REFLECTOR: fuel feed - my solutions

[michalk]

I don't understand this paragraph:

Jim Sower wrote:
...
> My point in this whole discussion is and has been to neutralize what I 
> believe to be a design defect in  the factory vent system.  I propose to 
> accomplish this by interrupting the scenario above at Step-6.  If the 
> sump vent is capped (the sump not allowed to vent) then the pumps will 
> create an overpressure in the now closed fuel storage system.  Instead 

Pumps making overpressure?  I thought you installed a selector valve in 
each tank line.

> of the "vacuum" caused by the pumps being vented by the sump vent, it is 
> transferred to the main tanks.  Now, instead of only head pressure 

I don't get what you are saying here.

> trying to crack your check valve,  you have the added suction capability 
> of the pumps.  Your 0.5 psi check valve will now *tend to* work better.  
> You will still have *at least* a .5 psi underpressure in the tanks.  
> Since the vapor pressure of Avgas is over 7 psi and that of Mogas maybe 
> 10 psi or perhaps more) it will work less and less well as the 
> atmospheric pressure declines with altitude.  All of this will be 
> further degraded by the fact that the fuel pump will not draw *anything 
> remotely like* a perfect (14.7 psi at sea level, 7 psi at something 
> below 20k ft altitude) vacuum.  As you climb, the /vapor pressure/ of 
> your fuel /remains the same/, the /atmospheric pressure decays/ and the 
> /difference/ is /the *only* pressure you have/ to open your check valve.

I would have thought that in a closed system ... unless we are talking 
about the check valve vent, then never mind.  But if the system really 
is closed then atmosphere wouldn't have anything to do with it.  I'm 
also a little confused.  I have a manifold that ties all three tanks 
together.  If there is a relief valve (check or other), then it would 
vent all three equally.

Assuming a perfect 0 PSI restriction valve, which may or may not exist, 
it seems to me that the optimal solution to solve everything is this 
check valve in the sump vent that does not allow the sump suck in 
air(only allows it to burp), and a check valve in each tank line to 
prevent cross-feeding.