REFLECTOR:Fuel and vent system
Chuck Jensen
reflector@tvbf.org
Sun, 27 Jul 2003 11:37:23 -0400
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You da man, Al, you da man. Real numbers instead of anecdotal information.
Good stuff.
Particularly interesting was your observation of the air bubble in the fuel
line that wouldn't go up and the down flowing fuel had a difficult time
pushing it back into the sump. And, this was under what I assume was ideal
conditions, in that the fuel line ran pretty much straight up from the sump
to strake; no humps and dips. Any hump/dip is virtually assured of causing
an air lock (anecdotal speculation), blocking/slowing fuel flow.
To those who are satisfied with to having to wiggle your wings occasionally,
ignore low-level warning lights, only do limited descents (then having to
reload your sump), flying around with who-knows-how-many gallons of unusable
fuel, having to remember to manually burp and vent the sump and so on ad
nauseum, I say NOT!
We all have quirks of one sort or another in our planes, but that doesn't
mean we should accept them, as they are neither dear or beloved. In some
cases, they are innocuous, in others, dangerous and potentially deadly.
They need to be fixed, not tolerated. The fuel flow discussion is headed in
that direction even though we don't seem to be getting there very fast.
Scott B., your example of the siphoning fuel flow through the sump (header)
vent was illustrative, but easily avoidable. By interconnecting the vent
lines from both tanks and the sump to a common manifold, then venting that
manifold, you can not establish a siphoning effect because all of three
system components are under the exact same pressure, beit negative or
positive.
Even if the single vent line pointed backward, the vacuum created would have
to overcome the 1' of head from the tank to the sump to impede flow. If it
pointed forward, it would slightly pressurize all three tanks, thus having
no effect. If one of the caps is leaking air, no big deal because the air
pressure is the same in both mains since they are interconnected. By making
the atmosphere common to all three tanks, this issue of negative/positive
pressure goes away and we rely on gravity to do it's job, which it does with
admirable consistency.
Chuck
-----Original Message-----
From: reflector-admin@tvbf.org [mailto:reflector-admin@tvbf.org]On Behalf Of
alventures
Sent: Sunday, July 27, 2003 12:29 AM
To: reflector@tvbf.org
Subject: REFLECTOR:Fuel and vent sytem
This has been an interesting discussion, and a lot of it focused on the vent
system. Someone questioned the validity of the fuel system design. I think
there is an element of the design which is an inherent weakness, and which
is at the root of these observed problems.
Consider that the bottom of our fuel tank is about at the bottom of the
spar. The top of the sump tank is just below the spar. Our friendly float
valve just a couple inches below that. Now note that a 1" head of fuel is
only about 0.03 psi. Yeah, 3 one hundreds of a psi. When our tanks are
full we have about 0.3 psi of pressure driving the fuel to the sump when the
sump is full. When the strake is nearly empty, that pressure is about 0.05
psi. This increases just slightly as the sump drains down to expose the
inlet from the strake, but that's as high as it's going to get no matter how
far the sump drains - the head that drives the fuel can only go as far the
exit at the sump.
So how much of a disturbance does it take to cause a problem. Damn little -
like a little cap leak, or partially plugged vent, or whatever. And the
float valve is right there at the top of the sump, so if the sump drains
down a little, no matter how temporary, the light goes on. Does this
blinking light issue come up more frequently on Velocitys with IO540 engines
that are using fuel at a higher rate?
So one obvious physics reason for not having a vent to the sump tank is that
without the sump vent we don't have to rely on that very small gravity head
for fuel feed. The fuel pump suction head is there to assist if needed.
(of course if the fuel is very warm and the vapor pressure gets high there
won't be much suction ability either, but that is not likely an issue).
