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PTFE Fuel line near exhaust

L

lr172

Well Known Member
I would like to route my -4 fuel line from the servo to the spider up between the #1 & #3 cylinders (it currently runs up the back). I have a vertical sump with a vetterman exhaust. I need to run the -4 line between the sump and the exh cross-over pipe right in front of it. I have about 1.5" of clearance. The fuel line is a PTFE type with integral firesleeve. the hose will cross the exh pipe at a 90* angle

I can make a little stand-off to keep the hose about 1/2" away from the pipe. I can also wrap the hose with heat reflective material and/or additional firesleeve. Will this be enough to keep it from melting? Any other ideas to protect it? I can make a heat shield, but the hose will be touching the heat shield, so not sure it helps.

I appreciate any guidance or suggestions you may have.

Larry
 
I have interest in your question also....how close is safe? Where's our fire wall flame thrower tester? Dan.......want to play with gasoline and high pressure?
 
I wouldn't do it. The problem is not so much a matter of melting the hose, but rather, constant vapor bubble problems. The fuel in the hose between the controller and the fuel divider is NOT at pump pressure, but at metered pressure. For example, if your idle fuel flow is 3 GPH, you would have 18 lbs per hour, and the charts say that equals 0.5 PSIG nozzle pressure. So, line pressure at that location would be 0.5 plus the spring pressure in the divider (can't remember at the moment; maybe one lb). Think you might boil some fuel at 1.5 psi?

My own is a lot more distant, but it's still insulated (over the integral fire sleeve) with some fuzzy firesleeve, and the nearest pipe has a big radiant heatshield.

 
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Tom of TSFlightlines has done several like you suggest. I plan to do the same, with his lines and assistance.

Give him a call.
 
DanH said:
I wouldn't do it. The problem is not so much a matter of melting the hose, but rather, constant vapor bubble problems. The fuel in the hose between the controller and the fuel divider is NOT at pump pressure, but at metered pressure. For example, if your idle fuel flow is 3 GPH, you would have 18 lbs per hour, and the charts say that equals 0.5 PSIG nozzle pressure. So, line pressure at that location would be 0.5 plus the spring pressure in the divider (can't remember at the moment; maybe one lb). Think you might boil some fuel at 1.5 psi?

My own is a lot more distant, but it's still insulated (over the integral fire sleeve) with some fuzzy firesleeve, and the nearest pipe has a big radiant heatshield.

Click to expand...

Thanks Dan. I was considering this to reduce heat in that line (still have moderate idle issues). This routing would reduce my hose length by 8-10" and would put more of it in the cooler upper plenum air. Hard to say if the 1-2" of exposure to high heat would be worse than 3' of exposure to the 200* air near the firewall. I suppose that pipe is running 800-1000* at the point of contact with the hose.

Maybe I'll just stay with what I have.

Larry
 
lr172 said:
Thanks Dan. I was considering this to reduce heat in that line (still have moderate idle issues). This routing would reduce my hose length by 8-10" and would put more of it in the cooler upper plenum air. Hard to say if the 1-2" of exposure to high heat would be worse than 3' of exposure to the 200* air near the firewall. I suppose that pipe is running 800-1000* at the point of contact with the hose.

Maybe I'll just stay with what I have.

Larry
Click to expand...

I should clarify that the ones Tom has done may not be the same as your setup. Still worth the free call.
 
In certified aircraft, the minimum distance a fuel line should be from an exhaust pipe is 3 inches. You could use a heat shield attached to the pipe to insure the fuel in the hose doesn't get hot enough to cause vapor lock.
 
Your fighting Vertical sump with cross over exhaust. I believe the only option will be to the back if it is a Silverhawk servo. While it is advantageous to keep this line as short as possible, routing it out the back, up and through the back of the baffle might be the best compromise.
 
Integral firesleeved hose

Larry---integral firesleeved hose is wonderful. BUT, you still need to use good practices to protect the cover from direct heat, like laying it on exhausts, or very close. Heat shields certainly help.
The spec is 15 minutes fire proof, and 5 minutes fire resistant. personally---I'd see what options you have for re-routing away from the heat.
Tom
 
TS Flightlines said:
Larry---integral firesleeved hose is wonderful. BUT, you still need to use good practices to protect the cover from direct heat, like laying it on exhausts, or very close. Heat shields certainly help.
The spec is 15 minutes fire proof, and 5 minutes fire resistant. personally---I'd see what options you have for re-routing away from the heat.
Tom
Click to expand...

Thanks for the input Tom. I think I will avoid this routing. I think the heat shields need some air gap to be effective and I just don't have enough space. It's not worth the risk and it may not even help much.

Larry
 
lr172 said:
I would like to route my -4 fuel line from the servo to the spider up between the #1 & #3 cylinders (it currently runs up the back). I have a vertical sump with a vetterman exhaust. I need to run the -4 line between the sump and the exh cross-over pipe right in front of it. I have about 1.5" of clearance. The fuel line is a PTFE type with integral firesleeve. the hose will cross the exh pipe at a 90* angle...

I appreciate any guidance or suggestions you may have.

Larry
Click to expand...

Any chance you have a picture?

Would a SS hardline give you more clearance?
 
Toobuilder said:
Any chance you have a picture?

Would a SS hardline give you more clearance?
Click to expand...

I'll snap one in a couple of days. At lunch, I just partially pulled the exhaust and the #1 cylinder on my way towards my mini-top o/h.

A hard line is a good idea and would mitigate any damage from heat. I could wrap the area near the exhaust pipe to keep down the heat transfer. The smaller diameter would leave me a bit more room to fab an heat shield as well.

