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Vent line not blocked, but tank pressurized?

sbalmos

Well Known Member
I'm trying to think this one through, whether it's just a nuisance, or I have to pull and fix my petulant cranky left fuel tank again before final inspection.

Currently, my hangar is 70-ish inside. Both left and right tanks have approximately 11 gallons in them, definitely nowhere near the top where fuel would readily enter the vent line. But my left tank is already dripping fuel out my vent. I'm almost certain this is fuel leaking through from the vent line bulkhead fitting inside the tank, probably the vent line fitting Just Loose Enough [tm].

Question 1 - is there enough normal outgassing pressure from fuel in the tank to push fuel in the vent line up the J-U loops of our vent lines inside the fuselage, clearly above the level of the tanks, until the fuel in the line vents overboard?

Now, the more important question... When I raise the tab on the fuel filler cap, and thus unseal it without removing the cap, I get a large hiss. I'm almost certain this is excess outgassing being released from the tank that didn't make it through the vent line in the fuselage, since the vent line was full of fuel.

This pressurization built up over 2 days, on the ground in my hangar. I'm wondering whether I should be concerned with it in a flight scenario? I'm thinking not, because when fuel is being used up from the tank, now we're dealing with a vacuum situation in the tank, and the vent line is used to pull in air into the tank. It's just going to happen to pull that "leaked" fuel back into the tank.

The real concern I have is whether the vacuum in the tank, in normal flight, will become too much for the tank because the vent line is filled with fuel, and maybe compromise the tank? I'm thinking not, but I'm just posing the scenario. Too many memories of reading too many posts here relating to balloon leak tests and pulling vacuums to pull in Locktite to seal weeping rivets, and posts saying that too much pressurization or vacuum will rupture the tank.
 
The tank should never have pressure or vacuum - this is the definition of being ?vented?. Something is wrong and wrong enough to ground the plane.

I?d first take off the vent line and see if it is blocked. You would not be the first to have a bug clog it.

From there move on to the rest of the tank.

Carl
 
I would think bug or blockage too. But I've got fuel dripping overboard through the vent on the underside of my fuselage, as if it was a full tank sitting out in the hot sun. I wouldn't think I'd be getting fuel through and overboard if there was a big enough blockage like a bug, would I?
 
Scott

I had an RV4 that I purchased that did this for years. I had to leave the fuel caps loose in the hangar or it would constantly drip fuel from the vents when the morning sun shined thru the hangar door on the wings. I finally removed both tanks. One vent line had no flare at the bulkhead connection inside the tank, the other tank had a loose b-nut on the line inside the tank at the bulkhead. Fixed both and it never leaked another drop.
 
vent

Might be a dirtdobber they put dirt in lines to build nest like concrete left my valve cover off while adjusting valves and got one in oil return had to make a new return line.Take cap off and see if you can blow through vent. I use large pipe cleaners in my vents while sitting in hanger.
Bob
 
Question 1 - is there enough normal outgassing pressure from fuel in the tank to push fuel in the vent line up the J-U loops of our vent lines inside the fuselage, clearly above the level of the tanks, until the fuel in the line vents overboard?

Is your vent line built so that it holds fuel inside it while on the ground and/or in flight?

The vent line should drain itself when your aircraft is parked... right? Either drain to the ground or drain back to the tank. You don't want liquid sitting in your vent line... what if you get some sort of water in there and it freezes in flight?
 
Here's my theory:

A leak as you suspect will allow fuel into the vent line at the tank fitting. This fuel settles into the low spot in the vent line, essentially forming a liquid plug. As things heat up and the resulting pressure builds, some of that fuel is pushed up the vent line and then out, causing the dripping. But there is still fuel leaking into the vent, so the line remains "plugged" and the rest of the pressure builds up in the tank itself.

As suggested you can try blowing into the vent tube (you may need a piece of flexible tubing to attach to it). You could also drain fuel to where it is below the level of the vent line fitting on the tank and see if the problem stops.

Chris
 
Exactly, Chris. The vent line essentially becomes a water column manometer. Looks like Tommy used to have the same issue. I'm not concerned about leaving the cap unsealed on the ground. I'm more concerned whether or not the tank will build up an unacceptable level of vacuum while the fuel is used up in flight, or whether enough vacuum will suck the fuel back out of the low point, allowing air to fill the tank.
 
The vents under the fuselage point forward so the airflow pressurizes the tank in flight and should draw the fuel ?plug? out of the vent line. Once I did notice fuel dripping from the vents, on a hot day with full(ish) tanks. 225 hours, only noticed that once.

I also noticed weeping rivets one time, with no fuel exiting the vent, so the pressure in the manometer per se was greater than the seal around that rivet. (That happened after a topping the tanks in winter, and rolling into a heated hanger, perhaps 60 degree temp difference)
 
The tank should never have pressure or vacuum - this is the definition of being “vented”. Something is wrong and wrong enough to ground the plane.

