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  #1  
Old 01-19-2012, 11:26 AM
Omega232Devils's Avatar
Omega232Devils Omega232Devils is offline
 
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Location: Catawba, NC
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Default Why is my tank pressurizing and NOT venting?

I have searched the forums and found some good information, but unfortunately has not corrected my problem. Any additional help is appreciated.

I have been plagued with a tank leak in my quick build tank. (Left one only) I removed the tank about two months ago and did a complete resealing of the inboard rib section that I could get to through the access panel. I was able to clean, prep and re-proseal the inside and outside of the tank. I pressure tested the tank for 36 hours with no leaks (1psi Balloon). Filled the tank with gas and let it sit for 48 hours, no leaks. The tank flew fine for about 2 weeks with no issues. In the meantime I started noticing that my left tank would build up pressure, both in flight and sitting (temperature changes most likely). I found this when I popped the fuel cap and could heard the pressure releasing.
After the two weeks, my tank began developing a small leak in the lower corner closest to the fuselage. (Same spot as the first leak) I pulled the tank off yesterday to do the repair and did the baloon pressure test and soapy water. After 6 hours with pressure, there was no evidence of a leak. (Frustrating) After I cleaned the old proseal off, and prepped the surface for new seal, the balloon was still inflated. When I put the new proseal on, I noticed a small air bubble between the 2nd and 3rd rivet from the back. (I have to admit, I didn't catch it right away, as I just kept adding more proseal trying to cover up the bubble and it kept coming back) The light bulb finally came on, that this is the source of my leak and explains why my tank leaks when I forget to pop the fuel caps and relieve the pressure.
So that is the good news, I have identified my leak and when it is leaking. (Under pressure = leak / No pressure = No leak) So I looked to my vent system. I have never had fuel come out of the vent ports, regardless of level of fuel. I was able to take my air hose and blow air through the vent on the bottom of the fuselage and feel it on the connector going to the wing tank. I was able to blow air through the vent tube on the tank and feel with my finger the air coming out of the tube by the fuel cap. So my vent lines are clear.
So I can seal the leak pretty easily, but until I identify and correct why my tank is pressurizing and not venting, my leak is going to continue to return.

Where do I go from here?

Dan
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Dan Thompson
RV-6A - N426JM - 180hp / C.S. / dual PMags
NC26 - Long Island Airpark, NC

Last edited by Omega232Devils : 01-19-2012 at 11:52 AM. Reason: Spelling. Thanks 'Mike'
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  #2  
Old 01-19-2012, 11:37 AM
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Mike S Mike S is offline
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Default

Quote:
Originally Posted by Omega232Devils View Post
I was able to take my air hose and blow air through the vent on the bottom of the fuselage and feel it on the connector going to the wing tank. I was about to blow air through the vent tube on the tank and feel with my finger the air coming out of the tube by the fuel cap. So my vent lines are clear.

Dan
Do you mean able, instead of about??

Following is assuming you meant "able"

I can only think of a couple things that could be causing this, one is fuel trapped inside the low point of the vent, other is if a blob of sealant is sitting on the inner tank end of the vent line, and is acting like a one way valve, like a flapper.

Try to suck on the vent line, while it is hooked to the tank, with the cap off----if you can not get air to come out the vent line, but you do get air to go in when you blow on it, then I would look real close for the blob I mentioned above.

I had this happen to one of my tanks, otherwise it would never have occurred to me to even think of such a thing.
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  #3  
Old 01-19-2012, 11:42 AM
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jdeas jdeas is offline
 
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Default test the vent

Remove the gas cap and try to blow through the vent tube to see if it's clear.
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  #4  
Old 01-19-2012, 11:44 AM
gasman gasman is offline
 
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Default

Quote:
Originally Posted by Omega232Devils View Post

So that is the good news, I have identified my leak and when it is leaking. (Under pressure = leak / No pressure = No leak) So I looked to my vent system. I have never had fuel come out of the vent ports, regardless of level of fuel. I was able to take my air hose and blow air through the vent on the bottom of the fuselage and feel it on the connector going to the wing tank. I was about to blow air through the vent tube on the tank and feel with my finger the air coming out of the tube by the fuel cap. So my vent lines are clear.
So I can seal the leak pretty easily, but until I identify and correct why my tank is pressurizing and not venting, my leak is going to continue to return.