Now don't let these low pressure head numbers scare you too bad. I did some
flow tests a couple of years ago with a 3/8" vinyl tube where I could watch
what happened, and it convinced me of a couple of things. One is that,
because of the low viscosity, fuel flow, even with only a few inches of
head, was "pretty fast". I didn't measure what that meant in gph, but I'm
going to guess that two tubes can run over 10 gph just fine. And experience
bears that out. Surely someone has measured the gravity flow from the
tanks, right? What is it?
The second thing was that air bubbles, for the most part, didn't seem to be
an issue. With very low flow they would migrate up stream, but it didn't
take much flow for them to easily get carried with the flow and into the
container. These bubbles varied from very small to maybe an inch or so long
in the tube.
I say 'for the most part' because there was one thing that caught my
attention. With the bottom end of the tube submerged in fuel, and air in
the tube, adding fuel to the tube which trapped a big (long) bubble, the
fuel was very reluctant to flow when that bubble came to the end of the
tube. It took a bit of head to get that air to bubble into fuel.
Apparently has to do with the surface tension at the air-fuel interface. I
don't know if this is an issue or not, but the entrance to the sump is an
end of a tube (though a different geometry) and it is likely to be
submerged. And one can envision that with low fuel level in the strake, and
we do a little nose down for awhile, the line could fill with air. It may
take a little while for the air to resolve where it's going; upstream or
into the sump tank. And of course, with no vent in the sump it will hang
around at the top of the tank until the next preflight when we open the
purge valve.
I can believe that air trapped in a sump without a vent would find its way
back to the strake; but, don't forget, that won't happen until the air
builds in the top of the sump down to the level of the tube from the stake
tank - at which point the low fuel light will probably be on or blinking.
So the inherent weakness in the system design is relying on gravity feed in
an airplane where the design doesn't give us much of a gravity head
(pressure). Gravity feed is also a plus - simplicity. We can always do as
Jean suggested, and all the low-wing planes do; pump the fuel from the
strakes. Or we can be meticulous about sealing the fuel caps, and maybe
valving off the sump vent.
One further note for those of us with fuel injection engines where fuel is
bypassed back from the pressure regulator to the sump; we may need to vent
the sump to avoid the buildup of bubbles of vapor fractions that form as a
result of the sudden expansion coming out of the pressure regulator. Maybe
they will re-condense; or maybe we are precluded from valving off the sump
vent.
Sorry to ramble on. Someone please respond so I know if this message got
posted, because some e-mail bug still prevents me from getting a copy of
what I post.
Al
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<DIV><SPAN class=779353914-27072003><FONT face=Arial color=#0000ff size=2>You da
man, Al, you da man. Real numbers instead of anecdotal information.
Good stuff.</FONT></SPAN></DIV>
<DIV><SPAN class=779353914-27072003><FONT face=Arial color=#0000ff
size=2></FONT></SPAN> </DIV>
<DIV><SPAN class=779353914-27072003><FONT face=Arial color=#0000ff
size=2>Particularly interesting was your observation of the air bubble in the
fuel line that wouldn't go up and the down flowing fuel had a difficult time
pushing it back into the sump. And, this was under what I assume was ideal
conditions, in that the fuel line ran pretty much straight up from the sump to
strake; no humps and dips. Any hump/dip is virtually assured of causing an
air lock (anecdotal speculation), blocking/slowing fuel
flow.</FONT></SPAN></DIV>
<DIV><SPAN class=779353914-27072003><FONT face=Arial color=#0000ff
size=2></FONT></SPAN> </DIV>
<DIV><SPAN class=779353914-27072003><FONT face=Arial color=#0000ff size=2>To
those who are satisfied with to having to wiggle your wings occasionally,
ignore low-level warning lights, only do limited descents (then having to
reload your sump), flying around with who-knows-how-many gallons of unusable
fuel, having to remember to manually burp and vent the sump and so on ad
nauseum, I say NOT!</FONT></SPAN></DIV>
<DIV><SPAN class=779353914-27072003><FONT face=Arial color=#0000ff
size=2></FONT></SPAN> </DIV>
<DIV><SPAN class=779353914-27072003><FONT face=Arial color=#0000ff size=2>We all
have quirks of one sort or another in our planes, but that doesn't mean we
should accept them, as they are neither dear or beloved. In some cases,
they are innocuous, in others, dangerous and potentially deadly. They
need to be fixed, not tolerated. The fuel flow discussion is headed in
that direction even though we don't seem to be getting there very
fast.</FONT></SPAN></DIV>
<DIV><SPAN class=779353914-27072003><FONT face=Arial color=#0000ff
size=2></FONT></SPAN> </DIV>
<DIV><SPAN class=779353914-27072003><FONT face=Arial color=#0000ff size=2>Scott
B., your example of the siphoning fuel flow through the sump (header) vent was
illustrative, but easily avoidable. By interconnecting the vent lines from
both tanks and the sump to a common manifold, then venting that manifold, you
can not establish a siphoning effect because all of three system components are
under the exact same pressure, beit negative or positive.
