Fuel and vent lines flooding when filling

[muncedog]

Hi all, I am fairly new to boating and have been having issues when fulling my fuel tank. When I first filled the tank fuel flooded out what I thought was the filling line. The second time I filled more slowly I realized actually it was flooding out the vent line (perhaps it was the filling line originally at full speed!) This is difficult to avoid even at slow filling speeds as the petrol pump doesn't cut out on its own - I guess because the vent is mounted lower than the filler plug? Anyway, looking inside the boat there is no upwards u-bend which I understand there should perhaps be on the vent line - to stop water entering/fuel exiting. I also noticed that odly the filling line was completely empty but there was still fuel upto 3/4 the way on the vent line. So my questions are: 1)is the upward U-bend neccisary - could my fuel get contaminated etc? 2)shouldn't the vent let air in and therefore the fuel level drop in the vent line or does it have some pressure resistance such that it only lets air in under a stronger vacuum? e.g. is there something wrong with my vent? 3)Other than filling super slow and listening for the full mark is there anyway to avoid this issue (I noted you can buy clip on bottles over the vent screw). Or is something wrong with my setup?

 

[Watermann]

You can try one of these and loose the loop that doesn't work.
[h=1]Fuel Vent Line Surge Protector[/h]

 

[muncedog]

So I guess it's a common problem given these exist!
Mine doesn't actually have the upward u bend.
Also just seemed odd that the fuel line is bone dry but the vent line is filled!
Nothing wrong with that?

 

[Watermann]

Yeah that's not right, the vent line shouldn't have standing fuel in it all the time.

 

[JASinIL2006]

If the vent line is too low, couldn't gas slosh up into from the tank? Then when add gas via the fill tube, the pressure in the tank would eventually launch the gas in the vent tube. Maybe just elevating the vent tube would do it...

On the other hand, I regularly have problems with fuel spill over when I'm fueling at a pump, and I've read lots of posts from others who experience that as well. My problem is avoided by fueling slowly, but it sounds like that doesn't make a difference in your case.

 

[JaCrispy]

You definitely want the U bend in the vent line and have the vent line angle down to the tank with no bend to catch water/fuel. I had to reconfigure the vent in my boat from stock to get this right. The fuel system is sealed so the only vent is the actual vent line. Once that's clogged you will create a vacuum in the fuel system and eventually stall. It's normal for fuel to spit out the vent. You can try to gauge it by only adding a certain amount or listening close to the hoses when fueling. If your vent has the upward bend, the excess fuel will flow back to the tank or out the hull and won't clog the vent. My fuel gauge is somewhat accurate so at 1/2 tank (45 gal) I only add ~20 gallons at the pump.

 

[jbcurt00]

So I guess it's a common problem given these exist!
Mine doesn't actually have the upward u bend.
Also just seemed odd that the fuel line is bone dry but the vent line is filled!
Nothing wrong with that?

Old specs:

New specs:

Fuel spilling out isn't the only reason you want a bend in the vent pipe. You also want any water that gets in the tube to come out not go down to the tank.

IMO, no, there is something wrong if the vent stays full of fuel......... It NEEDS the upward bend, or a P-trap at the thru hull.

 

[64osby]

Maybe the vent line is too low in the tank. We don't know how it is set up or constructed.

 

[muncedog]

So mine is likely the original from 1990. So should have the upward u bend. I wondered if it has trapped fuel because it's not an open vent but a pressurised relief? Alike holding thumb over a drinking straw full of water?

Also just doesn't make sense why no fuel to same level in fill line? They both reach the tank at roughly the same height and the vent fuel line is a good 2 ft above the top of the tank!
Other than my straw idea I just can't see how it's held there!
Any thoughts?

The boat is on a slope backwards so fuel will head towards pipes

 

[muncedog]

Also, when it does come out it's quite violent and sudden. And a good half litre comes out with no warning sign. Almost like a relief valve going!
I also swear it only starts flooding out the second I stop pumping fuel in - as if the fuel pump is holding the pressure back?
Are there baffles in the tank or ways for air to trap until I stop pumping they then burp out?

 

[Woodonglass]

What Make, Model and Year boat do you have? What size tank? Have you confirmed that there are no Kinks or weird sharp turns in the vent tube? Can you provide a drawing of your particular Fill Pipe Vent Tube configuration?

 

[muncedog]

Hi Woodonglass,

So its a Bucaneer 510 1990 ( same as link below)

Its a 110L tank.

No kinks but from an observation, it looks like the vent line drops down very slightly.

Drawing attached, blue line is vent line. First thinking is add an upwards u bend and remove the slack at the bottom.

Im wondering, when I fill up at the station, as its on a trailer the front of the boat is sloping down, e.g. bubbles in the tank will be at the opposite end to the vent. Does that not cause this problem? Im basically filling a tank at the bottom!

