Air in Fuel

[jonmhammond]

If anyone has a suggestion or input for this puzzling problem i will be a very much appreciative. The symptom: air bubbles can be seen entering the racor which is shared by two gensets. There is a manifold on the fuel supply line from which there are three valves, one for each racor (two for propulsion engine and one for two different gensets). Each genset has it's own dedicated return to the return manifold where the engine and both gensets meet and are directed back to the tank. There are non return valves on the lines from both the propulsion engine and generator, which I assume are there to prevent fuel siphoning from one return to another. I noticed last week that the valves were actually backwards and probably have been that way for a very long time. This means that the returns have not been going back to the tanks. Now I have switched the valves the right way round and the main engine is perfectly fine, not drawing air. But while the genset is running there is air coming into the supply. I do not understand why, but when I turn the one way valve back the wrong way round, restricting the fuel return, the air leak stops and everything seems fine. Also, when the second generator is running, there are air bubbles in the racor, even though the return from that genset is dedicated and unrestricted back to the manifold. Maybe it's too complicated a description without having a drawing, but if anyone has had a similar problem or a suggestion please let me know. I have been at this for a week now and the technicians i've had on board are unable to provide any valuable insight.

[DeepFrz]

A diagram would help. Also, do you get bubbles when only one generator is running without the main engine running? Obviously you have an air leak but without a diagram I can't figure out where it could be.

[8000hrs]

can you provide a diagram? also is the fuel being heated before passing the filter? is it a diesel fuel then change over to heavy fuel?

[GordMay]

Greetings and welcome aboard the CF, 8000hrs.

[Sailmonkey]

Originally Posted by 8000hrs

can you provide a diagram? also is the fuel being heated before passing the filter? is it a diesel fuel then change over to heavy fuel?

I don't believe anyone on this forum will be heating heavy fuel oil. We discuss small boats, usually sail with some exceptions. Not ships.

[8000hrs]

Originally Posted by Sailmonkey

I don't believe anyone on this forum will be heating heavy fuel oil. We discuss small boats, usually sail with some exceptions. Not ships.

sorry, i just joined this forum not knowing that majority is discussing systems on small boats,.

[Sailmonkey]

Originally Posted by 8000hrs

sorry, i just joined this forum not knowing that majority is discussing systems on small boats,.

No worries, I'd hate to think of cleaning the centrifuge while pitching on a 32' sailboat!!!

[8000hrs]

Originally Posted by GordMay

Greetings and welcome aboard the CF, 8000hrs.

thank you!

[banyandah]

Hi,
Air in fuel lines is almost always a poor fitting on the suction side. This is more prone to happen in systems that draw fuel from keel tanks where a poor hose clamp or faulty compression fitting allows a tiny bit of air to enter as the suction is quite great.
Check your fittings before the lift pump. Double up on hose clamps with the screws located opposite to each other.
Hope that helps.

On Banyandah, we pump out fuel into a gravity feed day tank of 50 litres that has a sight tube so we can measure fuel usage. Be cautioned, some injector pumps do not like gravity feed. Check with the manufacturer. But for us, this system has worked marvelously for 40 years. The day tank has a tap at the lowest point to draw off consensation, and a small ball valve to shut off flow while changing fuel filter. Purging anf priming are not a problem. We use the engines lift pump to fill the day tank.

Cheers from Australia.