REFLECTOR: fuel feed - my solutions

[Jim Sower]

<... 1 atmosphere versus vacuum will open nearly any check valve ...>
*Of course* a vacuum will open a check valve.  Nobody's arguing that.  
The (false) implication from the factory, etc. is that the head pressure 
of the fuel gravity feeding the fuel pump would open it, which it 
clearly would *not*.  The fuel pump would clearly *suck* it open, but 
that would put an underpressure at the pump inlet. 

I guess it was on another list that we went into so much detail around 
vapor lock which is not a real issue here (unless somebody is interested 
in a recirculating MPFI engine (auto conversion)).  However, if you have 
a vented sump tank, pretty much any check valve alive would prevent 
gravity flow to the sump, and you would be left with < 5 gal usable 
fuel.  Actually, you'd probably be under 3 gal and clueless as to the 
actual amount you had by the time you figured out that the low-sump 
indication was for real).

I think it's a recipe for disaster and you won't see it coming ... Jim S.

Brett Ferrell wrote:

> As soon as the pump pulls some fuel out of the sump with a blocked 
> vent line, the check valve will open and vent some air into the 
> tanks.  1 atmosphere versus vacuum will open nearly any check valve.  
> I've done some process safety work and seen industrial tanks collapsed 
> from under-pressure, and I think it would be foolish not to install 
> the factory-recommended check valve and allow your tanks to routinely 
> run under vacuum.
>
> Brett
>
> ----- Original Message ----- From: "Jim Sower" <canarder at frontiernet.net>
> To: "Velocity Aircraft Owners and Builders list" <reflector at tvbf.org>
> Sent: Sunday, February 27, 2005 3:53 PM
> Subject: Re: REFLECTOR: fuel feed - my solutions
>
>
> My figures indicate that .5 psi is the equivalent of about 165 kias. Bad
> news: they aren't going to vent a damned thing if the vent ices over.
> Good news: the fuel pump will generate plenty enough suction to overcome
> that (of course that underpressure would add to your chances of vapor
> forming in the vent line and/or cavitation at the pump face).
> Check valves are not the panacea some folks would have you believe ...
> Jim S.
>
> Wayne wrote:
>
>> I'm using two of the .5 jobs for an alternate air source in the top 
>> of the loop before the vent line goes to the bottom of the fuselage. 
>> If the fuel vent iced over I hope they will provide cabin air to the 
>> fuel tanks. If the bird is upside down they shouldn't allow fuel to 
>> leak inside the cabin.
>> Wayne
>>
>