</FONT></SPAN></DIV>
<DIV><SPAN class=779353914-27072003><FONT face=Arial color=#0000ff
size=2></FONT></SPAN> </DIV>
<DIV><SPAN class=779353914-27072003><FONT face=Arial color=#0000ff size=2>Even
if the single vent line pointed backward, the vacuum created would have to
overcome the 1' of head from the tank to the sump to impede flow. If it
pointed forward, it would slightly pressurize all three tanks, thus having no
effect. If one of the caps is leaking air, no big deal because the air
pressure is the same in both mains since they are interconnected. By
making the atmosphere common to all three tanks, this issue of negative/positive
pressure goes away and we rely on gravity to do it's job, which it does with
admirable consistency.</FONT></SPAN></DIV>
<DIV><SPAN class=779353914-27072003></SPAN> </DIV>
<DIV><SPAN class=779353914-27072003><FONT face=Arial color=#0000ff
size=2>Chuck</FONT></SPAN></DIV>
<BLOCKQUOTE dir=ltr style="MARGIN-RIGHT: 0px">
<DIV class=OutlookMessageHeader dir=ltr align=left><FONT face=Tahoma
size=2>-----Original Message-----<BR><B>From:</B> reflector-admin@tvbf.org
[mailto:reflector-admin@tvbf.org]<B>On Behalf Of
</B>alventures<BR><B>Sent:</B> Sunday, July 27, 2003 12:29 AM<BR><B>To:</B>
reflector@tvbf.org<BR><B>Subject:</B> REFLECTOR:Fuel and vent
sytem<BR><BR></FONT></DIV>
<DIV class=Section1>
<P class=MsoPlainText><FONT face=Verdana size=2><SPAN
style="FONT-SIZE: 11pt; FONT-FAMILY: Verdana">This has been an interesting
discussion, and a lot of it focused on the vent system. Someone
questioned the validity of the fuel system design. I think there is an
element of the design which is an inherent weakness, and which is at the root
of these observed problems. </SPAN></FONT></P>
<P class=MsoPlainText><FONT face=Verdana size=2><SPAN
style="FONT-SIZE: 11pt; FONT-FAMILY: Verdana"></SPAN></FONT> </P>
<P class=MsoPlainText><FONT face=Verdana size=2><SPAN
style="FONT-SIZE: 11pt; FONT-FAMILY: Verdana">Consider that the bottom of our
fuel tank is about at the bottom of the spar. The top of the sump tank
is just below the spar. Our friendly float valve just a couple inches
below that. Now note that a 1” head of fuel is only about 0.03
psi. Yeah, 3 one hundreds of a psi. When our tanks are full we
have about 0.3 psi of pressure driving the fuel to the sump when the sump is
full. When the strake is nearly empty, that pressure is about 0.05 psi.
