid]

What head loss calculator were you using?

Shane

 

[ChrisRD]

Hey Dave,

Question 1 : How can 1/2" pipe have no friction loss?

It can't. In fact, the friction losses using 1/2" pipe in this scenario vs. using 3/4" pipe would be a LOT greater. IMO, 1/2" pipe is too small for these flow rates.

Question 2 : I am assuming a Mag 9.5 isn't going to push 950 gph through a 1/2" pipe. Is this correct? Can't seem to find anything about how much pipe diameter affects flow rate (other than changing velocity).

You are correct, it will not. The 950 GPH rating is at 0' of head. Once a pump has to push fluid through any pipes/fittings there are losses - thus, you are no longer in a 0' head configuration. To make matters worse, most aquarium pump manufacturer's seem to overstate their pumps capabilities.

As pipe diameter gets smaller at a given flow rate, flow velocity increases, thereby increasing friction forces (termed friction/form losses). Since your pump has a fixed amount of pumping power, as you increase friction (ie. reduce pipe diameter) you get less total flow because some of the pump's power is being wasted overcoming those losses.

An illustration of this effect can be seen when drinking through a straw. Much harder to drink through the skinny little cocktail straw, than a large one, right?

Question 3: I feel really dumb for not being able to figure this out, but what size should my return line be?

As the saying goes - the only "dumb" question is the one that never gets asked! :wink:

I would suggest a minimum of 3/4". If you want the maximum flow you can get from this pump, use 1".

 

[anna]

Just as a qualitative comparison, I just set up at 75G with a 9.5Mag in a sump with 3/4" pipes with several right angle bends.

When I opened up the pump full throttle, it blew all of my sand out of the bed even when I aimed the outlet valve.

I have had to use a valve to cut down on the flow.

 

[ChrisRD]

Another reason to go with the 1" line - lower flow velocity.

Going from a 3/4" outlet to a 1" outlet will almost cut your flow velocity in half.

 

[madrefkepr]

Thanks Chris and Anna!

I had originally planned using 1", as I just happened to have a bunch of 1" flex pipe laying around. I figured I could reduce the outlet if I needed a little more velocity. Then I started playing with the head loss calculator....

I wish pump manufacturers would use "real world" data. How many people do you know that would use a pump with no pipes? :wink: It sure would be nice to see graphs showing flow rates listed by pipe diameters over a given distance. Too complicated to account for all variables I guess.

 

[liquid]

Have you seen this page yet? http://www.reefs.org/library/pumps/ ?

Shane

 

[madrefkepr]

Have you seen this page yet? http://www.reefs.org/library/pumps/ ?

Shane

Yes I have, Shane, and refer to it quite a bit. I am guessing the excel spreadsheet would have helped me with my question? I'll have to upgrade my excel to use it. I have 97 installed now, but do have office xp. Some day I'll get around to installing it :roll: . Thanks!

 

[liquid]

I am guessing the excel spreadsheet would have helped me with my question?

Yup, plug in the pump you want to use, your plumbing diagram, click Solve and you're done.

Shane

 

[madrefkepr]

Did my office upgrade and downloaded the spreadsheet. Cool tool! Wish I would have checked it out much sooner. Thanks again, Shane.