I am refurbishing a Hawker Hunter for a friend and have solved most of the issues that were wrongly done by original builder. Fuel system comprises a BVM UAT, followed main tank then 2 saddle tanks in parallel coupled with 1 vent line exiting from bottom of fuselage.
Last Saturday, we encountered a new issue. while filling up the tanks, system seemed over pressured.
In addition when i tried to purge the fuel system to free the bubbles from filter, the pump worked erratically and could not cope....
to rule out a problem with the fuel pump, i fed the input directly from the Fuel jug, and all seemed ok as pump was running at various speeds without any hiccup
Furthermore, after emptying the saddle tanks the draw from main tank seemed fine.

Problem is that saddle tanks have been glued by initial builder and stoppers are not accessible. vent lines are T'ed with 4mm clear tube and there is only 1
4 mm line attached to a short 3 mm brass tube exiting bottom of fuse.

Would appreciate ideas on how to solve this problem trying to avoid any damage to the airframe.

I had a similar problem or at least I thought. I checked everything out and never found a problem. The turbine ran fine. In your case access being a problem I would break the line to the pump. Run a line outside the air frame and see if the fuel will siphon out. If you get good flow your probably OK there.

The fuel system will only deliver to the limit of the most restrictive component. Are you using a low-speed setting on the filling station's pump? I always fill complex fuel systems with the fill pump set at about 1/3 of it's capacity or lower.

On my UltraFlash with a main and twin saddle tanks, I used 3/16" vent lines on the saddles and a 3/16" vent fitting on the vent outlet. I drilled out the vent outlet fitting a bit. I also used larger brass tubing in the saddle tanks' fuel stoppers with 3/16" tygon back to the main tank. Then at the main tank, I used the larger brass tubing in its stopper with 6MM festo line to the fuel pump. I can't remember if I used a barbed reducing fitting to 4mm just before the pump or not. Also, I always use BVM Large Clunks and 3/16" tygon with a short brass tube inside each tank. The wrong Clunks can be restrictive too.

Before I did he above, the P140 would not spool up fully. Once I made the above fuel system changes the P140 would spool up to full RPM. To test the system run the turbine at full speed for 20 seconds or so. But if your filling station,s pump is pressurizing the system at a low speed, you have excessive restriction.

Thank you both for your input.
I already know that pump is working normally by feeding it from outside the airplane.
I know that main tank is also ok as i was able to access it and checked/changed tubes inside.
Problem definitely relies in saddle tanks and vent line.
Issue is that those tanks were glued to the airframe by initial builder and stoppers are not accessible....
I am not comfortable with the tubing inside the saddle tanks to start with.
I could eventually change the vent lines to larger ones, but unsure if this would solve completely the problem.

Your problem could be with the vent line. Replace the tee with one that will accommodate two smaller lines going into a larger line. Then use a larger diameter line from the tee to the atmospheric vent.

Dany

Can you access the feed and vent pipes on the saddle tanks? If so treat each tank as an individual tank, fill in the feed and see what it fills like. Do the same on the opposite side. Do they fill/drain the same, any creaking from the tanks ( showing a restriction on vent and likely pressure) if it loads the fill pump filling the restriction could be in the feed line.

a single 4mm vent for two tanks to share is woefully inadequate. Our fuel systems really don't like restrictions, especially on the vent side, so anytime I have more than a single tank sharing a vent or a turbine larger than a 140, i use a Hi-flow vent system. The extra cost is negligible and i have had zero fuel feed or refuel problems.

That's basically what I was trying to tell him in post #5. The 4 mm vents are probably adequate for each saddle tank. Once they are teed together the single vent needs to be capable of handling the flow of the two saddles. Get a 4-4-6 mm tee and run a 6mm line to a brass 6 mm tube exiting the fuse.

You can easily see whether or not this is the problem by disconnecting both vents from the tee and fill the tanks (not too full) to see if they over-pressure or not.

it seems that problem is solved. overflow line was partly squeezed between inlet duct and saddle tank. Anyway, I have changed overflow line to a tygon 6mm line. in addition, it looks like there was some sort of partial clogging that was freed.
Turbine ran flawlessly on the ground, and 2 nice flights were done without any event.
Yet there are still two issues that need to be solved:
A) The BVM UAT seem to get compressed while turbine is running (don't know if this is acceptable/normal) as i have not experienced this in other setups.
B) After the flight UAT was partially empty implying that fuel system is somehow leaking air which i need to attend to.

Thanks for all for your input.

Usually I’m doing two vents with use of saddle tanks, better ventilation and just in case my tanks don’t filled evenly I can close one vent to fill the other one until level is equal, plus I never do overflow lines underneath my jet just because one’s I clogged one up with sponge that was under my jet for transport, from that moment the vents are on the lower side usually at 30-40 degrees, much easier to plug overflow tank , my vents are at least 6-8 mm lines and never smaller, remember if uat is collapsing it means vacuum that means fuel or air vents restriction!

I replace all my internal and exterior hoses about ever year and half. I always find the hoses are getting loose in side the tanks even though they are safety wired. You stated the the tanks are glued in. You need to make them removable to inspect the stoppers and hoses. Sooner than later they will need maintenance . Dan

Does it also expand when filling?

Blockage on either line in main tanks can do it. Air in UAT is probably an air leak in a Joint because the system is working hard. Or filler plug is leaking air for the same reason

sysiek,
I was going to do as per yours but after seeing that tanks were filling and emptying equally i opted for one vent line. As for your suggestion to place vent exit at 30- 40 degrees is definitely a good idea for next installs. i still have to find where system is producing restriction. UAT monitoring is the key for a clean and flawless fuel flow.

DAN,
Fully agree with you. I never glue my tanks and always make sure they are easily removable or accessible for servicing and changing hoses. Unfortunately, in this case, tanks were heavily glued by original builder, who also did many mistakes which needed major modifications.

Dave,

Yes the UAT expands (although much less then before) when filling but isn't this a little bit normal due to having one large main tank followed by 2 saddle tanks and 1 vent line?
i will try to change the filling plug (which is on the tygon fill Line, remove the festo 6-4 converter and replace it by a brass 6-4 coupler and check if this would make a noticeable difference

Finally thank you all for your precious input. The learning curve comes from all the inputs that each experienced modeler is providing .
Great Community!!