BTW Mike, If your tanks are cross connected with a simple line
I see no need to return the fuel to more than one tank.
The only thing you need to worry about is a slip toward
a near empty tank.

Paul Lamar

I would think that for the cross connect line to work as described, it
would have to flow roughly the max rate of BOTH pumps (takeoff with full
tanks), but do it at only 6 or 8 inches of 'head.' How big a pipe would
it take to gravity feed 60 to 80 GPH with less than 8" of fall?

Charlie England

Hey Charlie why 60 GPH???? We don't have 1000 HP here?
360 pounds of fuel at .5 pound BSFC equals 720 HP.

Paul Lamar

I'm working on the assumption (important word) that if each pump is
capable of pumping 30 to 40 GPH @ 40 PSI, you run both on takeoff for
safety, you're burning ~15 GPH, & the regulator is bypassing most of the
rest of the volume to the return line. What is the actual flow in the
return line?

Charlie

Obviously 80-15 or 65 gallons per hour based on that assumption.
So the cross connect should be twice the area of the return line
I would guess. So far no one has suggested running both pumps
at the same time. The size of the line feeding the fuel rail will 
restrict that volume of flow anyway as will the return line size.

You must also be assuming that all the return flow goes to one thank.
In an extreme case like that you might want to put a check valve in
the vent line like Vlad's setup to prevent pumping fuel over board.

If both pumps are running it matters not if all the return flow goes to one
tank providing there is a check valve in the vent line.

As a worse case scenario it is a good point and worth considering.


Paul Lamar
 
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