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In a message dated 09/23/02 8:48:30 PM Pacific Daylight Time, glcasey@adelphia.net writes:
I thought the whole deal with the check valves was to add a second vent in
parallel with the first one.
The original post in this thread was by Mark Quinn (psteelepbq@aol.com) on 9/17/02 in which he asked a question about check valves in the wing tip but didn't state what they were for. On 9/19/02 Mark clarified that he was talking about a check valve in series with the vent to prevent overboarding fuel when parked on an uneven ramp (I'm paraphrasing).
Clearly, Andair intended that their fuel vent check valve be put in series. Otherwise, why would they drill two holes in it and discuss fuel expansion on their webpage?
As far as putting a check valve in parallel, this was discussed by at least two other submitters (Chris Zavatson on 9/19/02 and Dan Schaefer on 9/22/02) and I don't think there is any argument that this is an improvement. However, I'm not aware that an alternate fuel vent source is either required or installed on certificated planes but I'm no expert (or even knowledgeable) in that subject. I'm quite sure my Mooney didn't have them.
During this thread someone stated that Lancair was furnishing the Andair valves with the Legacy. I would be interested to know how Lancair intended that they be installed, in parallel as a backup or in series to prevent(?) overflow. The more I think about the latter, the less likely it seems to me that they will work. IMHO, parking a plane with an unobstructed fuel vent on a side slope (assume slope is sufficient for fuel to cover the vent opening) will probably not result in much fuel coming out unless there is heating of the tank and its contents. This is because the air space in the tank is no longer vented to atmosphere. It is the same effect you get when you dip a straw into a soda. Cover the top end of the straw with your finger and pull the straw out and it will still be full of liquid. Only when the tank gets heated by the sun should fuel come out. In other words, it takes two conditions, parking on a slope and heating of the tank.
If we assume that the tank gets heated from 50F to 150F (may be extreme but 100 delta F is easy for my tired brain to deal with), the fuel expands by about 7% (from some generic hydrocarbon data), the air in the tank expands by 20% (ideal gas law) and the vapor pressure of the fuel increases by some unknown (to me) amount. All of this tends to increase the pressure inside the tank and force fuel out the vent and it will continue to do so until the air pressure inside the tank becomes equal to local atmospheric pressure. It seems to me that if the vent has a check valve with a couple of holes drilled in it, it won't make any difference. The same volume of fuel will be spilled, it might just take a little longer.
Sorry to prattle on about such minutiae but I wonder if anyone has ever thought this through before.
Jim McKibbin
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