REFLECTOR:Fuel and vent sytem

Sun, 27 Jul 2003 00:28:39 -0700

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Al,
Good post, with some good thought behind it.  When I built my sump tank =
(for a fixed-gear XL) I placed the fuel inlets at the extreme top corner =
of the tank, so I don't think I would have any significant volume of =
trapped air even if the vent line were eliminated completely.  Since the =
vent line is already installed, I plan to leave it in the system with a =
valve to shut it off.  Then I'll have the option to use it if my =
theories all turn out to be bogus!  I'll be a little surprised if I have =
to exercise the "burp" valve to release trapped air, though.=20
Keith
=20
  ----- Original Message -----=20
  From: alventures=20
  To: reflector@tvbf.org=20
  Sent: Saturday, July 26, 2003 9:29 PM
  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.=20

  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. =20

  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. =20

  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><FONT face=3DArial size=3D2>Al,</FONT></DIV>
<DIV><FONT face=3DArial size=3D2>Good post, with some good thought =
behind it.&nbsp;=20
When I built my sump tank (for a fixed-gear XL) I placed the fuel inlets =
at the=20
extreme top corner of the tank, so I don't think I would have any =
significant=20
volume of trapped air even if the vent line were eliminated =
completely.&nbsp;=20
Since the vent line is already installed, I plan to leave it&nbsp;in the =
system=20
with a valve to shut it off.&nbsp; Then I'll have the option to use it =
if my=20
theories all turn out to be bogus!&nbsp; I'll be a little surprised if I =
have to=20
exercise the "burp" valve to release trapped air, =
though.&nbsp;</FONT></DIV>
<DIV><FONT face=3DArial size=3D2>Keith</FONT></DIV>
<DIV>&nbsp;</DIV>
<BLOCKQUOTE dir=3Dltr=20
style=3D"PADDING-RIGHT: 0px; PADDING-LEFT: 5px; MARGIN-LEFT: 5px; =
BORDER-LEFT: #000000 2px solid; MARGIN-RIGHT: 0px">
  <DIV style=3D"FONT: 10pt arial">----- Original Message ----- </DIV>
  <DIV=20
  style=3D"BACKGROUND: #e4e4e4; FONT: 10pt arial; font-color: =
black"><B>From:</B>=20
  <A title=3Dalventures@msn.com =
href=3D"mailto:alventures@msn.com">alventures</A>=20
  </DIV>
  <DIV style=3D"FONT: 10pt arial"><B>To:</B> <A =
title=3Dreflector@tvbf.org=20
  href=3D"mailto:reflector@tvbf.org">reflector@tvbf.org</A> </DIV>
  <DIV style=3D"FONT: 10pt arial"><B>Sent:</B> Saturday, July 26, 2003 =
9:29=20
  PM</DIV>
  <DIV style=3D"FONT: 10pt arial"><B>Subject:</B> REFLECTOR:Fuel and =
vent=20
  sytem</DIV>
  <DIV><FONT face=3DArial size=3D2></FONT><FONT face=3DArial =
size=3D2></FONT><BR></DIV>
  <DIV class=3DSection1>
  <P class=3DMsoPlainText><FONT face=3DVerdana size=3D2><SPAN=20
  style=3D"FONT-SIZE: 11pt; FONT-FAMILY: Verdana">This has been an =
interesting=20
  discussion, and a lot of it focused on the vent system.&nbsp; Someone=20
  questioned the validity of the fuel system design.&nbsp; I think there =
is an=20
  element of the design which is an inherent weakness, and which is at =
the root=20
  of these observed problems. </SPAN></FONT></P>
  <P class=3DMsoPlainText><FONT face=3DVerdana size=3D2><SPAN=20
  style=3D"FONT-SIZE: 11pt; FONT-FAMILY: =
Verdana"></SPAN></FONT>&nbsp;</P>
  <P class=3DMsoPlainText><FONT face=3DVerdana size=3D2><SPAN=20
  style=3D"FONT-SIZE: 11pt; FONT-FAMILY: Verdana">Consider that the =
bottom of our=20
  fuel tank is about at the bottom of the spar. &nbsp;The top of the =
sump tank=20
  is just below the spar. &nbsp;Our friendly float valve just a couple =
inches=20
  below that.&nbsp; Now note that a 1=94 head of fuel is only about 0.03 =

