Building Tip - Fuel Tank Vent Line

[mspenc45]

I know this, I believe the Van's design is part of the elusive fuel tank paint blister problem. On my RV, as long as I leave the tank caps cocked open on a hot day while on the ground or hangar, no blisters. If I leave the tanks closed up, blisters. When I pop the fill caps on a hot day, I can hear the pressure release from the tanks. That small amount of pressure seems to induce fuel vapor migration through the sealant. With a good impermeable paint paint layer on top of that, one that cannot pass this vapor fast enough, and shizam, a paint blister! A venting system that does not allow liquid fuel to enter it and create head pressure when the tank heats up and fuel expands and even vaporizes and tries to escape through that already fuel filled vent tube.

[rv8ch]

Is there a chance that your vent is blocked?

[DavidHarris]

I didn't love the intricate routing of the fuel vent line through the fuselage, and saw this thread and built my vent for an RV-7A according to this thread.

The DAR looked at it today and wouldn't sign off on it. He pointed out that there is a low point in the loop where sludge or water could accumulate. If it then froze at altitude, the vent line would be blocked and it would cut off fuel flow from that tank. I'll have to go back and rebuild according to plans.

Just a heads up for others considering this tip.

David

[RV7A Flyer]

Originally Posted by DavidHarris

I didn't love the intricate routing of the fuel vent line through the fuselage, and saw this thread and built my vent for an RV-7A according to this thread.

The DAR looked at it today and wouldn't sign off on it. He pointed out that there is a low point in the loop where sludge or water could accumulate. If it then froze at altitude, the vent line would be blocked and it would cut off fuel flow from that tank. I'll have to go back and rebuild according to plans.

Just a heads up for others considering this tip.

David

I love the random arbitrariness of DAR inspections. Mine saw it, said it looked good and that a lot of Rocket guys do that, and moved on.

Sludge or water? Uh, yeah, so? It's the VENT line, how's it supposed to get in there in the first place? Osmosis? One end is below the tank, the other is at the high point in the tank itself. Hard to see ho water would get in there in the first place.

I swear...

BTW, isn't there a low point in ANY vent line that doesn't have a continuous downhill run? You know, like the plans-shown line that goes into the cockpit, UP and then back down? Wouldn't that have a low point prior to the upward portion, between the tank and the top of the uppermost section? Couldn't water or some other mysterious substance magically appear there, as well?

[Carl Froehlich]

Here is a set up I did for the RV-10 and RV-8. It mitigates mud dabblers and ice blocking the fuel vents.

http://www.vansairforce.com/communit...d.php?t=165701

Carl

[ian_mac]

Originally Posted by f1rocket

Over 30 Rockets flying with this set up. The Rocket plans show this as the standard way to vent the tank. I thought the RV folks might be interested in using this. If not, that's okay by me.

great post thank you

[DanH]

Originally Posted by lr172

When pumping cold fuel into a warm, sunlit tank, expansion forces (expanding fuel displacing available space is different than the effect that atmospheric pressure creates) easily push the fuel up 10's of feet in the right circumstances.

Yes. In hard numbers, gasoline has a coefficient of thermal expansion around 0.0005 per degree F. The change in volume for a temperature rise is dV = V * 0.0005 * dT. So, 21 gallons in an RV-7-8 tank filled to the brim will expand roughly 1/10 of a gallon for every 10 degree F rise in temperature. Although the vent line itself provides some expansion volume, it's not much, thus some fuel is pushed out of the full tank when it warms in the sun.

I am sure the coiled line works fine as a tank vent. Given the numbers don't lie, I'm also pretty sure the coiled line spits some fuel on the ground if the owner tops off to the neck with cool fuel, then leaves the airplane in the sun.

Is it better to run the vent line up high in the fuselage per plans? Again, the numbers say it will spit fuel if topped, then warmed.

There is variation between airplanes. Least fuel on the ground will be a case in which the builder got the outboard end of the vent line hard up against the top tank skin, in the small airspace higher than the bottom edge of the T-406 filler neck flange.

[f1rocket]

Originally Posted by ian_mac

great post thank you

You?re welcome. Been 15 years since I first posted this but glad some folks are still benefiting.

[N5AD]

I know this is an old article. However! After recently purchasing a used RV6A and finding fuel vent lines inside the cabin (passenger side) that have been compromised likely from a nervous passenger in the past, this seems an excellent retrofit to eliminate the additional source of fuel even potentially entering the cockpit. Of course all engineered considerations above i.e. parking slope, lateral vent line to tip slope etc. Looks to me to be a great option.

[chopperchops]

Thanks Randy for sharing, and a big thanks to Steve Riffe for helping me with the RV-8 aspects.