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Frozen tank venting

Tomas J

Tomas J

Active Member
I have the retrofit lockable fuel caps from Vans. I ran into a thing that made me worry, and I wonder if others have the same experience.
I flew half an hours in freezing temperatures. Everything was normal, and after the flight I set the machine in a hangar that holds a few degrees above freezing. By coincidence I opened the fuel caps after the machine has been in hagaren a few hours. When I lifted the cap of one of the tanks there was a puff, indicating there was an excess pressure in the tank.
This got me worried ... there are not supposed to be an overpressure ni the tank, as the venting should let it out. I can not understand other than that it must have been water in the aeration system that froze in minus degrees during the flight. That could have creat a vacuum in the tank, and if I have flown longer I might not come up with fuel from the tank. The lockable fuel cap is wholly tight, unlike the original fuel caps.
When the air in the tank warmed up in the hangar, instead it created a positive pressure that caused it to puff when I opened the fuel cup.

I loosened the pipes between the body and the tank and tried to catch up the liquid that was in the pipes. However, I was only getting a small portion of it and it was pure gasoline. But there may have been some water that I did not manage to catch. I can of course make sure that the pipes are empty, but I have no guarantee that there will not be more condensed water that freezes. I ponder to remove the lockable caps and use the originals that are not completely sealed and can allow air to vent if pipes would freeze again.

Does anyone have similar thoughts as me?
Sorry for my bad english :cool:
 
Most set-ups have the vent line go up into the cockpit 12" to 14" above the fuel tank level before turning down and going to the vent. If you have full tanks then the vent line will fill up with fuel and produce pressure equal to the height of the vent line loop. You would be surprised how much air will be released out of the fuel cap when venting off that accumulated pressure. - Was the vent line dripping fuel?
 
Thanks for the comment!
No, I did not see any dripping fuel, but it was pretty much fuel in the vent line when I drained it. Perhaps I shall fill up the loop line to see if it can really make that kind of pressure release. Would be good to know, and if so it would indicate that everything could be normal.
 
Original caps

Not to steal this thread, does vans stock caps actually have the ability to vent? I am using them on my unfinished 8 and am curious to know that.

Bird
 
my std Van's caps do not vent

on a hot day when removing the cap when the tank is totally full I can get significant overflow. my conclusion is the std Van's cap does not vent.
 
The filler caps are designed to seal and all venting is through the vent tubes.

I remember when Barry Ward wrote about his RV-6 ferry flight from Texas to France, he experienced fuel starvation in light icing conditions almost certainly due to the vent tubes icing up. This only alleviated when he descended out of icing conditions. The article used to be on the VAF archives, but doesn't appear to be there now.

I did think about lining up the vent tubes behind the exhaust to prevent icing and also give a little extra tank pressurising, but as the exhaust is pointing at a slight down angle, it could be difficult to achieve.
 
pa38112 said:
Most set-ups have the vent line go up into the cockpit 12" to 14" above the fuel tank level before turning down and going to the vent. If you have full tanks then the vent line will fill up with fuel and produce pressure equal to the height of the vent line loop. You would be surprised how much air will be released out of the fuel cap when venting off that accumulated pressure. - Was the vent line dripping fuel?
Click to expand...

My vents top out about 18" above the fitting on the fuselage. With a little math, that works out to about 0.52 psi if the vent line were flooded with avgas. Easily enough to cause a little hiss/pop when the cap is removed.
 
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I have the "rocket style" vents (coiled tube inside the wing root) and have had this happen a time or two, as well. Disconcerting at first, but no apparent ill effects. (They've "blurped" a bit of fuel once or twice, also, when the tanks are full and got warmer after filling up).
 
Mark Albery said:
?...
I did think about lining up the vent tubes behind the exhaust to prevent icing and also give a little extra tank pressurising, but as the exhaust is pointing at a slight down angle, it could be difficult to achieve.
Click to expand...

Fuel vents, hot exhaust....I guess that might cure any icing issue. Might be able to roast marshmallows too. :eek:
 
Just a thought if it is something you worry about. What about a T on the vent line in the cockpit that connects to a small valve and runs back to the wing root area. Just like an alternate static, why not an alternate fuel vent?

I also think it may be value added to interconnect the vents at their high-point in the cockpit. That way you still can use both tanks if one vent is frozen.
 
Check valve

I actually did this on one of my airplanes. I used a check valve so it was one way---fuel would not "burp" into the cockpit but if the vent was frozen/blocked it would be able to suck air from the cockpit.

Vic
 
I think that's an excellent idea Vic! I've seen/replaced colapsed tanks due to blockage from mud daubers and vent icing could definatly be a problem for IFR guys.
 
vic syracuse said:
I actually did this on one of my airplanes. I used a check valve so it was one way---fuel would not "burp" into the cockpit but if the vent was frozen/blocked it would be able to suck air from the cockpit.

Vic
Click to expand...

I know several people, including me, who have check valves/alternate vents in the wing root area of their -10's.
 
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I once stumbled into something similar to this flying night cargo and can say it ain't fun. :D. I like the idea of a check valve. Vic, do you have anymore info on where to purchase these valves or who makes them? I was even thinking of making a recessed area on the bottom side of the floorboard to keep the vents out of the airstream.
 
I recall that Bruce Bohannon collapsed a tank trying to set an altitude record in his modified RV when his water injection system iced over the tank vent. Don't know if it will help, but I put a small hole on the back side of the vent tube
 
As I only fly VFR I'm not so worried about ice on the outlet of the vents. Especially as they are placed behind the warm air outlet from the engine. I'm more concerned about freezing in the tubes between the cockpit and the tank. There it might be condensed water added during many flights, and it only need to be some droplets of water in the tubes, and freezing temperatures, to stop the air ventilation.
Therefore I have isolated that tubes, and before a flight i freezing conditions I put a hose on the vents outlet and blow into it, with the fuel cap removed, to empty the tubes.

I think Keith and Colin have a good point in saying that gasoline in the vent tubes can build up a surprising high pressure in the tank, causing the little "pup" when opening the cap.
 
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