REFLECTOR: Vent Check Valve No-Yes

[Douglas Holub]

Interesting. Sometimes when I remove a fuel cap I hear a swoosh of air 
coming out.  How much pressure was in the tank?  I'm repairing a fuel leak 
in the right strake that wasn't there before. Maybe the pressure caused it.

Maybe I should remove the check valve and run the vent line down near the 
bottom of the sump (which is mounted behind my back seat) so that if fuel 
ever escapes from the vent line it would drain out the hole in the fuselage 
for the sump valve.

Doug Holub
Standard FG
----- Original Message ----- 
From: "Scott Derrick" <scott at tnstaafl.net>
To: "Velocity Aircraft Owners and Builders list" <reflector at tvbf.org>
Sent: Sunday, March 13, 2011 2:42 PM
Subject: REFLECTOR: Vent Check Valve No-Yes


> I'm not too sure about the vent check valve that has been added to the 
> kit.
>
> The general description of the vent system is to plumb all vents, L&R 
> tanks and sump into a block high on the cabin side of the fire wall.  A 
> feed line is plumbed into the same block that travels down to the bottom 
> of the fuselage and is  positioned or set up to apply positive pressure to 
> the vent system in flight.  Mime is plumbed to a little NACA scoop on the 
> bottom of the fuselage.
>
> At some time after 2000,  a one way valve was added to the feed line to 
> only allow air into the system. Why?  I'm assuming it was to prevent fuel 
> from leaking out of the vent system in case of an accident or something? 
> Or was there a different reason?
>
> I bought a valve recently and have yet to add it as I've flown the plane 
> for 600 hours with nary a problem with the fuel system.
>
> The problem with adding this valve is it prevents pressure from escaping 
> the system.  Fuel tanks are vented to allow air in and to allow vapor out 
> when the fuel expands from heat. We've all filled up an airplane, parked 
> it on the ramp, only to discover it puked a puddle of fuel onto the ramp 
> as the cool fuel heated up in the sun.
>
> Now that valve won't let that happen, and if your system is tight you will 
> be pressuring it as the fuel heats up. Is that a problem?  I don't know.. 
> A couple of months ago I calibrated my fuel tank sensors, adding fuel into 
> each tank in 5 gallon increments until full. I had planed on flying 
> immediately after words but something came up.  I pushed the plane back 
> into the hanger but the sun was shining in on the plane. I was futzing 
> with something on my workbench when I heard dripping.  I looked arouns and 
> saw a stream of fuel dripping from the bottom of the fuselage!   Just like 
> I've seen from Bonanzas, Cessnas, etc..  I got all excited and looked into 
> the cabin thinking I would find a cabin full of fuel, nothing.  I looked 
> under the fuselage and could then see it was coming out of the vent  It 
> then stopped.  I popped a fuel cap and left it un compressed so anymore 
> pressure wouldn't build up while the tanks were completely full!  Now if I 
> had that vent in there, the pressure would have built up, and up?
>
> How high, I don't know.
>
>
> Scott
>
> -- 
>
> Stand porter at the door of thought.
> Mary Baker Eddy
>
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