Twin tanks and fuel return

[matthewriches]

My boat has 2 tanks and when I purchased it, had an issue whereby both engines were taking from their own tanks but returning to a single tank. This very quickly caused an imbalance although never actually overflowed. There were no valves anywhere to rectify the issue either and no balance pipe or transfer pump.

I've resolved the issue for now but will need to revise the system again as new generator coming onboard anytime soon.

 

[NormanS]

Each tank goes via a racor filter first which is assume would stop one levelling into the other. So as the fuel returns to the main tank it just keeps adding until it overflows. That's my thinking anyhow.

Well maybe, but as the level of fuel rises in the tank being supplied by the return, its increased pressure would mean that it would preferentially supply the engine. Anyway, on a sailing boat, I would never leave tanks able to cross level when under way.

 

[elenya]

We also have multiple fuel tanks supplying the engine. Our spill return line feeds into the fuel supply line between the tank valves and first fuel filter. Therefore the fuel returns to the supply line not the tank. This set up has been in place for almost fourty years with same engine, thousands of hours and no issues so far.

I would prefer the ability to return to a tank of my choice as this would allow fuel rotation through the filters but I am not going to change at present.

 

[Polly1]

That is a good idea.

 

[Polly1]

I think that I have understood the system now.
The tanks can be interconnected but can also be shut off separately, I get the comments about the hazard this could pose on a heeling boat, one tank could pressurise the other and dump the contents out of the breather. Thanks for that
This is the set up, the inflow is at the top and the returns are at the bottom
I cant seem to rotate the picture correctly

 

[seaangler23]

Be wary about your set up with twin tanks, the returns on mine seem to t into the tank breathers above the tank.
My first trip to sea on my boat was at night in a force 7 and somehow while setting the boat up I’d taken the supply from the port and return was set to starboard which shouldn’t of been an issue as we had plenty fuel.
Anyway we could smell diesel at night but it was too rough to get the engine hatch opened or get out on deck so come morning we noticed the breather which is on the wheel house side was leaking diesel, the tank had pressurised and the return was largely just spilling out the breather, opening the fillet cap released a lot of pressure, lessoned learnt

 

[NormanS]

Be wary about your set up with twin tanks, the returns on mine seem to t into the tank breathers above the tank.
My first trip to sea on my boat was at night in a force 7 and somehow while setting the boat up I’d taken the supply from the port and return was set to starboard which shouldn’t of been an issue as we had plenty fuel.
Anyway we could smell diesel at night but it was too rough to get the engine hatch opened or get out on deck so come morning we noticed the breather which is on the wheel house side was leaking diesel, the tank had pressurised and the return was largely just spilling out the breather, opening the fillet cap released a lot of pressure, lessoned learnt

I hope that the lesson learned was that it's better to have the excess fuel returning to the tank in use. The fact that it is teed into the breather pipe above the tank, is neither nor there.

 

[pcatterall]

With having two tanks I rather like the idea of a 'working tank' with fuel going to engine and returning and a 'slave' tank quite separate. As I have said we top up when required via a separate filter.
We have had trouble with BLACK fuel ( not bug but due to' ashphaltine separation' ) one contributory factor is considered to be fuel being heated when passing through the pumps and then being returned. At least the fuel in the slave tank is unaffected.

 

[seaangler23]

I hope that the lesson learned was that it's better to have the excess fuel returning to the tank in use. The fact that it is teed into the breather pipe above the tank, is neither nor there.

Not to me the fact is that it shouldn’t be t’d into the breather as it will vent to atmosphere, I’d like to be able to pump between tanks using the engine, sure it can be done if I open the cap but il sort it when I get to the tank cleaning next.
Boat gets used by a few of us and it’s better to have it sorted.

 

[Vicarage]

We have three tanks (550L, 225L, 300L). We run from the main tank, leaving the 225L as reserve, and the genset/heater etc run from the 300L tank.
The engine can draw from any of the tanks but always puts back into the main tank. It takes a bit more thought as you have to plan for 20L an hour fuel consumption (and make sure there's space in the main tank), but it's not the end of the world and saves having another set of valves to go wrong. We've also not got any fuel transfer pumps so the engine is the only way of moving the fuel from the saddle tanks to the main.

 

[rogerthebodger]

This is the 4 position 3 way valve I made up to switch the fuel supply and return to the correct tank of the 3 I have.