REFLECTOR:Fuel and vent sytem

Sat, 26 Jul 2003 21:29:28 -0700

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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 class=3DSection1>

This has been an interesting discussion, and a lot =
of it
focused on the vent system.&nbsp; Someone questioned the validity of the =
fuel
system design.&nbsp; I think there is an element of the design which is =
an
inherent weakness, and which is at the root of these observed problems. =

<p class=3DMsoPlainText><font size=3D2 face=3DVerdana><span =
style=3D'font-size:11.0pt;
font-family:Verdana'>&nbsp;</span></font></p>

Consider that the bottom of our fuel tank is about =
at the
bottom of the spar. &nbsp;The top of the sump tank is just below the =
spar. &nbsp;Our
friendly float valve just a couple inches below that.&nbsp; Now note =
that a 1&#8221;
head of fuel is only about 0.03 psi.&nbsp; Yeah, 3 one hundreds of a =
psi.&nbsp;
When our tanks are full we have about 0.3 psi of pressure driving the =
fuel to
the sump when the sump is full.&nbsp; When the strake is nearly empty, =
that
pressure is about 0.05 psi. &nbsp;This increases just slightly as the =
sump
drains down to expose the inlet from the strake, but that&#8217;s as =
high as it&#8217;s
going to get no matter how far the sump drains &#8211; the head that =
drives the
fuel can only go as far the exit at the sump.

<p class=3DMsoPlainText><font size=3D2 face=3DVerdana><span =
style=3D'font-size:11.0pt;
font-family:Verdana'>&nbsp;</span></font></p>

<p class=3DMsoPlainText><font size=3D2 face=3DVerdana><span =
style=3D'font-size:11.0pt;
font-family:Verdana'>So how much of a disturbance does it take to cause =
a
problem.&nbsp; Damn little &#8211; like a little cap leak, or partially =
plugged
vent, or whatever.&nbsp; 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.&nbsp; 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=3DMsoPlainText><font size=3D2 face=3DVerdana><span =
style=3D'font-size:11.0pt;
font-family:Verdana'>&nbsp;</span></font></p>

So one obvious physics reason for =
not
having a vent to the sump tank is that without the sump vent we =
don&#8217;t
have to rely on that very small gravity head for fuel feed.&nbsp; The =
fuel pump
suction head is there to assist if needed. &nbsp;(of course if the fuel is very =
warm and the
vapor pressure gets high there won&#8217;t be much suction ability =
either, but
that is not likely an issue).

<p class=3DMsoPlainText><font size=3D2 face=3DVerdana><span =
style=3D'font-size:11.0pt;
font-family:Verdana'>&nbsp;</span></font></p>

Now don&#8217;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&#8221; vinyl tube where I could watch =
what
happened, and it convinced me of a couple of things.&nbsp; One is that, =
because
of the low viscosity, fuel flow, even with only a few inches of head, =
was &#8220;pretty
fast&#8221;. I didn&#8217;t measure what that meant in gph, but =
I&#8217;m going
to guess that two tubes can run over 10 gph just fine.&nbsp; And =
experience
bears that out.&nbsp; Surely someone has measured the gravity flow from =
the
tanks, right?&nbsp; What is it?

<p class=3DMsoPlainText><font size=3D2 color=3Dblack =
face=3DVerdana><span
style=3D'font-size:11.0pt;font-family:Verdana;color:black'>&nbsp;</span><=
/font></p>

<p class=3DMsoPlainText><font size=3D2 color=3Dblack =
face=3DVerdana><span
style=3D'font-size:11.0pt;font-family:Verdana;color:black'>The second =
thing was that
air bubbles, <b><span style=3D'font-weight:bold'>for the most =
part</span></b>,
didn&#8217;t seem to be an issue.&nbsp; With very low flow they would =
migrate
up stream, but it didn&#8217;t take much flow for them to easily get =
carried
with the flow and into the container.&nbsp; These bubbles varied from =
very
small to maybe an inch or so long in the tube.&nbsp; </span></font></p>

<p class=3DMsoPlainText><font size=3D2 color=3Dblack =
face=3DVerdana><span
style=3D'font-size:11.0pt;font-family:Verdana;color:black'>&nbsp;</span><=
/font></p>

I say =
&#8216;for the
most part&#8217; because there was one thing that caught my =
attention.&nbsp; 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. &nbsp;It took a bit =
of head
to get that air to bubble into fuel.&nbsp; Apparently has to do with the
surface tension at the air-fuel interface. &nbsp;I don&#8217;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.&nbsp; 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. &nbsp;It may take a little while =
for the
air to resolve where it&#8217;s going; upstream or into the sump =
tank.&nbsp; 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.

<p class=3DMsoPlainText><font size=3D2 color=3Dblack =
face=3DVerdana><span
style=3D'font-size:11.0pt;font-family:Verdana;color:black'>&nbsp;</span><=
/font></p>

<p class=3DMsoPlainText><font size=3D2 color=3Dblack =
face=3DVerdana><span
style=3D'font-size:11.0pt;font-family:Verdana;color:black'>I can believe =
that air
trapped in a sump without a vent would find its way back to the strake; =
but,
don&#8217;t forget, that won&#8217;t happen until the air builds in the =
top of
the sump down to the level of the tube from the stake tank &#8211; at =
which
point the low fuel light will probably be on or blinking.&nbsp; =
</span></font></p>

<p class=3DMsoPlainText><font size=3D2 color=3Dblack =
face=3DVerdana><span
style=3D'font-size:11.0pt;font-family:Verdana;color:black'>&nbsp;</span><=
/font></p>

So the =
inherent
weakness in the system design is relying on gravity feed in an airplane =
where
the design doesn&#8217;t give us much of a gravity head =
(pressure).&nbsp;
Gravity feed is also a plus &#8211; simplicity.&nbsp; We can always do =
as Jean
suggested, and all the low-wing planes do; pump the fuel from the =
strakes. &nbsp;Or
we can be meticulous about sealing the fuel caps, and maybe valving off =
the
sump vent.

<p class=3DMsoPlainText><font size=3D2 color=3Dblack =
face=3DVerdana><span
style=3D'font-size:11.0pt;font-family:Verdana;color:black'>&nbsp;</span><=
/font></p>

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. &nbsp;Maybe they will =
re-condense;
or maybe we are precluded from valving off the sump =
vent.

<p class=3DMsoPlainText><font size=3D2 color=3Dblack =
face=3DVerdana><span
style=3D'font-size:11.0pt;font-family:Verdana;color:black'>&nbsp;</span><=
/font></p>

<p class=3DMsoPlainText><font size=3D2 color=3Dblack =
face=3DVerdana><span
style=3D'font-size:11.0pt;font-family:Verdana;color:black'>Sorry to =
ramble
on.&nbsp; 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=3DMsoPlainText><font size=3D2 color=3Dblack =
face=3DVerdana><span
style=3D'font-size:11.0pt;font-family:Verdana;color:black'>&nbsp;</span><=
/font></p>

<p class=3DMsoPlainText><font size=3D2 color=3Dblack =
face=3DVerdana><span
style=3D'font-size:11.0pt;font-family:Verdana;color:black'>Al</span></fon=
t></p>

<p class=3DMsoPlainText><font size=3D2 face=3DNimrod><span =
style=3D'font-size:11.0pt'>&nbsp;</span></font></p>

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