Fuel Tank Vent Question

[jibes138]

The fuel tank in your boat needs to be vented while fuel is being drawn from the tank, but how about when it is sitting in storage for the winter? It seems like valuable fuel is evaporating causing pollution, wasting money, and making the additives in the fuel a higher concentration than originally designed. Should our tank vents have shut off valves to seal up the tank when not in use? A tank you use to fill your lawnmower is a sealed container until you actually go to use it. This would also limit the condensation issue to any moisture already trapped in the tank when it is sealed shut. Thoughts?

 

[Tim R.]

Do not seal your tank. You can if you really need to but you must allow for contraction and expansion. Not much fuel will evaporate over the winter. I doubt that you could even measure it.

Smaller poly gas tanks like you have on a lawn mower do not contract and expand enough to cause this problem. The poly is also very forgiving. Monel and aluminum are not.

Take a 6 gallon fuel can and fill it with gasoline. L eave it out in the sun for a few hours to see how much it expands. Release the pressure and then put it in a cold place and see what happens.

 

[Alan]

Plugging the vent line on your fuel tank over the winter is probably a good idea for preventing condensation. I would remove the plug in late winter before any real expansion happens. Many skippers forget to replace the rubber gasket on the fuel fill cap which will leak over time allowing water in through the deck fill.
Evaporation is not really an issue for diesel fuel. Gasoline is far more prone to evaporation. Automobile fuel systems are closed systems with designed vent pressure release. If the fuel cap on your car is not installed properly a 'Check Engine' light will appear on the dashboard signaling a failure in the pressure integrity.

 

[higgs]

I would not mess with the vent due to the expansion issue. At what point would you go unplug your vent as the weather warms? Misjudge and you could have a split tank. Ain't worth the chance to me.

 

[Patrick17430]

Don't be messing with the vent. Its there for a reason. If you keep your fuel level full, you would minimise the chance of condensation contamination. Be careful when you wash your boat not to spray the water directly at the vent cover. Your filter will pick up any moisture that is present. I only use fuel from land based stations. Its better quality than dock fuel so the motor run better w/less smoke, alot less expensive and alot less prone to have any water in it. Its kind of a pain in the ass, but I'm used to it. I top it off with my 5 gal. emergency can I keep in the lazarett so it's always fresh. I have never had more than a spoon full of water in my separator, filter. Another tip is, don't get fuel or gas when the delivery truck is present . It stirs up the holding tanks and some of the crud and water is going home with you. Go to a different station. I'm just weird that way.

 

[MoonSailer]

Gasoline and diesel are different!!!! If you seal a gasoline tank that is half full at 80 degrees and then it gets cold (30) you will probably collapse the tank!!!! Gasoline has a much higher vapor pressure than diesel. Air doesn't contract that much over a temperature change but gasoline goes from being a gas to being a small amount of liquid. Car vents are one way letting air in and prevent gasoline fumes from leaving. Cars steel tanks can withstand the pressure when the gas warms. Also as the pressure rises the gasoline vaporises less.

 

[RichH]

Unless you tank is rated/designed for FULL VACUUM .... DO NOT BLOCK or SHUT OFF the vent !!!!
If the tank wasnt DESIGNED to withstand the partial vacuum you will risk (partly) IMPLODING the tank.

Condensation is a symptom that the fuel in the tank is ***ALREADY*** SATURATED with water (vapor). To prevent 'condensation', empty the tank (remove the diesel fuel, take it home and burn it in your oil furnace).
The 'tech solution' with a 'non vacuum rated' tank is to apply a desiccant chamber (silica gel or activated alumina) to the vent line .... or continually 'pad' the top of the tank with a slow 'purge' of very low pressure nitrogen or other inert/dehydrated gas. Letting fuel sit around in contact with the atmosphere (even through a 'vent' is risking water vapor uptake and saturation via 'chemical equilibrium').

Empty tanks dont 'condensate'. IF they did we wouldnt need to dig wells as all we'd have to do is put out 'empty tanks'. :-o