I’d first take off the vent line and see if it is blocked. You would not be the first to have a bug clog it.

From there move on to the rest of the tank.

Carl

The tanks can build a small amount of pressure in certain situations. As the gas heats it expands and air must be displaced. If it can't be removed that air will compress (i.e. pressurize). If the vent line is submerged in fuel, the now pressurized air will push the gas out the vent circuit to relieve/equalize itself. However, almost two feet of head pressure exists, requiring enough pressure to build in the tank in order to push the gas against gravity to clear the uphill loop in the cockpit of a side by side. Due to this head pressure, some pressure will be maintained in the tank until the vent line is cleared of fuel or the gas temp decreases or you pull fuel out via vacuum. This is not significant or dangerous pressure, but certainly enough to hear it escape when the cap is removed. This phenomenon has no effect in reverse and creates no restriction on the vacuum side of things.

Having the the vent leak at the inner tank bulkhead would leave the vent line submerged with as little as 1/3-1/2 full. Most people only see this with VERY full tanks and in that case there is a very small volume of compressed air.

Larry
 
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I absolutely agree with Tommy Turner. I had a hiss on removal of one of the fuel caps, and "spitting" of fuel on hot days, and it also turned out, on inspection, to be a loose fitting of the vent line at the fitting at the root end.
 
So be it. As long as I?m not going to have a vacuum issue in flight where the tank can?t equalize, I have no problem just keeping the cap unsealed (in the hangar) or having some spitting (on the ground somewhere). And dealing with the probable loose fitting becomes a deferred maintenance item for my condition inspection next winter. :) Thanks all!
 
I had an interesting experience the other day away from home. Fuel cap was hard to open. Very unusual. I blew to the vent it's passable but something was there. I flew to another airport using the opposite tank and got some compressed air there. Retrieved a fossilized bug from the vent :)
 
I found five dead bugs under the floor pan. they get around.

I had an interesting experience the other day away from home. Fuel cap was hard to open. Very unusual. I blew to the vent it's passable but something was there. I flew to another airport using the opposite tank and got some compressed air there. Retrieved a fossilized bug from the vent :)
 
interesting.........i posted questions about venting a month ago and really never read about problems like i am seeing on this thread. i am about done with the custom tank i built for my 12 . behind the passenger seat i put a 1/4npt bung for possible vent site or who knows what. i didn't use it as a vent line site but put a small valve on it. i easily have an ''alternate source of vent'' by reaching behind passenger seat. never heard of that before but venting is as vital as fuel.
 
I had an interesting experience the other day away from home. Fuel cap was hard to open. Very unusual. I blew to the vent it's passable but something was there. I flew to another airport using the opposite tank and got some compressed air there. Retrieved a fossilized bug from the vent :)

A good reminder that all tank vents should be protected from bug injestion. If anyone needs proof of this concept, stop by the hanger and I'll show you a colapsed RV8 tank that was so badly damaged from the vaccum being pulled on it by the fuel pumps it had to be replaced :eek:
 
How to keep bugs out and prevent ice blockage of fuel vents

I have a modified fuel vent system on my planes. There are two elements:
- A transition from 1/4? to 3/8? tubing to facilitate the vent end to have a screen to keep bugs out.
- A vacuum breaker valve to allow air into the tank in the event that the exposed vent end gets iced over. The vacuum breaker lets air in, but does not let fuel go out. It is related for gas.

The vaccuum breaker is a McMaster part.

Test once a year by putting a plastic hose over the vent and sucking to make sure air comes in the valve.

Carl
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That's a lot of vibratory mass to be supported by the aluminum lines - I would feel more comfortable with a hard mount somewhere. YMMV.
 
Yep - no harm to do that but the valve itself is very light and the line run is short.

Carl

I could not find that valve on the MC web site Carl. Would you happen to have a part number or catagory you found it under? Also,,,is it a steel or plastic valve? Thanks!
 
I could not find that valve on the MC web site Carl. Would you happen to have a part number or catagory you found it under? Also,,,is it a steel or plastic valve? Thanks!

Part number is 5492K51. It is plastic - I just weighed one and it comes in at 0.8 oz.

Carl
 
I would think bug or blockage too. But I've got fuel dripping overboard through the vent on the underside of my fuselage, as if it was a full tank sitting out in the hot sun. I wouldn't think I'd be getting fuel through and overboard if there was a big enough blockage like a bug, would I?

If you have any pressure in the fuel tank bet is something is blocking it not fully but almost causing the slight pressure. If a vent line is open fully and the line is full of fuel it takes very little pressure to force fuel out of it and there should be no pressure in the tank.

This comes from many years of experience with fuel, tanks and leaks.
 
Possibly an internal issue with the vent line? Gee Scott, I hope not. But sure sound like a venting issue.

Tom
 
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