----- Original Message -----
Sent: Saturday, February 12, 2005 5:18
AM
Subject: [FlyRotary] Re: Vapor lock in
sump/header tank.
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)
No virus found in this incoming message.
Checked by AVG
Anti-Virus.
Version: 7.0.300 / Virus Database: 265.8.7 - Release Date:
2/10/2005