The flow though a gravity fed drainage line that discharges to atmospheric pressure can be roughly calculated using the continuity equation as it is a product of flow velocity and cross sectional area.

Q = A * V

Where Q is flow rate, A is cross sectional area of the pipe, and V is velocity.

The units work out this way:

M^3/sec = M^2 * M/Sec
OR
Ft^3/sec = Ft^2 * Ft/Sec

You can almost neglect head pressure as this will be offset by frictional losses in the pipe.

Remember you need to switch your time units from sec to minutes and convert to Gallons to get you gpm rate.

A good article for this can be found here:
http://www.agf.gov.bc.ca/resmgmt/pub...s/590304-5.pdf

Hope this helps.

so what's the velocity?

Well ideally you would measure this lol, but I dont see that being practical in this scenario. You could do this the way the pdf I posted goes about it, but this would be tough to find tables for as our elevation changes are small, or so I assume. Another way to do this would be to estimate the velocity based on gravitational acceleration:

9.81m/s^2
OR
32.2ft/s^2

Time how long it takes from the first overflow trickle till when its out of the piping. For example say it takes 2.5 sec to flow through, this would translate into a velocity of 24.5m/sec. This can give a ballpark figure. The more water volume on top of the intake though will create some head pressure, which then changes all of this. This assumes gravity fed drainage into atmospheric pressure.

Im sure if you google it someone has done the math!

Yeah I was attempting to point out that equation isn't useful here, it's only for sizing pipe when you have a desired flow rate and velocity or solving for a different variable with the other two known. The velocity will actually depend on a few things including pressure difference, density (which is variable as well due to an inconsistent mixture of air and water), major loses from pipe friction and minor loses from pipe fittings. While it's completely possible to calculate this all it would be a complete waste of time especially since RC has been nice enough to make a simple calculator that estimates the required pipe size based on flow rate.
http://reefcentral.com/index.php/dra...flow-size-calc

Well to state that the equation isn't useful here I believe is wrong. You are correct that it depends on a few things, such as air water mixtures, whether or not it is laminar/turbulent flow. You could easily solve for a Reynolds number and jump into a whole bunch of fluid mechanics equations. But the RC calculator would of course be the fastest and simplest way to go. Good find.

Nah it's pretty much useless, it depends on more than a "few" things.

Hah ok... well that was a good chat.

Im sure the RC calc has accounted for all these variables then.

Have a good one.

drain size from RC (gravity)

Beananimal's siphon calculator

I think 1" drain on a 50g is adequate. Have a valve on the discharge side of your pump and use it to tune your flow. If you are getting lots of bubbles from the discharge pipe into your tank close the valve until they stop, likewise if the return section of your sump keeps fluctuating, as it should remain at a constant level (with evaporation as the exception of coarse). It's that easy!

Hey! Im using a Hydor seltz pump (800 gph model). I calculated the loss in the gph from the head and it equals out to about 500 gph.