psi.&nbsp; Yeah, 3 one hundreds of a psi.&nbsp; When our tanks are =
full we=20
 have about 0.3 psi of pressure driving the fuel to the sump when the =
sump is=20
 full.&nbsp; When the strake is nearly empty, that pressure is about =
0.05 psi.=20
 &nbsp;This increases just slightly as the sump drains down to expose =
the inlet=20
 from the strake, but that=92s as high as it=92s going to get no matter =
how far the=20
 sump drains =96 the head that drives the fuel can only go as far the =
exit at the=20
 sump.
 <SPAN=20
 style=3D"FONT-SIZE: 11pt; FONT-FAMILY: =
Verdana">&nbsp;
 <SPAN=20
 style=3D"FONT-SIZE: 11pt; FONT-FAMILY: Verdana">So how much of a =
disturbance=20
 does it take to cause a problem.&nbsp; Damn little =96 like a little =
cap leak,=20
 or partially plugged vent, or whatever.&nbsp; And the float valve is =
right=20
 there at the top of the sump, so if the sump drains down a little, no =
matter=20
 how temporary, the light goes on.&nbsp; Does this blinking light issue =
come up=20
 more frequently on Velocitys with IO540 engines that are using fuel at =
a=20
 higher rate?
 <SPAN=20
 style=3D"FONT-SIZE: 11pt; FONT-FAMILY: =
Verdana">&nbsp;
 <SPAN=20
 style=3D"FONT-WEIGHT: bold; FONT-SIZE: 11pt; FONT-FAMILY: Verdana">So =
one=20
 obvious physics reason for not having a vent to the sump tank is that =
without=20
 the sump vent we don=92t have to rely on that very small gravity head =
for fuel=20
 feed.&nbsp; The fuel pump suction head is there to assist if=20
 needed<SPAN=20
 style=3D"FONT-FAMILY: Verdana">. &nbsp;(of course if the fuel is very =
warm and=20
 the vapor pressure gets high there won=92t be much suction ability =
either, but=20
 that is not likely an issue).
 <SPAN=20
 style=3D"FONT-SIZE: 11pt; FONT-FAMILY: =
Verdana">&nbsp;
 <SPAN=20
 style=3D"FONT-SIZE: 11pt; FONT-FAMILY: Verdana">Now don=92t let these =
low pressure=20
 head numbers scare you too bad. I did<SPAN=20
 style=3D"COLOR: black"> some flow tests a couple of years ago with a =
3/8=94 vinyl=20
 tube where I could watch what happened, and it convinced me of a =
couple of=20
 things.&nbsp; One is that, because of the low viscosity, fuel flow, =
even with=20
 only a few inches of head, was =93pretty fast=94. I didn=92t measure =
what that meant=20
 in gph, but I=92m going to guess that two tubes can run over 10 gph =
just=20
 fine.&nbsp; And experience bears that out.&nbsp; Surely someone has =
measured=20
 the gravity flow from the tanks, right?&nbsp; What is=20
 it?
 <SPAN=20
 style=3D"FONT-SIZE: 11pt; COLOR: black; FONT-FAMILY: =
Verdana">&nbsp;
 <SPAN=20
 style=3D"FONT-SIZE: 11pt; COLOR: black; FONT-FAMILY: Verdana">The =
second thing=20
 was that air bubbles, for the =
most=20
 part, didn=92t seem to be an issue.&nbsp; With very low =
flow they=20
 would migrate up stream, but it didn=92t take much flow for them to =
easily get=20
 carried with the flow and into the container.&nbsp; These bubbles =
varied from=20
 very small to maybe an inch or so long in the tube.&nbsp; =

