How to seal threads inside combustion chamber

[cebaero]

Hello, hopefully this is the last question from me for awhile.

I cannot figure out how to seal the threads on my nozzle inside the combustion chamber. Cant use teflon tape/pipe dope because of heat. Cant get compression fittings to work. Am I going to have to weld this? I was trying to not do that so that I can have interchangeable parts but it’s looking like I’m going to have to.

Any thoughts? Using a nozzle with a 1/8 npt male connector. The shaft from the nozzle to outside the combustion chamber can be anything, I just don’t know how to seal the nozzle.

Thanks in advance for any help

[racket]

The fuel nozzle is liquid cooled , it shouldn't get overly hot

[cebaero]

Aug 16, 2024 21:16:27 GMT -5 racket said:

The fuel nozzle is liquid cooled , it shouldn't get overly hot

So even the pipe dope/teflon tape around the nozzle should be fine? It says it’s only rated for 400F

[racket]

Should be OK , you can only try :-)

[CMDR_Boom]

While for me it depends on what time of year I'm working on prospective welding jobs, (for instance where I live, it's typically around 102*F by noon, I will actively look for alternatives that preclude me from dragging welding gear outside til about October!), I've had various levels of success in brazing brass fittings (couplers/nuts, etc) or if I'm really desperate, something like the alumaweld rods and a gas torch into areas around the combustor that will either be shielded in a recess or exposed in an area that's a good ways from the hottest parts, so you can put a moderate amount of strain on things for strength like tightening plugs/sensors/gauges, etc for air-tightness. Truth be told, any threads that are beyond the area of engagement, I wouldn't worry about too much anyway except for areas of build-up that would be prohibitively difficult to remove with like a bristle brush or ultrasonic cleaner for tear-down reasons. If you're going to melt threads, everything attached to it will be a molten puddle too, which means something in your design probably needs a tweak. 😉

[cebaero]

Aug 17, 2024 11:36:42 GMT -5 CMDR_Boom said:

While for me it depends on what time of year I'm working on prospective welding jobs, (for instance where I live, it's typically around 102*F by noon, I will actively look for alternatives that preclude me from dragging welding gear outside til about October!), I've had various levels of success in brazing brass fittings (couplers/nuts, etc) or if I'm really desperate, something like the alumaweld rods and a gas torch into areas around the combustor that will either be shielded in a recess or exposed in an area that's a good ways from the hottest parts, so you can put a moderate amount of strain on things for strength like tightening plugs/sensors/gauges, etc for air-tightness. Truth be told, any threads that are beyond the area of engagement, I wouldn't worry about too much anyway except for areas of build-up that would be prohibitively difficult to remove with like a bristle brush or ultrasonic cleaner for tear-down reasons. If you're going to melt threads, everything attached to it will be a molten puddle too, which means something in your design probably needs a tweak. 😉

Im not worried about melting the threads, Im worried about sealing the threads in my fuel nozzle so that it doesnt leak. I am hesitant to use pipe dope/teflon tape as im not sure it could handle the heat of the combustionchamber, but i also dont want to weld the seals shut so that i can replace the nozzle if i need to

[CMDR_Boom]

Ah, in that case (trying to figure out how to word it most efficiently) most of the small nozzles I've used thread kind of like a spark plug where the bulk of it is up top, well out the major heat zones and are sufficiently cooled with passage of fuel. With a traditional fuel nozzle for a jet engine, the older types I've gotten either from salvage lots and repurposed or scrap for study are inserted into the combustor through the top and are fastened well away from any internals, like on the external walls of the case or to a central fueling ring with ports and diverters. Visual reference, like a gooseneck shower arm of sorts, that sit just inside the combustor with a bit of protrusion, no fittings internally if that makes sense.

Except for base startup when you have a relatively low volume of airflow, most of the air in a good combustor design pushes the vast volume of charge air around the combustor and relatively little bleeds into the central combustor area where the flame front is not directly touching the nozzle. If you really need it, I would look at something like LocTite's copper-infused anti-seize. One of the variants I use is high heat tolerant, in the area of 1500*F+/900*C. I'll update this if the product number is incorrect as I don't have the bottle in front of me, but LOC38650 visually looks like the one I use with a slightly updated label. I use it on spark plugs too for our vehicles and all manner of things, it's never given me any problems.

