most flow through pipe

[ravedood]

I am working on a closed loop system. I have one 1" inlet and two 1/2" returns to the tank. I know they should have been bigger but this is they way I received the tank and I don't want to start drilling. Whats the most flow that I can push through the loop? Also which would be the best pump for this setup?

[BeanAnimal]

In theory there is no limit until the pressure exceeds that of the pipes capacity.

In other words, the smaller the pipe, the more pressure it takes to move a given volume of liquid through that pipe in a given amount of time.

A pressure rated pump (less efficient at low pressures, but capable of pumping higher pressures) will be your best bet.