Diesel return fault?

[tolhurstorganic]

My Pinky Ketch has a 28hp Betamarine connected to two diesel tanks, custom built by Tex Tanks, these are deep in the bilge below the level of the engine, capacity is #1 190 litre and #2 100 litre. The return pipe from the diesel pump is routed back to the tanks via a “T” and then into each tank (#1+#2), there are no separate valves to turn off the return fuel to each tank. I usually run the engine on one tank until near empty then switch over to other tank, closing down the near empty one. Last week went for a sail to Portsmouth from Emsworth used the engine going as it was a dead header and sailed back.
When I left Emsworth I had equal amounts in each tank-half full. On returning from the trip and checking my fuel level in the tanks I find that tank #1 which I ran on was empty, whereas the tank #2 that was not used was full. It would appear that the return fuel all went back to #2 tank instead of to its original tank. I used to run on both tanks, which would go down at an equal rate and never had problems with the return fuel going to the wrong tank.
How do I ensure that return fuel goes back to its original tank, put a separate valve in on each return pipe to route the fuel when I change over the tank? Any ideas will be much appreciated.

 

[boguing]

Here's one thought.

Using longer tubes from the T, push them into the tanks so they vent at the bottom.

That way the pressure needed to pump into a full tank will make the flow go the empty one. That will carry on until they equalise.

 

[penfold]

You can get duplex T valves to do this automatically(that is to say 2 separate valves operated by the same spindle, usually in one complex cast body); is that not what you have?

 

[pvb]

Here's one thought.

Using longer tubes from the T, push them into the tanks so they vent at the bottom.

That way the pressure needed to pump into a full tank will make the flow go the empty one. That will carry on until they equalise.

Doesn't that rather negate the purpose of having 2 separate tanks?

 

[Tomahawk]

Here's one thought.

Using longer tubes from the T, push them into the tanks so they vent at the bottom.

That way the pressure needed to pump into a full tank will make the flow go the empty one. That will carry on until they equalise.

not sure if that will work ... the tanks are vented so any return pressure will simply be vented out via the breather..

i am inclined to think that if the system changes the way it works, then something has changed somewhere. most likely one of the swich over valves has failed in some way... or something has blocked somewhere..

 

[tolhurstorganic]

Yes tanks are vented separately so don't think pipes to bottom of tank from each return would make any difernce.
Duplex valves sounds interesting not heard of this, the valve at present only shut off supply from tanks

 

[KenMcCulloch]

Yes tanks are vented separately so don't think pipes to bottom of tank from each return would make any difernce.

It's surely nothing to do with the venting, if one tank is fuller than the other the diesel return will be pushing against a bigger head of pressure from the fuel in the tank, and therefore the return flow from the T junction will go to the tank with the lesser head?

 

[tolhurstorganic]

Yes that could be right, but I need to check if the return pipe to tank goes to bottom of tank or just drops it from the inlet at the top of the tank. The fuel feed is from a dip pipe. I am not on boat at present so can't check. I will look at Tex tanks site for any info.
Anybody know answer to this?

 

[pvb]

If you want to control which tank the return fuel goes to, you'll have to add valves. In your original post, you said:-

I usually run the engine on one tank until near empty then switch over to other tank, closing down the near empty one.

With no valves on the return pipes, return fuel would go to both tanks, gradually filling up the one which you are not drawing fuel from. This is what happened, as you said:-

On returning from the trip and checking my fuel level in the tanks I find that tank #1 which I ran on was empty, whereas the tank #2 that was not used was full. It would appear that the return fuel all went back to #2 tank instead of to its original tank.

I don't imagine all the return fuel went to #2 tank, it would have gone to both. You also said:-

I used to run on both tanks, which would go down at an equal rate and never had problems with the return fuel going to the wrong tank.

This is logical, because if you select both tanks to draw fuel from, the level in the tanks will automatically equalise by gravity, regardless of where the return fuel is going.

 

[tolhurstorganic]

Yes thanks for that, reckon you are right. So if I want to be able to choose which tank I run on I need to be able to shut off return fuel and divert to operating tank. As Penfold suggest duplex valves would be the simplest.A websearch has revealed little apart from very expensive aircaft components.
Anybody know where i may find such a thing, need two for 8mm flexible fuel pipe.

 

[nigelmercier]

... duplex valves would be the simplest...

I'd be inclined to have a separate valve, that way you could choose where the return goes. Might be handy for cleaning etc.

 

[tolhurstorganic]

Thanks Nigel-good idea it means I could empty a tank separately

 

[nigelmercier]

Thanks Nigel-good idea it means I could empty a tank separately

Just remember to switch both, or you could end up with a lot of fuel in the bilge!

 

[TQA]

Just remember to switch both, or you could end up with a lot of fuel in the bilge!

I think having a manual switch that could result in the above is a bad idea. Murphy's law applies and even with a careful owner and a checklist one day you will have that return switched to a full tank.

 

[tolhurstorganic]

Yes the possibility is real these things do happen. But would the tank overspill? The diesel would have to go to the top of the breather pipe which is almost 2metres higher and the breather from each tank is teed into the one breather/no smell filter, I would have thought the overspill would then go back into the other tank?

 

[NormanS]

I have a similar set up, with two tanks. The fuel return pipes have shut-off cocks fitted at their tank entrys. If I am using (say) the port tank, I open the port tank return cock. It works for me.

 

[prv]

I need to check if the return pipe to tank goes to bottom of tank or just drops it from the inlet at the top of the tank. The fuel feed is from a dip pipe. I am not on boat at present so can't check. I will look at Tex tanks site for any info.
Anybody know answer to this?

I believe Tek Tanks normal practice is to include a dip pipe on the return only for large engines as used on motorboats, as the rate of return on these is enough to cause foaming if it were just dropped in the top. On sailing boats they wouldn't normally include a dip pipe; there certainly isn't on mine.

Pete

 

[tolhurstorganic]

Yes you are correct, I remember the return pipe is connected to tank via a banjo only.

 

[Oldhairy]

A suggestion that might address two issues in one move.
#1; The fuel return problem as posted.
#2; Fuel Bugs allegedly thrive in warm to hot fuel.
Suggested solution; Instead of returning the fuel to the tanks, return it to the primary fuel filter. In most installations the CAV types have at least one spare inlet port.
Works well on a V/P 2003.

 

[penfold]

Here's an electrically actuated duplex valve, and there's a mechanical one here and here.

If you did go with Nigel's suggestion, it would be important to make sure the overflow from one tank to the other can cope with the fuel return rate from the engine at full throttle to avoid dumping it over the side/on deck.