Closing fuel vent

[shorebird]

This may have come up before, but I can't help wonder about the age-old mantra of topping up fuel tanks before winter layup, the idea being to prevent moisture laden air from entering the tank and condensing water within. Although this has some merit, I wonder why simply closing the vent line wouldn't be a lot easier. The vent is primarily intended to allow air to vent during fill-up, and allow a vacuum to replace fuel as it is burned. Looking at a typical out board motor, for example, the vent is simply closed when not in use. Pressure variations during temperature changes would be very unlikely to build enough pressure to cause any problems. Thoughts?

[Dsanduril]

Outboard motor tanks are generally plastic and change shape easily with pressure variations.

It's not the temperature swings (at least with diesel), it's the atmospheric pressure swings. Working in old school (nonsensical) units a 40 millibar swing in air pressure (which would be quite reasonable between a typical winter low and a following clear weather high) results in an atmospheric pressure change of 0.58 psi. That means that for each square foot of tank surface (since the tank is surrounded by air it is all the surfaces, not just the top) there's 83 pounds of force being applied to the tank.

Think about that another way, for the typical 25-40 gallon diesel tank that's someone standing on the top, and another person "standing" on the bottom, and a couple more people spread about "standing" on the sides of the tank. What will your tank think of that? Some won't care, others.... Now imagine those people climbing on and off the tank 10 or 15 times over the winter, flexing it at all the joints/welds/corners...

(Sorry for the imperial units, just easier for old brains - the pressure delta is about 4 kPa and the force on the tank is about .4 N/mm2)

[deblen]

I live on an island in the Bay of Fundy @ N45deg. My boat & about 200 other diesel boats are never more than a few hundred yards from salt water & humidity year round. There are also several hundred diesel vehicles as well as many diesel storage tanks. You can assume that these boats,vehicles & tanks all have open vents & various levels of fuel. I know my boat has been stored for up to 8mos. with anywhere from 10% to 100% tank level & vented.

Water in fuel-Due To Condensation_ is not a problem in this area. I do try to remember to put some stabilzer in to maintain it's cetane(octane?).
When water shows up locally,there will usually be several cases at the same time(water in fuel at delivery) or a storage tank that got a dose of rain,etc. somehow. Being a small community,word gets out on these problems.


So-my personal experience is that,in my area,it doesn't matter with storage of diesel fuel.


Gasoline,especially ethyl gas,is another story. Don't bother trying to store it more than a couple mos. Dump it in your vehicle,give it away - use it up.It cause trouble after 3 mos.


Len

[thinwater]

Originally Posted by Dsanduril

Outboard motor tanks are generally plastic and change shape easily with pressure variations.

It's not the temperature swings (at least with diesel), it's the atmospheric pressure swings. Working in old school (nonsensical) units a 40 millibar swing in air pressure (which would be quite reasonable between a typical winter low and a following clear weather high) results in an atmospheric pressure change of 0.58 psi. That means that for each square foot of tank surface (since the tank is surrounded by air it is all the surfaces, not just the top) there's 83 pounds of force being applied to the tank.

Think about that another way, for the typical 25-40 gallon diesel tank that's someone standing on the top, and another person "standing" on the bottom, and a couple more people spread about "standing" on the sides of the tank. What will your tank think of that? Some won't care, others.... Now imagine those people climbing on and off the tank 10 or 15 times over the winter, flexing it at all the joints/welds/corners...

(Sorry for the imperial units, just easier for old brains - the pressure delta is about 4 kPa and the force on the tank is about .4 N/mm2)

Actually, temperature alone is a big factor over a full season. Assuming -40F (420R) to 70F (530R), just with air you are looking at a 4 psi (560 pound/ft^2) difference.