I had considered SS, but have had a difficult time flaring it. What wall thickness would you recommend in an application like this?

This is an interesting idea and appreciate you sharing.

Larry
 
I use this stuff from Spruce:

http://www.aircraftspruce.com/catalog/mepages/stainlesstubing.php?clickkey=3505

It is .035 wall which as WAY overkill for the application but is quite damage tolerant and pretty inexpensive. I've also been able to flare/bend it with normal hand tools, but it does take a bit more muscle to do it. Lubrication helps a bunch too.

As an alternate, you might take a look at good old auto parts store steel fuel line. Just flare for AN fittings and press on. Not like it is going to rust.
 
I am strongly considering the steel fuel line from the Servo to the spider. Do you guys see a problem with this steel tube 1" from the exhaust? I plan to build a heat shield and wrap the tube with fiberglass header wrap to protect from heat. I am thinking that the 1' + of hose eliminated will more than offset the short exposure to the exhaust pipe.

Larry
 
Larry, its similar to the governor line from Lycoming. Both accessories are mounted to the engine. Trick will be bending the tubing to get everything aligned. As Michael said, 1/4, .035 is fine, and fairly easy to work with.
Tom
 
lr172 said:
I am strongly considering the steel fuel line from the Servo to the spider. Do you guys see a problem with this steel tube 1" from the exhaust? I plan to build a heat shield and wrap the tube with fiberglass header wrap to protect from heat. I am thinking that the 1' + of hose eliminated will more than offset the short exposure to the exhaust pipe.

Larry
Click to expand...

Larry - not sure I would agree. 1' of hose in an area of comparatively low heat doesn't necessary equate to a shorter run exposed to high heat. I simply don't know. Keep in mind, you can shield from some radiated heat, but eventually any insulation will get heat soaked.You also now have a maintenance item as you should remove the insulation at intervals to inspect for any heat damage to the stainless line. 1" is way to close for me for even a control cable, let alone a fuel line. But again, I don't have any solid data to support my personal opinion or "gut".
When I was contemplating the same thing years ago, I just couldn't find a comfortable solution other than running it behind. 700 hours and no issues.
If you can find someone who has routed the line through the cylinders with cross over exhaust and vertical induction and has enough time on that setup to be a qualified example, I would search them out.
 
Re-reading some of your post, .... have you spoken with Don at AFP? He has helped others with idle issues with injector size changes. I probably have this backwards but smaller injectors increase pressure in the lines from the spider to cylinder. This is where you are likely getting the boiling fuel, not in your fuel line to the spider.

I chased an idle issue and all symptoms pointed to boiling fuel. I did the "clear tube" test and no bubbles where present. It turned out to be a completely unrelated issue.

It is pretty common to have some stumbling in FI set ups when hot at idle. If they are stopping your engine on the ground or inflight, that is a problem I wouldn't want to live with. I just get a small variance in idle rpm, when heat soaked that is very manageable but, some day, I will get Don involved as I am pretty sure it is boiling in the injector line from the spider and can be fixed with injector resizing.
 
JonJay said:
Your fighting Vertical sump with cross over exhaust. I believe the only option will be to the back if it is a Silverhawk servo. While it is advantageous to keep this line as short as possible, routing it out the back, up and through the back of the baffle might be the best compromise.
Click to expand...

IMG_1776.jpg

I'm dealing with this issue and didn't feel like there was clear consensus on how to proceed. I'm using the Bendix with vertical mount with cross over exhaust. It doesn't seem prudent to to me to run the line between the cross over exhaust and the oil pan. (the ruler shows the common routing direction and distance from the servo outlet to the 1-3 inner cylinder baffle penetration.) I'm trying to minimize teh fuel line to keep things cool IAW airflow performance's recommendation.

It does not seem like it would pass the airworthiness certification inspection.

It seems to me with 1000's of rv's flying there must be a lot of vertical mount fuel servo and the cross over exhausts flying. I'm thinking I'd like a different exhaust option to fix this problem. What are all of you doing?

-Is the common routing up and around the back of the servo?
-Are we ditching, or modifying the cross over exhaust?
-is it OK and am I just fabricating a problem that does not exist?
 
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I consulted with Kyle at Airflow Performance. Where I saw nothing but road blocks, Kyle saw lots of different paths. He was able to see and help brainstorm alternatives.

He showed me an alternate "fuel Out" port on the opposite side of the Bendix RSA servo to the left fo the data plate. Removal of the plug and inserting an AN fitting there provides for a two good paths away from the exhaust. One of those paths is as short as the option next to the exhaust tube. I brainstormed other options with a piece of tubing and found one additional good/acceptable alternatives with good clearance out the front that uses the inner cylinder baffle penetration hole. It exits out the front with the elbow turned abbot 10 o'clock and goes around the right side of the oil pan. The other two penetrate the baffling. One goes up the back of the accessory case and will require turning the flow divider around to change the facing of the "fuel in" port.

So for now, I have three options. I will install the baffling and see how things look.

Thanks fo listening. Maybe this will help others problem solve who come across this issue.
 
The problem you have is the crossover exhaust. All the examples I have seen running the from the Silverhawk up between the cylinders have used a non crossover exhaust, so avoiding the problem you have. I have a crossover exhaust and routed the line from the Silverhawk servo to the spider out the rear port on the servo and up through the rear baffle. Spruce sells a very nice metal circumference grommet with a flexible centre rubber that rivets to the baffle and fits the line very nicely. Good clearance can be maintained from the exhaust.
Bill Brooks
Ottawa, Canada
RV-6A finishing
 
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