 

[muncedog]

Sorry, front slopes up I meant due to height at tow ball! Just struggle to visualise how air can get out that way!
I also noticed yesterday that when it does come out it pulses a little alike water sloshing back and fourth in a bath tub. Probably why it starts when I stop pumping as backed pressure comes back!

 

[Daniel1947]

From your drawing: What is the black line? Are the black and blue lines going into the tank on the top of the tank or on the sides near the top? Only observations I can offer from what I have read through out your thread are these: 1. No part of the vent line should be lower than the top of the tank; or 2 be lower than the top of the fuel level in the tank; and 3. Laws of physics...the amount of fuel going into the tank through the fuel filler tube/hose will cause a pressure imbalance. The volume of fuel will be trying to displace the same volume of air which is inside the tank. The air has to escape for the fuel to enter the tank; if the air can't vent/escape it will take forever to fill the tank.

Some of us old timers can remember years ago when changes were being made to vehicles because of new laws about the environment. Many vehicles had smaller filler openings and you just couldn't pump the gas as quick, because the fuel system was a "closed" system; no fumes were allowed to escape into the atmosphere; the vents ran through a "charcoal canister" which was supposed to absorb the fumes, but where not designed to release a lot of pressure all at once. So when you opened the valve on the pump hose, the air pressure had to be released through the filler neck and many of us ended up with fuel on the sides of the vehicles as well as our shoes. Any fuel that ends up in your vent tube is going to be forced out when you attempt to fill your tank.

Like I said, law of physics...nature hates imbalance. You are forcing fuel down a 1-1/2 inch? ID hose, and you are venting through a 3/8"? ID hose. Hope it helped.

 

[muncedog]

All of that makes sense.
FYI the fuel line is the black line.
They both come into the side of the tank, almost at the same height. For that reason it might be very difficult to not get any fuel in the vent line.
Other than the mess of fuel coming out, would trapped fuel there cause any problems?
I can't think of any, the tank can still breathe..

 

[JaCrispy]

All of that makes sense.
FYI the fuel line is the black line.
They both come into the side of the tank, almost at the same height. For that reason it might be very difficult to not get any fuel in the vent line.
Other than the mess of fuel coming out, would trapped fuel there cause any problems?
I can't think of any, the tank can still breathe..

If the vent line has a dip in it, it can collect fuel or water, which will create a vacuum on your fuel system. So anything trapped in the vent is a bad thing. The tank can only breathe through the vent.

 

[achris]

Other than the mess of fuel coming out, would trapped fuel there cause any problems?
I can't think of any, the tank can still breathe..

Apart from the fire hazard, in a lot of places now there are hefty fines for fuel spillage... Especially marinas... And if you drop fuel in a petrol shop, they ban you.


Had the same problem with mine. The vent connection point on the tank is at the front end of the tank, so having a continuous upward path for the vent hose was impossible. I also had the vent hose fill with fuel... And at $1.50/litre it gets too expensive to be throwing on the ground.

My solution was to build a S/S canister and mount it to the inside of the transom next to the through-hull vent fitting. The vent line from the tank enters the canister, into a welded in pipe that reaches more than halfway down the canister, on the top. Another line goes out of the top of the canister to the through-hull vent fitting, but not before completing a upward loop to stop water entering. The 'clever' part of the system is the hose that comes out of the bottom of the canister. It goes to a fitting in the fill hose, also fabricated out of S/S. The canister also has a baffle plate welded across it about halfway... Any fuel in the vent line that is forced up by the outgoing air during filling goes straight into the canister, into the bottom and out, back into the fill tube (which acts as a venturi and actually sucks the fuel liquid back in)... The excess air is expelled through the top vent hose, clear of any fuel it was carrying.... I haven't lost a drop of fuel since building and installing it....

Chris.......

 

[Woodonglass]

This ^^^ Sounds GREAT but...My Old Dumb Okie Brain is gunna need a drawing/diagram/pic to "SEE" what you're talking about. It would greatly help others too, IMHO!!!

 

[achris]

Q & D drawing...

The baffle across the middle of the canister has small holes in it. Above the baffle is filled with SS 'wool', like drill swafe, as a flame arrester. The return fitting in the fill line is cut so the incoming fuel creates a venturi effect as it passes over the tube...



HTH

Chris.......

 

[Woodonglass]

Velly Intellesting!!!! Did you Build this or have it done. Very nice engineering and well thought out.

I'm wondering if this will work on his set-up since his fill tube and vent tube are right next to each other. At least his drawing seems to indicate they are. And he said that when he fills at the pump on the trailer it's tilted forward causing the fuel to pour into the fill tube and the vent tube simultaneously preloading them with fuel. I'm NOT by any means knowledgeable about this cuz I've NEVER owned a boat with this style of tank. I'm anxious to learn and this is extremely interesting to me. You seem to have a GREAT design and method for dealing with this. I've been researching it and found a LOT of people with this problem but your's is the first solution that I've found that stated they had GREAT success.