Fuel and vent lines flooding when filling

[achris]

Designed and built by my own fair hand. (Yes I have a TIG welder)....

 

[Daniel1947]

Considering your statement that both filler hose and vent hose are located in the side of the tank, I still believe the problem can be completely corrected by relocating the vent to the top of the tank. I am not an artist, but let me see if this helps: A graphic representation of your system as it stands now. You can see that as you fill the tank and as the fuel level reaches the top of the tank, fuel will have to be in the vent. Also, as the fuel sloshes around it will also get into the vent.

If you can relocate the vent to the top of the tank, there wouldn't be any way for fuel to remain in the vent for any reason. And to address anyone's idea as to installing an elbow on your existing vent, it should be obvious that fuel would still be in the lower portion of the side vent.

Atmospheric pressure/gravity is forcing the fuel down towards the bottom of the tank. While I think that Moderator Achris has a good design, I still think it misses the point that your vent is on the side of the tank and not the top. There is absolutely no way to keep fuel out of your vent when the tank is full, and it will remain in the vent until the fuel level drops.
And as that old Okie says...it is your boat, just my humble opinion and just my 2 cents.

 

[achris]

My vent line IS on the top of the tank, as per your second drawing. The vast majority of them are... That doesn't stop fuel getting in them... Hence my canister...

 

[Daniel1947]

[FONT=&quot]Achris you are correct, however the only way fuel will stay in the vent line is if the tank has been overfilled, allowing fuel to remain up inside the filler neck. With that assumption, your vent system would work. However, if the tank is not overfilled, fuel would not "remain" in the vent tube. The fuel that does slosh up into it would flow back as long as the other end of the vent is open to atmospheric pressure.

Muncedog stated that his vent was "lower" than his filler tube; with that in mind, nothing is going to keep fuel from getting into his vent. The fuel will remain in the his side vent until the fuel level in the tank drops below the vent. He gets fuel in the vent when he fills quickly because fuel splashes around inside the tank and he continues to have fuel in the vent until the fuel level drops below the vent. Everyone wants to assume that the vent is there to release pressure when in all actuality it is there to vent the tank to allow fuel flow to the engine. No vent, no fuel to fuel pumps.[/FONT][FONT=&quot] His system may also be allowing some venting through the filler cap. Not meant to be a science lesson...but "liquids will seek out their own level," and "nature hates a vacuum."[/FONT]

 

[gm280]

I've had my shares of the same issues with my Glasstream HydroBass boat years ago. The real problem we all seem to share is that fact that boat fuel systems are not like vehicle fuel system being that boats don't have the room that is really needed to fuel from a higher fuel hose to a much lower tank. And then you have shallow tanks too many times and makes it very hard to refuel at regular gas pumps. I had to turn the nozzle upside down and just trickle the gas in with mine or I would gas back up and shut off the gas pump at the station all the time. If you can't vent the air out of the tank as fast as the gas comes in, you will have problems. So there are any number of reasons you have issues. What you really need is for the vent to actually vent the air out as fuel goes in....DUH! But too many times the vent was not really correctly laid out to allow that to work right or even large enough to keep up. There are some good ideas floating around on these comments and you should try to implement some to see how that fixes your problem. But understand you can't supply more gas then the little diameter vents lines can remove the air...it will never work that way!

 

[achris]

The vent line is also there to allow air out of the tank as new fuel is added, and this is where the problems occur. Most vent lines do not take a smooth continuously rising path from tank to vent fitting, they come off the tank, then lay along it for some distance before running up to the vent fitting. Some even must lay down the side of the tank, or run under a bulkhead...

So, what happens is as you're cheerfully driving your boat around, or even if you're stopped to do a bit of fishing, or babe watching, is the fuel continues to slosh around in the tank, some of it goes up the vent hose, and as the hose is laying down, it will remain there. This happens even if the tank isn't overfilled. Even if it's only 3/4 filled, it will do this (ask how I know).... Even if you start the engine and drive home, the fuel in the vent line will not be drawn back into the tank, as the air required to fill the void left by the fuel used in the engine is drawn in so slowly that it just bubbles through the fuel laying in the vent line (ask me how I know this too)....

So, your next stop is the local petrol shop.. Your vent line has a reasonable quantity of fuel trapped in it. You open the fill cap, drop the nozzle in and squeeze the trigger. One of two things will happen, often both. The fuel in the vent line stops enough air from being vented (purpose of the 'vent' line) and fuel in the fill hose is blown back up the fill hose and all over the person holding the nozzle (ask me how I know this one).... The next thing that happens is the operator fills more slowly, and the fuel trapped in the vent line is eventually forced out, and all over the ground, causing the manager to run out with a bucket of sand and a fine for you to pay! (Don't ask me how I know this! :facepalm

Hope this clarifies why I built my 'fuel return canister'....