Where do I go from here?

Dan
So did you do this? You didn't say. Just connect a rubber hose to the vent fitting on the tank or the belly and blow through it. Sounds like the vent line in the tank is clogged. If the vents are clear, the tank will not pressurize.
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  #5  
Old 01-19-2012, 11:57 AM
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Omega232Devils Omega232Devils is offline
 
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Default Correction

Thanks guys. Man, one word really can change the meaning of a paragraph huh? haha


I did blow air through the vent system, both the fuselage portion and the tank portion. (The tank is off the plane for the repair.) How much pressure can I put through the tube in an attempt to 'dislodge' any material that may be acting as the one-way valve?

The tank tube is not totally block, because I am able to inflate the balloon during my pressure tests.

Thanks again,

Dan
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  #6  
Old 01-19-2012, 12:35 PM
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jdeas jdeas is offline
 
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Default Make sure the cap is off

If the cap is off you can hit it really good. 3003 tube can take a good deal of air pressure. Last tank I looked at was pressure tested to only 3 psi and leak tested and 1.5psi so double check that the cap is off the tank!
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  #7  
Old 01-19-2012, 12:49 PM
Squeak Squeak is offline
 
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Location: Indiana
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Default leaking vent line inside of tank

You might have a leaking vent line inside of tank at the fitting. Have found this before and when fuel is over the fitting fuel it will get into the vent line and cause pressure to build in the tank. Usually burping out fuel with full tank. Can be found by air testing vent lines.
Squeak
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  #8  
Old 01-19-2012, 01:23 PM
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Omega232Devils Omega232Devils is offline
 
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Default Now what?

Ok I just put 80-100psi through the vent lines. The fuselage was clear, strong pressure out. Vent line on tank with fuel cap and drain valve removed air passes through. I cant measure exactly, because of vent line location in tank, but appears to not be blocked.

Some have mentioned blowing from fuel cap side, but how did you do this? My vent line is straight on bulkhead and not bent toward the cap.

I have not pulled the access panel yet to look at inside fitting on vent line. Since there has never been fuel out the vent drain could a hole here even be an issue? My tank is pressurizing and the fuel is not venting, but rather making its way out through a hole created by the pressure inside?

Any other thoughts?

Thanks guys.

Dan
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  #9  
Old 01-19-2012, 01:37 PM
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Mike S Mike S is offline
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Get a helper to run the blow gun, get a flashlight, and an inspection mirror.

Take the fuel cap off, insert the mirror and using the flashlight, find the end of the vent line.

Have your helper blow pressure into the vent line from the other end of the tank.

Look for a blob of sealer acting as a one way valve on the inner end of the vent line.
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VAF 909

Rv-10, N210LM.

Flying as of 12/4/2010

Phase 1 done, 2/4/2011

Sold after 240+ wonderful hours of flight.

"Flying the airplane is more important than radioing your plight to a person on the ground incapable of understanding or doing anything about it."
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  #10  
Old 01-19-2012, 01:55 PM
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Omega232Devils Omega232Devils is offline
 
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Default all clear

Mike,
Did the mirror check and no evidence of proseal valve. Good airflow.

Any other ideas? I am stumped.

Dan

UPDATE: I put fuel cap back on, installed Vans fuel tester valve in drain port. Covered fuel line and left vent line OPEN. I slowly put air in tank using very little pressure and felt for air coming out vent. Air flowed with increase of air going in. I.e. when I increase inflow air pressure, exit flow increased. So doesn't appear to be blockage or I have removed it somehow.

Any other suggestions or tests?

Dan
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Dan Thompson
RV-6A - N426JM - 180hp / C.S. / dual PMags
NC26 - Long Island Airpark, NC

Last edited by Omega232Devils : 01-19-2012 at 02:13 PM. Reason: update
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