If you ran all three vents to one seperator then the back flow could go to the wrong tank and fail to trigger the shutoff at the pump and cause the fuel
to spill on the deck
. It might tend to enourage the flow to go to each tank in turn until all tanks were filled. Normally with one tank the resistance of the fuel flow will back up the filler and trip an auto shutoff quickly based on the size of the reservoir in the seprator (see review notes). The size of the racor
adds to the ability to do this reliably. Since the seperator won't totally block the vent the fuel may not back up properly if one tank was less than full. If another tank did not have its fill open then it would tend to deter the flow into an unvented tank so it might still work. The configuration of the vent lines might be the telling factor.
If you used a fill manifold to have one deck
fill for three tanks then you would want a similar manifold for the vent too. It also means when you switch from one tank to another you would switch the supply line, the return line, and the vent line. Having all the controls together could save the cost of two more seperators and two more deck fillers but may eat up savings in a manifold, fittings, and extra tubing. If you have to crawl around to get at them all then you may be making a setup that could cause you trouble if you forgot one manifold valve. 3 seperators means you can't screw it up.
You don't want any loops or low spots in the vent lines. It appears the seperator specs call for no less than 60 degrees from horizontal for the vent lines. You really do want fill and vent lines to be as short as possisble too and have as few fitting as possible. Fuel fittings are not cheap
so the money
side of this may be more conclusive.
I would think you could combine the outlet for multiple seperators assuming you don't fill more than one tank at a time. That could save the cost of three exterior ventsc but adds a manifold or two Y fittings. The flow on the seperator should handle a high speed deisel fill pump where the big trawlers fill 100's of gallons assuming a 5/8 inch vent line. Flow volume is not a problem with this unit and why the reviews
seemed to like it the most.
For me I have two tanks one far forward and one all the way aft so combining lines is not practical at all.