OK, I'm finally done with my tank stand and the tools / materials for drilling the tank are in the mail to my house.
I've planned on going with the 90 elbow and strainer in the corner of the tank since the time it was suggested in this thread, but i have one more question before i start drilling.
they have the same setup in one of the lfs for there centralized filtration system and i noticed that the water flows 1/2 way up the strainer. now I'm worried if i drill too high I'm going to end up with water all over the floor! i would like the water level to be above the trim at all times, so anyone know how low or how to figure out how low to drill the bulkhead? all help appreciated!
If you are drilling on the side, I would say put the bulkhead as high as you can because that will determine the waterlevel. If you want the waterlevel above the trim you'll have to put your bulkhead in the trim. How high the water gets above the bottom of the bulkhead opening is determined by the rate of flow relative to the sized of the opening.
Oh, wait did you mean a 90° elbow on the inside strainer pointed upward? If that is the case I'd set the height of your opening to your strainer either at or just a hair below the bottom edge of your trim. A large bulkhead and large plumbing coupled with a slow rate of flow from a small pump will keep the height above the bottom of the strainer opening to a minimum.
As for how to calculate it I'm not sure of the top of my head, and if I remember correctly it was a complex calculation. The book I have handy on flow measurement does not list the equasions that show relation between Flow Rate, Pressure, and Restriction Diameter. In this case the restriction diameter would be your overflow, Flow rate would be your pump speed, and pressure would be your head height usually expressed in the pressure unit Inches of Water column.
There may be some simplified equasions out there that eliminiate the other variables such as Temperature, Fluid Density, viscosity etc, if they just assume water is what is flowing at normal room conditions. I would think that at some point the physicists would have had to simplify things for plumbers, since plumbing doesn't require a PHD.