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Hi
Paul;
Caution this is a long story relating my previous vapour
lock issues and how they may relate to yours. Recent comments have differed
about the need to vent or not to vent a sump/header tank. As I understand it you
have a sump tank in which it is gravity fed from your mains, then pumped up to
your engine. However IIRC a previous post in which you said you had copied Ed's
system which is a header tank that has boost pumps feeding it. (I may have that
incorrect as I couldn't find the post in question) Now I went through a couple
different versions of a header tank before scrapping the idea altogether. My
first one was always vented, but this required level control, via the boost
pumps as it required fuel to be pumped up to the tank, then it gravity fed my FI
pumps. If not filled by pumps, it would just drain the tank due to the vent. If
pumps were left on it would over fill and fuel would go out the vent lines which
were tied via a common header to the main tanks so negative draft would draw
most of the fuel back to the feed tank, but not all. I'd planned to use
an electronic level control system to help manage this but supplier problems
caused me to look closely at what was really an unnecessary failure mode.
At this time I decided to use a non-vented system much
like Ed's, however I was certain that I could improve upon it by increasing the
size of the header tank and having it located on the cool side of the firewall.
But I found that it required a vent as there was no other way to purge out the
air from the tank which would accumulate there when one would run a tank dry
before switching tanks (I have 6 tanks so it is important to completely empty
the aux tanks). So I installed a vent with a pilot accessible valve. This
allowed the venting of all air out of the tank at which time it would be closed.
Then the fuel would draw up from the main tanks to replace the fuel that had
been drawn out and not returned through the return line from the fuel reg,
without the need for pumps. I had a clear sight tube to see the level of the
header tank and another short clear section of hose on the vent line, so that I
could see whether I was venting air or fuel.
This seemed like a fine system, until it began seeing the
heat of flight operations. Ground runs seemed mostly successful and running a
tank dry until the header tank was empty resulted in approximately a 20 second
delay in getting fresh fuel to restart the engine and completely purging the air
within 2-3 minutes. But under flight conditions it was much different. Heat
would cause vapour problems that were very difficult to deal with. The pressure
would build within the header tank and the sight tube would not always give a
true reading as it should. Twice I ran a tank dry while at +10000' above the
airport and had a very difficult time in getting fuel back resulting in approx
6000' of glider alt loss each time. Another time I was doing circuits and was on
my downwind leg when the pressure in the tank caused a vapour lock even though I
had plenty of fuel in the tank(s) feeding the header tank. The pressure had just
built up to the point where even the 7 psi boost/transfer pumps couldn't
overcome it to keep fuel in the tank. In this case I was able to make an
emergency deadstick landing on an intersecting runway and fortunately I even
kept my speed up enough to coast all the way to my tie-down spot. That's where
the firetrucks met me... kinda embarrassing!
In each of these cases, I found that when opening the vent
valve to release the pressure, I could see through the clear vent tube section
that I was venting a boiling fuel (air/fuel) mixture. And it took an
uncomfortably long period of time to relieve that pressure and refill the tank
with fresh fuel. I then installed optical fuel level sensors (hi/low) on the
header tank, a vacuum/pressure gauge and a temp sensor. I found that it would
begin by having a vacuum in the tank as the system would draw in new fuel to
replace that which was being consumed, but it would soon begin to build as a
pressure as the fuel heated and would then begin to push the level down. Opening
the vent relived this pressure but it couldn't be left open, or it would just
empty the header tank. Keeping the pump(s) running would over flow the tank.
Can you see where all this was going? I surmise that the
greater capacity of the header tank (as compared to Ed's) allowed some heat
absorption as it took longer to become affected, but once heated it was far less
manageable. My pilot workload was too great and became centred around fuel
management. I had so much time, effort and $$$ invested in making that header
tank work that I didn't want to let go of the idea, but one day I just had
enough and tore it out of there. I've now converted to a returnless system with
no header tank and I couldn't be happier.
What I'm trying to show is that there is more going on with
heat in the fuel system than expected. I'm not familiar with your system, but
the way I understand it, you have your main tanks draining by gravity into your
sump tank. Your FI return line feeds into this tank bringing heat from the
engine with it. I surmise that it is bringing enough heat to begin a vapour
build-up in your sump tank overcoming the gravity feed. How long after your
emergency landing did you restart your engine? If it was immediately and it ran
fine, then this theory may not be valid, however if there was sometime elapse
(while you kissed the ground & changed your shorts :-), then it could be
that the tank had time enough to cool and/or relive the pressure allowing more
fuel to enter the tank.
One way that I could see overcoming this would
be.....
A vent
line from the top of your sump tank up to the top of both of your main
tanks, but not tied into the main tank vent system. This would allow any fuel
vapours to immediately flow out of the sump tank, eliminating any chance of
vapour build-up, while allowing the cool fuel to continue to be gravity fed to
the sump tank. These fuel vapours would then flow into the main tanks where they
should immediately condense, preventing the loss of any fuel through a direct
atmosphere vent system. This would (may) only work if you have your main
tanks located above the sump tank. Having your FI pumps located at or below the
sump pump would surely be a help as well. But I would not expect the tank to
work without a vent or with a vent to atmosphere.
I hope some of this is relevant to you and
helps.
Todd
Bartrim (top posted all the way to the
bottom)
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