Vent Line Header Tanks

[DanH]

I'm told some airports, in particular those in Florida, are enforcing fuel spillage regulations. My RV-8 regularly spit fuel from the tank vents if left parked while full, so I decided to do something about it.

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-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 think there is another factor at work here also. Many have noted the escaping fuel sometimes spits rather than flowing out in a stream. It suggests the small ID of the -4 vent line allows alternating blobs of fuel and air to be pushed up the vertical section of the vent system. Separating them would be a plus.

So, I built header tanks for the vent system. They serve two purposes, (1) providing expansion volume, and (2) acting as a separator, allowing air to bubble past entrained fuel. These tanks are 2" OD by 8" long (IIRC), welded 0.049" 6061. Interior volume is about 0.1 gal, in theory enough to handle a 10F rise. It's possible to create a case with more fuel temperature rise, as we might by topping off from an underground tank on a hot sunny afternoon. However, it takes time for the fuel to warm. The headers should allow a fella to top off, then go take a leak without returning to find the airplane doing the same thing.

So far so good. I was going to fly them through a whole summer before reporting, but perhaps someone would like to duplicate the installation and compare performance notes.





Ignore the tygon tubes and temporary wiring. That's a different experiment.

[Mark Dickens]

I used the Rocket coiled tube vent approach and have never had a drop escape after refueling. If it has, I've never noticed it, but generally, I refuel right before I fly so it could be that the fuel never warms up enough to expand before I start burning it off. I dunno.

Looking at the photo, the first thing I think of is "how is he going to retorque those gear nuts with that thing in the way?"

[scard]

(next project : rotary welding table)

[DanH]

Originally Posted by Mark Dickens

Looking at the photo, the first thing I think of is "how is he going to retorque those gear nuts with that thing in the way?"

Two B-nuts and they are out on the bench.

[Jpm757]

I also used the Rocket coiled vent, 2 loops, has worked fine-no spills.

[PaulvS]

The vent header tank is an interesting concept and I'm wondering how it works for draining out the separated fuel? I don't know the configuration of the -8 tanks but presumably they are in the wings and when the pressure in the wing tank drops from fuel being pumped to the engine the header tank will flow back into the relevant wing tank. I guess you'd want to be absolutely sure of no chance of blockage in the backflow otherwise fuel flow to the engine could be restricted, or the wing tank might even implode?
If it was me I think I would hook up and test one side first, but I may be overly cautious, and I'm sure you know a lot more than I do!

[emsvitil]

When you fill a tank, how much air space is left over?

I would think that would handle expansion...........