This increases just slightly as the sump drains down to expose the inlet
from the strake, but that’s as high as it’s going to get no matter how far the
sump drains – the head that drives the fuel can only go as far the exit at the
sump.</SPAN></FONT></P>
<P class=MsoPlainText><FONT face=Verdana size=2><SPAN
style="FONT-SIZE: 11pt; FONT-FAMILY: Verdana"></SPAN></FONT> </P>
<P class=MsoPlainText><FONT face=Verdana size=2><SPAN
style="FONT-SIZE: 11pt; FONT-FAMILY: Verdana">So how much of a disturbance
does it take to cause a problem. Damn little – like a little cap leak,
or partially plugged vent, or whatever. And the float valve is right
there at the top of the sump, so if the sump drains down a little, no matter
how temporary, the light goes on. Does this blinking light issue come up
more frequently on Velocitys with IO540 engines that are using fuel at a
higher rate?</SPAN></FONT></P>
<P class=MsoPlainText><FONT face=Verdana size=2><SPAN
style="FONT-SIZE: 11pt; FONT-FAMILY: Verdana"></SPAN></FONT> </P>
<P class=MsoPlainText><B><FONT face=Verdana size=2><SPAN
style="FONT-WEIGHT: bold; FONT-SIZE: 11pt; FONT-FAMILY: Verdana">So one
obvious physics reason for not having a vent to the sump tank is that without
the sump vent we don’t have to rely on that very small gravity head for fuel
feed. The fuel pump suction head is there to assist if
needed</SPAN></FONT></B><FONT face=Verdana><SPAN
style="FONT-FAMILY: Verdana">. (of course if the fuel is very warm and
the vapor pressure gets high there won’t be much suction ability either, but
that is not likely an issue).</SPAN></FONT></P>
<P class=MsoPlainText><FONT face=Verdana size=2><SPAN
style="FONT-SIZE: 11pt; FONT-FAMILY: Verdana"></SPAN></FONT> </P>
<P class=MsoPlainText><FONT face=Verdana size=2><SPAN
style="FONT-SIZE: 11pt; FONT-FAMILY: Verdana">Now don’t let these low pressure
head numbers scare you too bad. I did<FONT color=black><SPAN
style="COLOR: black"> some flow tests a couple of years ago with a 3/8” vinyl
tube where I could watch what happened, and it convinced me of a couple of
things. One is that, because of the low viscosity, fuel flow, even with
only a few inches of head, was “pretty fast”. I didn’t measure what that meant
in gph, but I’m going to guess that two tubes can run over 10 gph just
fine. And experience bears that out. Surely someone has measured
the gravity flow from the tanks, right? What is
it?</SPAN></FONT></SPAN></FONT></P>
<P class=MsoPlainText><FONT face=Verdana color=black size=2><SPAN
style="FONT-SIZE: 11pt; COLOR: black; FONT-FAMILY: Verdana"></SPAN></FONT> </P>
<P class=MsoPlainText><FONT face=Verdana color=black size=2><SPAN
style="FONT-SIZE: 11pt; COLOR: black; FONT-FAMILY: Verdana">The second thing
was that air bubbles, <B><SPAN style="FONT-WEIGHT: bold">for the most
part</SPAN></B>, didn’t seem to be an issue. With very low flow they
would migrate up stream, but it didn’t take much flow for them to easily get
carried with the flow and into the container. These bubbles varied from
very small to maybe an inch or so long in the tube. </SPAN></FONT></P>
<P class=MsoPlainText><FONT face=Verdana color=black size=2><SPAN
style="FONT-SIZE: 11pt; COLOR: black; FONT-FAMILY: Verdana"></SPAN></FONT> </P>
<P class=MsoPlainText><FONT face=Verdana color=black size=2><SPAN
style="FONT-SIZE: 11pt; COLOR: black; FONT-FAMILY: Verdana">I say ‘for the
most part’ because there was one thing that caught my attention. With
the bottom end of the tube submerged in fuel, and air in the tube, adding fuel
to the tube which trapped a big (long) bubble, the fuel was very reluctant to
