Diesel tanks - Blowback!

[Scarron]

Our new [to us] boat came with both it's 600 litre wing tanks about half full. Yesterday I attempted a refill for the first time and of course it turned into a complete balls up.

I was directed to the 'faster' of the two nozzles at Lallows in Cowes and started to fill the port tank, well I was conscious of course that diesel delivered at high flow rates can foam up I only had the trigger half squeezed but within a minute or so I heard it gurgling its way back up the filler neck so I released the trigger.. after a couple of seconds a geyser of foamy diesel erupted from the filler neck and covered me and the teak side decks. I did the only thing possible and resorted to my best Anglo Saxon, I suppose it was probably only half a litre or so but a little goes a long way as they say.

Having shaken myself down, wiped the worst from my eyes and cursed the fact that my new oilies were now literally - oily.. I then proceeded to commence a cycle of slowly filling and waiting, filling and waiting all the while listening for the tell tale gurgle of an approaching geyser.

I reckon we had started with about 100 litres in each tank and I only wanted to add another 200 per side to take me up to half full but it took a good 20-30 minutes to do it. The port tank was a right bugger and I had at least another half dozen smallish geysers, starboard tank wasn't quite as bad as the port but I still had a couple of small blow-backs. By the time I was finished both I, the cabin sides and teak side decks were heavily splattered.

None of this was helped by the fact that we had been with the whole family to Cowes for lunch to celebrate my daughter's birthday so with a full compliment aboard I had an audience, the only one not bothered by the whole performance was the dog!

I had replaced the breather tube on the port tank as it didn't have a swan neck and it's probably only 12mm/ 1/2" hose which now strikes me as far too small a bore to allow enough air out while filling the tank fairly quickly, what do we think, should I increase the bore size of the breathers?

Also the tank filler necks go into the front of the tanks and there is a baffle halfway down the tank, so perhaps the holes in the baffle aren't' allowing the fuel filling the front half into the back quickly enough leading to a big build up of frothyness?

Advice gratefully received.

 

[Portofino]

Try the slower pistol .
Open both sides fillers while filling one side .Assuming the tanks have a connect / transfer pipe ?

I don’t think it’s the breather dia because gas compressors .

 

[BruceK]

I had replaced the breather tube on the port tank as it didn't have a swan neck

fuel blow back issues are nearly always related to a blocked or restricted vent hose. Typically, a venting hose from the top of the tank, runs parallel to the fill tube and connects to a port close to the fuel fill opening. This vent tube must always be parallel to and above the level of the fill tube. It must be routed in such a way that any fuel entering the tube will flow back into the tank. If there is any dip below the level of the fill tube, fuel that enters the vent tube due to reaching a top-off level will block air flow out the tube, much like a p-trap. That blocked air flow means trapped air from the tank can only escape through the filler neck, thus a blow back.

 

[Scarron]

fuel blow back issues are nearly always related to a blocked or restricted vent hose. Typically, a venting hose from the top of the tank, runs parallel to the fill tube and connects to a port close to the fuel fill opening. This vent tube must always be parallel to and above the level of the fill tube. It must be routed in such a way that any fuel entering the tube will flow back into the tank. If there is any dip below the level of the fill tube, fuel that enters the vent tube due to reaching a top-off level will block air flow out the tube, much like a p-trap. That blocked air flow means trapped air from the tank can only escape through the filler neck, thus a blow back.

The breather tubes on my tanks exit through the hull topsides above the top of the tank and didn't have a swan neck so I was concerned about spray or rainwater entering the tank. Thinking about it they must only be intended to operate as expansion breathers.

I think I might need to put a secondary larger filling vent hose in the top of the tank at the back and feed it forwards to a tee piece just below the deck filler to allow air to escape while fuel comes in from the nozzle?

Try the slower pistol .
Open both sides fillers while filling one side .Assuming the tanks have a connect / transfer pipe ?

I don’t think it’s the breather dia because gas compressors .

Both tanks and engine fuel systems are entirely independent with no balance/transfer pipe.

 

[Paul1962]

If memory serves, should not the vent pipe be in the order of 1.5 times the diameter of the fill pipe?

My Antares 7 would do precisely as described by Scarrons first post, until I added a filler to the top of the tank itself and used the transom filler as a vent when filling .

 

[Bandit]

Most diesel motor boats have about a 2 inch filler and a half inch or 3/8th id vent air can exit far faster through a small tube than diesel can.

Many people have had the blowback diesel facial experience.

Check the vent is not blocked sometimes the gauze cruds over and can be cleaned or replaced with new.

The vent needs a swan neck and a design that does not allow water in to your tank.

Also check the nitrile o ring on the filler cap is in good condition or replace.

On my boats you can hear the tank getting near full and its judgement.

Also wet your teak deck with seawater before filling then if you do spill any it will not soak in so easily.