Chris..........

 

[achris]

...Muncedog stated that his vent was "lower" than his filler tube; with that in mind, nothing is going to keep fuel from getting into his vent. The fuel will remain in the his side vent until the fuel level in the tank drops below the vent....

Wrong. As there is no direct communication between the fuel in the tank and the fuel in the vent line (because the vent line is in the top of the tank and does not have a spear to the bottom) hence no 'syphon' action can occur... Any fuel in the vent line will remain there, even when the tank is drained dry....

 

[Daniel1947]

If the vent tube is higher than the fuel tank...and continues to be higher than the fuel tank without any bends or kinks and is open to the atmosphere...fuel will not remain in the vent line unless the rest of the tank and fuel system is completely sealed. A slight upward angle on the vent tube/hose from the tank is all that would be necessary. Looking at my second drawing...if the tank has been overfilled and has forced fuel up into lets say 2 inches of the vent tube, and if the tube is unobstructed and open at the top, the fuel in the tube will flow back into the tank as the fuel level drops in the tank. Yes, if there is fuel in the vent and you attempt to fill the tank from a pump and you are forcing a larger volume of fuel into the tank than a similar volume of air can escape from, you are going to have fuel forced out of the vent. I am not saying that with the splashing around of fuel when filling the tank, or when motoring across the lake and the fuel is sloshing around, that you may not get fuel into the vent, all I am saying is if the vent is open and upwards, the fuel will not stay in the vent. I understand completely that what I am stating is accurate in a perfect world...we all know that most of our boats were not manufactured in a perfect world. Hence, vent lines ran improperly. We all have to adjust our conditions as we see fit; what works for some, won't work for others.

 

[Daniel1947]

One additional note....true the vent tube is there to assist with the air movement from the tank as it is being filled. However it is mainly there to allow fuel to be suction from the tank by the pump. If anyone doubts this, then close off all openings to your tank completely and see if your pump can pump fuel to the injectors or carburetors. If the pump was strong enough, it would collapse the tank.

 

[achris]

The vent tube has 2 functions, both equally as important, allow air in as fuel is used by the engine and air out as the tank is filled. And yes, in an ideal world the vent house would not fill with fuel, but the vast majority of tanks and vent hoses are not in an ideal world.... Your arguments are moot.

 

[muncedog]

Thanks all for your help.
Some real interesting discussion points there - and some very well (over? engineered) solutions!

I realised a couple of things that seem to help for those looking for the quick work around.

1) temporarily drop the trailer off the tow bar, lowering the front of the tank, helping the air rise to the vent tube at the back of the tank.
2) while filling (slowly) rock the boat gently like burping a baby.
3) if there are more than one of us (most times), one fills, one watches the fuel line (is easily visible, can see it froth up like a beer being poured).

So far these three steps have avoided fuel spillage, albeit painfully slow to fill.

I will try and raise the vent line some more. I think for me unfortunately a fuel catcher on the vent line won't be enough as its out of both the filler and vent lines at the same time so the ss cannister solution also wont suffice.

Tempting to 'blow out' the vent line before filling. Wonder if there would be some kind of suction cup to cover the vent then use some lung power to clear the passage.
I reckon once its clear and stationary at the gas station it would be plain sailing, unless being so close to the filling hose (side by side) that it would instantly refill.. Certainly would anyway near the full tank mark.

 

[muncedog]

Thank you all for your help - I lifted the vent line using a combination of duct tape and a brace support at the top of the upside down U-bend (N bend?).

This appears to make a world of difference!

Also, a simple fix is to actually watch the fuel lines (transparent) as they fill, so to alow time to stop pumping before a mess is made!

 

[kcassells]

I sent this over to someone earlier on a post. Sounds like you may have it licked. Those "transparent" lines are the regulation? just wondering. cause can cause trouble later.

 

[muncedog]

I sent this over to someone earlier on a post. Sounds like you may have it licked. Those "transparent" lines are the regulation? just wondering. cause can cause trouble later.

I simply meant watching the fuel fill line - see when its creeping up!

Things holding up so far although I note its better when a large amount of fuel has been used - otherwise residual amounts can sit in the breather hose and cause breathing problems. The hose still isn't really long enough to have an upwards u bend - lets hope the outside valve thing works to keep water out!