100LL Vapour Lock

[paul330]

I am building a RV-10. It has a known issue with high temperature in the tunnel where the fuel lines run. I am insulating various components to mitigate this but am also fitting a OAT probe to monitor the temperature on my Skyview. Despite trying to research this on the net, I am unable to come up with the answers I need.

There is a critical temperature, above which vapour lock is possible. As ambient pressure is lower, this temperature will obviously be lower at altitude. My feeling is to set the WARNING level at sea level temperature since these would mean vapour lock is a possibilty at any altitude. I would then set the CAUTION at the temperature for (say) 8000' since this would mean that vapour lock is possible, depending on the altitude.

Can anybody give me these numbers for 100LL?

[John Price]

Hi Paul330

No idea what it all means but if you follow this link it might give you a clue.

http://www.avgas.ru/law/def_stan_91-90.pdf

or

http://www.epc.shell.com/.../GPCDOC_Fue ... tion_Fuels_...

Look at it another way, if you cannot use AVGAS 100LL what can you use?

John.

[bertdeleporte]

Hello Paul,

I am not an expert, but vapor lock is a quite tricky behaviour of organic liquids. The question is unfortunately not so simple to answer: the liquid will turn to vapour depending on temp AND pressure.
It means on ground, the vapor lock can occur if for instance liquid is sucked through a restriction with
enough loss of charge in the line to bring the liquid to the conditions leading to create vapour.
Still on ground, the tank filling will also give variable surface height, which can lead to vapour lock
It means also in altitude liquide will be much more prone to the vapour lock if you are at limit T°&P° conditions.

instead of taking care of temp, on which you have to cope with, an other way to "pass the hurdle" and get rid of vapour lock is to design lines with big diameter before the first pump, avoid angle connectors, restrictions in connections, or to put a small electric feeder pump just out of the tank. This will help to get a positive pressure in the feed line in any condition. I however assume
-the pump is near the engine or forward of the firewall
-the line is sucked from the tank
-there is a return line flowing back the non used fuel to the tank

I am working in a shipbuilding yard, VL is an issue we look very closely during design. I can give you a method we use to get rid of VL. We usually design depressed feed lines at a section to get the max flow at a lower value than 0.05m/s.
For instance, at say a max fuel flow of 70L/h in the line (I don't know the caracteristics of your pump), it will give:
70 L /3600 s = (approx) 0, 02 L/s which is 2 cm3/s (One liter is 1000 cm3)
20 cm3 passing 5 cm in one second is a section of line of 4 cm2
4 cm2 corresponds to a radius of line of sqrt(4/3,14)= 0,4 cm. It means a minimum line diameter of approx 8 mm would allow a fuel flow to get rid of VL. The return line, pressurized, can be of a much smaller diameter.
Pay attention to the actual fuel flow of the pump (consumption + return or max flow given in the datasheet of the pump) to adapt this calculation.
I don't know the design of the RV 10 and if those features are possible. Insulating the fuel lines as you do is a good idea, if there is a return line, it will help the fuel to remain at a low temp with liquid flowing in and out the tank, cooling into the tank.

An other link giving formula to calculate T° vs P° for REID VP:

http://en.wikipedia.org/wiki/Reid_Vapor_Pressure

Hoping this will help,

Bertrand

[paul330]

Thanks Bertram

Probably just a bit too much information.......

I think I will step back a bit and forget about the vapour lock issue - doesn't seem like it can happen while the engine is running. I'll concentrate more on what are desirable tunnel temperatures and especially what might constitute a fire. There has been at least one tunnel fire leading to an aircraft loss and serious injuries - albeit with sub-standard fuel line fittings.

[ian herdis]

Paul

What is the heat source causing the high temps?

Ian

[paul330]

There are 2 issues. Radiant heat from the firewall and bottom skin (exhausts) and the hot air ducting running to the rear seats. I've insulated behind the heat box on the front of the firewall and the front face and bottom of the tunnel. I will also wrap the scat tube once it's fitted.

It's a well documented problem - just trying to manage it the best I can.

[ian herdis]

Thanks for the explanation Paul.

But I think when you get that big 540 up and running you will be sucking so much cool fuel from those wing tanks that you could probably use the fuel lines to set up a cool box for your soft drinks

Ha Ha yes sorry not much help I know

Ian