 <SPAN=20
 style=3D"FONT-SIZE: 11pt; COLOR: black; FONT-FAMILY: =
Verdana">&nbsp;
 <SPAN=20
 style=3D"FONT-SIZE: 11pt; COLOR: black; FONT-FAMILY: Verdana">I say =
=91for the=20
 most part=92 because there was one thing that caught my =
attention.&nbsp; With=20
 the bottom end of the tube submerged in fuel, and air in the tube, =
adding fuel=20
 to the tube which trapped a big (long) bubble, the fuel was very =
reluctant to=20
 flow when that bubble came to the end of the tube. &nbsp;It took a bit =
of head=20
 to get that air to bubble into fuel.&nbsp; Apparently has to do with =
the=20
 surface tension at the air-fuel interface. &nbsp;I don=92t know if =
this is an=20
 issue or not, but the entrance to the sump is an end of a tube (though =
a=20
 different geometry) and it is likely to be submerged.&nbsp; And one =
can=20
 envision that with low fuel level in the strake, and we do a little =
nose down=20
 for awhile, the line could fill with air. &nbsp;It may take a little =
while for=20
 the air to resolve where it=92s going; upstream or into the sump =
tank.&nbsp; And=20
 of course, with no vent in the sump it will hang around at the top of =
the tank=20
 until the next preflight when we open the purge =
valve.
 <SPAN=20
 style=3D"FONT-SIZE: 11pt; COLOR: black; FONT-FAMILY: =
Verdana">&nbsp;
 <SPAN=20
 style=3D"FONT-SIZE: 11pt; COLOR: black; FONT-FAMILY: Verdana">I can =
believe that=20
 air trapped in a sump without a vent would find its way back to the =
strake;=20
 but, don=92t forget, that won=92t happen until the air builds in the =
top of the=20
 sump down to the level of the tube from the stake tank =96 at which =
point the=20
 low fuel light will probably be on or blinking.&nbsp; =

 <SPAN=20
 style=3D"FONT-SIZE: 11pt; COLOR: black; FONT-FAMILY: =
Verdana">&nbsp;
 <SPAN=20
 style=3D"FONT-SIZE: 11pt; COLOR: black; FONT-FAMILY: Verdana">So the =
inherent=20
 weakness in the system design is relying on gravity feed in an =
airplane where=20
 the design doesn=92t give us much of a gravity head (pressure).&nbsp; =
Gravity=20
 feed is also a plus =96 simplicity.&nbsp; We can always do as Jean =
suggested,=20
 and all the low-wing planes do; pump the fuel from the strakes. =
&nbsp;Or we=20
 can be meticulous about sealing the fuel caps, and maybe valving off =
the sump=20
 vent.
 <SPAN=20
 style=3D"FONT-SIZE: 11pt; COLOR: black; FONT-FAMILY: =
Verdana">&nbsp;
 <SPAN=20
 style=3D"FONT-SIZE: 11pt; COLOR: black; FONT-FAMILY: Verdana">One =
further note=20
 for those of us with fuel injection engines where fuel is bypassed =
back from=20
 the pressure regulator to the sump; we may need to vent the sump to =
avoid the=20
 buildup of bubbles of vapor fractions that form as a result of the =
sudden=20
 expansion coming out of the pressure regulator. &nbsp;Maybe they will=20
 re-condense; or maybe we are precluded from valving off the sump=20
 vent.
 <SPAN=20
 style=3D"FONT-SIZE: 11pt; COLOR: black; FONT-FAMILY: =
Verdana">&nbsp;
 <SPAN=20
 style=3D"FONT-SIZE: 11pt; COLOR: black; FONT-FAMILY: Verdana">Sorry to =
ramble=20
 on.&nbsp; Someone please respond so I know if this message got posted, =
because=20
 some e-mail bug still prevents me from getting a copy of what I=20
 post.
 <SPAN=20
 style=3D"FONT-SIZE: 11pt; COLOR: black; FONT-FAMILY: =
Verdana">&nbsp;
 <SPAN=20
 style=3D"FONT-SIZE: 11pt; COLOR: black; FONT-FAMILY: =
Verdana">Al
 <SPAN=20
 style=3D"FONT-SIZE: =
11pt">&nbsp;</DIV></BLOCKQUOTE></BODY></HTML>

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