flow when that bubble came to the end of the tube. It took a bit of head
to get that air to bubble into fuel. Apparently has to do with the
surface tension at the air-fuel interface. I don’t know if this is an
issue or not, but the entrance to the sump is an end of a tube (though a
different geometry) and it is likely to be submerged. And one can
envision that with low fuel level in the strake, and we do a little nose down
for awhile, the line could fill with air. It may take a little while for
the air to resolve where it’s going; upstream or into the sump tank. And
of course, with no vent in the sump it will hang around at the top of the tank
until the next preflight when we open the purge valve.</SPAN></FONT></P>
<P class=MsoPlainText><FONT face=Verdana color=black size=2><SPAN
style="FONT-SIZE: 11pt; COLOR: black; FONT-FAMILY: Verdana"></SPAN></FONT> </P>
<P class=MsoPlainText><FONT face=Verdana color=black size=2><SPAN
style="FONT-SIZE: 11pt; COLOR: black; FONT-FAMILY: Verdana">I can believe that
air trapped in a sump without a vent would find its way back to the strake;
but, don’t forget, that won’t happen until the air builds in the top of the
sump down to the level of the tube from the stake tank – at which point the
low fuel light will probably be on or blinking. </SPAN></FONT></P>
<P class=MsoPlainText><FONT face=Verdana color=black size=2><SPAN
style="FONT-SIZE: 11pt; COLOR: black; FONT-FAMILY: Verdana"></SPAN></FONT> </P>
<P class=MsoPlainText><FONT face=Verdana color=black size=2><SPAN
style="FONT-SIZE: 11pt; COLOR: black; FONT-FAMILY: Verdana">So the inherent
weakness in the system design is relying on gravity feed in an airplane where
the design doesn’t give us much of a gravity head (pressure). Gravity
feed is also a plus – simplicity. We can always do as Jean suggested,
and all the low-wing planes do; pump the fuel from the strakes. Or we
can be meticulous about sealing the fuel caps, and maybe valving off the sump
vent.</SPAN></FONT></P>
<P class=MsoPlainText><FONT face=Verdana color=black size=2><SPAN
style="FONT-SIZE: 11pt; COLOR: black; FONT-FAMILY: Verdana"></SPAN></FONT> </P>
<P class=MsoPlainText><FONT face=Verdana color=black size=2><SPAN
style="FONT-SIZE: 11pt; COLOR: black; FONT-FAMILY: Verdana">One further note
for those of us with fuel injection engines where fuel is bypassed back from
the pressure regulator to the sump; we may need to vent the sump to avoid the
buildup of bubbles of vapor fractions that form as a result of the sudden
expansion coming out of the pressure regulator. Maybe they will
re-condense; or maybe we are precluded from valving off the sump
vent.</SPAN></FONT></P>
<P class=MsoPlainText><FONT face=Verdana color=black size=2><SPAN
style="FONT-SIZE: 11pt; COLOR: black; FONT-FAMILY: Verdana"></SPAN></FONT> </P>
<P class=MsoPlainText><FONT face=Verdana color=black size=2><SPAN
style="FONT-SIZE: 11pt; COLOR: black; FONT-FAMILY: Verdana">Sorry to ramble
on. Someone please respond so I know if this message got posted, because
some e-mail bug still prevents me from getting a copy of what I
post.</SPAN></FONT></P>
<P class=MsoPlainText><FONT face=Verdana color=black size=2><SPAN
style="FONT-SIZE: 11pt; COLOR: black; FONT-FAMILY: Verdana"></SPAN></FONT> </P>
<P class=MsoPlainText><FONT face=Verdana color=black size=2><SPAN
style="FONT-SIZE: 11pt; COLOR: black; FONT-FAMILY: Verdana">Al</SPAN></FONT></P>
<P class=MsoPlainText><FONT face=Nimrod size=2><SPAN
style="FONT-SIZE: 11pt"></SPAN></FONT> </P></DIV></BLOCKQUOTE></BODY></HTML>
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