Not quite a nuclear power plant, but...

All but the smallest boats usually have two fuel tanks, one to a side. And each tank needs to have both a supply and a return line. Add a daytank, fuel filters, and a fuel transfer pump and things can get really complicated.

I have found it useful to have a conceptual diagram of Eolian's fuel system, laminated and posted near the valving. Along with labeling the valves, the diagram helps to keep me from (but, sadly, does not prevent) making a valving error.