fuel tank anti siphon

[bobnoffs]

hi all,
this question is not about any specific fuel system but a general question.
i understand the theory of how an anti siphon set up works to keep a fuel tank from being emptied when the vent acts as a siphon but how can it have any practical value when the vent is on top of a tank 12'' tall. maybe a tiny bit of fuel worst case but the fuel level is almost immediately below the vent opening.

[airguy]

Originally Posted by bobnoffs

hi all,
this question is not about any specific fuel system but a general question.
i understand the theory of how an anti siphon set up works to keep a fuel tank from being emptied when the vent acts as a siphon but how can it have any practical value when the vent is on top of a tank 12'' tall. maybe a tiny bit of fuel worst case but the fuel level is almost immediately below the vent opening.

When the aircraft is perfectly level, yes. Parking areas are not always level, tires are not always fully and equally inflated, and gear legs may occasionally get "modified" by enthusiastic landings. All these can contribute to a non-level condition sitting on the ground. In addition to that the tanks still have to vent without dumping fuel in a non-coordinated flight condition.

[Carl Froehlich]

Correct - unless you park the plane on a slope and the vent line in the tank is significantly submerged.

The standard problem is fuel venting after fill up on a hot day. While you only loose a few ounces, some people frown on seeing this on the ramp. I defer tank fueling until the last in a cross country stop, and if doing a short hop back home I shift to the other tank before landing so I don?t land with a topped off tank.

I have been far more concerned about a fuel vent getting clogged and ending up with a dead engine, collapsed tank or both. I install a simple vacuum breaker valve in the fuel vent line as a precaution.

Carl

[bobnoffs]

greg,
if the plane is sitting not level and expansion causes an overflow it is going to run out of the tank regardless if there is an anti siphon, until the fuel level is below the vent.
the uncoordinated turn brings up another aspect. my take on that is that bubbles will enter the tank thru the vent line at 6 gph or whatever the engine is using. this brings up the idea of a 2nd vent line...... seems like a 2nd vent would allow fuel to run out the other line.sooooo..... is a 2nd vent line a good idea if both vent lines have anti siphon? and it would seem that where the vents enter the engine should be very close together so they are either both open to air or both submerged?

[airguy]

Originally Posted by bobnoffs

greg,
if the plane is sitting not level and expansion causes an overflow it is going to run out of the tank regardless if there is an anti siphon, until the fuel level is below the vent.
the uncoordinated turn brings up another aspect. my take on that is that bubbles will enter the tank thru the vent line at 6 gph or whatever the engine is using. this brings up the idea of a 2nd vent line...... seems like a 2nd vent would allow fuel to run out the other line.sooooo..... is a 2nd vent line a good idea if both vent lines have anti siphon? and it would seem that where the vents enter the engine should be very close together so they are either both open to air or both submerged?

Don't get me to preachin' on it... I don't have the standard tank vents in my plane anyway. My entire fuel system is somewhat different.

[terrye]

Originally Posted by Carl Froehlich

I have been far more concerned about a fuel vent getting clogged and ending up with a dead engine, collapsed tank or both. I install a simple vacuum breaker valve in the fuel vent line as a precaution.
Carl

Any pictures of the vacuum breaker valve Carl?

[bobnoffs]

a clogged vent was on my list, i just didn't get to it yet.
i am thinking about a second vent line. this line would have its own anti siphon and be completely separate from the other vent. both vent inlets into the tank would be next to each other.
with a single vent i would guess the tank will continue to vent thru the antisiphon. seems like an issue, no one i know of does a preflight that includes checking the vent line. and just checking the vent at the bottom of the plane would not catch something between the anti siphon and the engine.

sooooo......any thoughts about 2 vents, right next to each other?

[RV7A Flyer]

All of this seems like a solution in search of a problem to me.

Is the additional "stuff" like anti-siphon valves and dual vents *really* needed?

[Carl Froehlich]

Originally Posted by terrye

Any pictures of the vacuum breaker valve Carl?

Sure - PM me your email address and I?ll send. I got the valve itself from McMaster: https://www.mcmaster.com/#plastic-check-valves/=1ap4kcq

Carl

[bobnoffs]

well, that's what i am asking. i would be happy to forget the dual vents if more experienced builders say it is not needed.