[cebaero]

Aug 17, 2024 0:23:18 GMT -5 racket said:

Should be OK , you can only try :-)

I found a gas turbine engineer that was kind enough to do the math and apparently it would not cool it enough for the sealant I was looking at. Plus, he said that even if it did cool it down a bit that the nozzle would cook as soon as the engine turned off. I am now looking at maybe a combination of anti seize and stove gasket. I will keep you updated!

[richardm]

why dont you try seating the nozzle against a copper washer ? All you need is a flat surface on both the back of the nozzle and its fitting. No heat problem

[cebaero]

Aug 20, 2024 4:24:14 GMT -5 CMDR_Boom said:

Ah, in that case (trying to figure out how to word it most efficiently) most of the small nozzles I've used thread kind of like a spark plug where the bulk of it is up top, well out the major heat zones and are sufficiently cooled with passage of fuel. With a traditional fuel nozzle for a jet engine, the older types I've gotten either from salvage lots and repurposed or scrap for study are inserted into the combustor through the top and are fastened well away from any internals, like on the external walls of the case or to a central fueling ring with ports and diverters. Visual reference, like a gooseneck shower arm of sorts, that sit just inside the combustor with a bit of protrusion, no fittings internally if that makes sense.

Except for base startup when you have a relatively low volume of airflow, most of the air in a good combustor design pushes the vast volume of charge air around the combustor and relatively little bleeds into the central combustor area where the flame front is not directly touching the nozzle. If you really need it, I would look at something like LocTite's copper-infused anti-seize. One of the variants I use is high heat tolerant, in the area of 1500*F+/900*C. I'll update this if the product number is incorrect as I don't have the bottle in front of me, but LOC38650 visually looks like the one I use with a slightly updated label. I use it on spark plugs too for our vehicles and all manner of things, it's never given me any problems.

Thank you very much. Do you think the anti seize will provide a seal over tapered threads though? 

Also, what if I used the anti seize and then also some sort of furnace cement or stove gasket thats rated for 2000F? That way I could atleast get it off in the future without destroying threads.

Do you think that would work?

[racket]

Simply isolate the nozzle from direct flame contact .

I've never had problems , ...........without any drawing/pics of your setup its hard to understand your problem , LOL...... you should have asked you gas turbine engineer for suggestions

[ericthegreatish]

Thread sealant should be fine. I have mine threaded right onto the flame tube with it and it hasn't leaked even during an over temp run. Worst case, it leaks a tiny bit and you find an alternative...

Good luck,
Eric

[cebaero]

Aug 20, 2024 16:54:52 GMT -5 racket said:

Simply isolate the nozzle from direct flame contact .

I've never had problems , ...........without any drawing/pics of your setup its hard to understand your problem , LOL...... you should have asked you gas turbine engineer for suggestions

Yeah thats my fault, I know the question was vague without more info on my set up. I really appreciate all of your help so far in my build though

[cebaero]

Aug 20, 2024 18:51:28 GMT -5 ericthegreatish said:

Thread sealant should be fine. I have mine threaded right onto the flame tube with it and it hasn't leaked even during an over temp run. Worst case, it leaks a tiny bit and you find an alternative...

Good luck,
Eric

Thank you for the reply. Do you know what kind of thread sealant by chance?

[CMDR_Boom]

I would avoid the furnace cement and just stick to anti-seize. I've tried a number of commercial ceramic sealants for experimenting on plug and sensor sealing duty. Even with the best prep, they don't handle vibration well and flake off, crack, or will generally fail at the worst possible time (especially around anything that produces net positive pressure). I've made a few ceramic-buffed phenolic resin putties that work quite well, but they are definitely not meant for being able to remove the fittings, to the point that they bond so strongly that you would have to destroy any substrate to get them off.
I can, unfortunately, send you pictures of that exact case stated above with a broken ignitor plug that gave up trying to remove it before the resin broke.