How do I calculate this?

[007]

I'm going to have my tank drilled in the next couple weeks. I'm going to use a single internal overflow with dual drain lines and I'm going to have the return line T'd off to two returns. I estimate that the vertical distance the water will have to travel from the sump to the tank will be roughly 4 feet.
I'm looking for water movement of about 300 gph total through the sump . . . how do I calculate a) the size of the drain lines, b) the size of the returns, and c) the size of the pump?

 

[sly]

For the pump, you need to look at the specs on each one. They should tell you the flow at different levels of head pressure. You want a pump that can push at least 300gph at a 4 foot head pressure.
One problem with teeing your return lines is water cavitation. When the water from both lines mixes you may have problems with microbubbles getting in the system and they can be hard to get out. I think most people simply keep the return seperated and let them individually flow into the sump.... but I may be wrong.
300gph is not very much. IMO you will not need anymore than 3/4 inch drain lines if you are using a dual drain setup. You may even be fine with 1/2 inch drains... but I don't know how to calculate this.
A mag drive 5 is sized just about perfect for this application. It puts out 310 gallons at a 4 foot head pressure. Do a search for mag drive and click on the first link.

 

[007]

I've decided on a pump already (eheim 1260 which pumps about 400 gph at 5' of head) . . . i was wondering what size bulkheads to drill basically.
Also, can you explain the cavitating thing again? Whats the problem with T'd return?

 

[sly]

Cavitation is a process where very turbulant water is rapidly mixed (such as in a tee) and air bubbles form as the water quickly changes from positive to negative pressures. It is more of a problem in high flow tanks. It's like miniature explosions going on in the water to produce damage to the tubing and to create these bubbles.
It's kind of like opening a soda can and it spewing bubbles. A sudden drop in pressure and the gasses come out of solution. Running water through a tee causes a back pressure before the two pipes join, then after the tee the water immediately experiences a vacuum as it passes into the sump. You may have problems with air bubbles if you mix the two overflows in a tee before it gets to your sump. That is why many people don't use the tee and simply let the water drain from both overflows independantly into the sump.
I can't really tell you what size bulkhead to dril. 3/4 inch sounds about right but don't quote me on that...

 

[007]

I think there's a bit of confusion here.
I will keep the two DRAIN lines separate as they drain to the sump. The RETURN lines will be T'd off into two separate returns.