Has anyone any ideas about a suitable thread sealant to make an M8
thread on a boron-nitride tube gas tight in a stainless steel block (to
roughly 10" of water), whilst the stainless is cooking at at least 400
Centigrade ? I am at a complete loss ! I have tried Zypcoat boron
nitride paint, but once the carrier has dispersed, the thing still
leaks.
Thanks
Steve
If I could help it, I wouldn't use a thread sealant at 400C (or a thread
seal even at low temperature). Hard solder another fitting to it, like
Swagelok?
TorrSeal is pretty tough stuff. I don't know what temperature it's rated
for, but it could surely handle 400C for brief periods.
--
"The preferred method of entering a building is to use a tank main gun
round, direct fire artillery round, or TOW, Dragon, or Hellfire missile to
rated
I used TorrSeal once to repair a auto radiator in a pinch. It worked
for a short while enough to get me home, but under a 400C environment
it is not going to cut it. The ceramic cements typically just dont get
dense enough to form hermetic seals. Youre best best is a low Tm glass
or low temperature braze. Wetting of the BN may be a problem with the
braze, but there are many brazes you can work with. You might try
water glass cements ( I have had good sucess with sodium silicate +
aluminum hydroxide cements for special applications)
I'd assumed he's going to 400C for an hour or so to bake out a vacuum
system, and then would run at room temperature.
If it's 400C for sustained use, forget the thread sealants, lose the
thread, and replace it with conflat, VCR, or some other all-metal bakeable
fitting. Or replace it with a hard solder joint and plan to take a
propane torch to it every time the tube goes in or out, but forget
threads and sealants.
--
"Experiments are the only means of knowledge at our disposal. The rest is
poetry, imagination." -- Max Planck
Nope, the device runs (intermittently) at upto 1750 C, about 5.5 inches
from the stainless steel parts. The stainless bit is a block around 6"
long and 1" diamteter, with a water cooling system for about 1/3rd of
the length. The temperature of the cooled part is almost ambient.
Still have to join airtight to Boron Nitride,and what will let me do that ?
Thanks
Steve
It's easy to come up with possibilities since I won't be the one
implementing them. I don't know where you're getting your boron nitride
parts but you could think of a glass-to-metal seal giving a steel tube
that can be sealed with Swagelok, have a VCR gland welded to it, or just
be brazed or hard soldered into place. Add a few $$$ with each buzzword.
Does it have to be boron nitride? It would be more convenient if you
could solder directly to the ceramic, then you could use solder as the
thread locker. And sure you'd need a propane torch to replace it, but
what the heck. But I don't know anything about soldering or brazing to
ceramics, except that I think they usually vacuum deposit a metal base
coat.
High, sustained temperatures make this a hard problem.
--
"'No user-serviceable parts inside.' I'll be the judge of that!"
Hi Gregory,
Back to my work persona, instead of posting from the field from a lab in
Egypt where I was trying to commission the equipment, and needed some
quick advice....
I will post a couple of pictures of the rig mounted over the furnace
system on the company website for any interested folks.
It has to be boron nitride, Molybdenum would work, but we could't get
lengths of tubing for anything like a sensible price, and Mo becomes
incredibly fragile once it has been fired at these temperatures. I doubt
BN can be metal coated, since its not very hard.
No other ceramic is compatible with the melt. It dissolves pure alumina
in a fraction of a second.
We fabricate the BN tubes from BN bar stock in our own workshops. The
tubes are nominally 3/8 OD, we bore down 1/8" almost all the way
through, and then pierce the ends 0.5 and 1.0 mm, for the bubbles of gas
to escape...
Thanks once again.
Steve
Anything can be metal coated. But possibly at considerable effort and
expense for in-house production of consumeable parts. It would be
different if you just need one. There are deposition businesses that
would coat parts for you, but I don't know what they would charge. And
get more expert advice than mine before you decide it would solve the
problem. My advice is worth everything you paid for it.
Harsh. But I don't understand why they can be mounted on a stainless
steel block. Can the tubes be made longer and the block cooler?
Glass frit is another possibility, again requiring a torch. But high
temperature seals are not my strong point. Good luck with that.
--
"You're not as dumb as you look. Or sound. Or our best testing
indicates." -- Monty Burns to Homer Simpson
The BN stock is only available in relatively short lengths, and I don't
fancy the chances of survival of long, fragile pieces of BN out of the
furnace !
I think it would help greatly if we had more, well any, radiation
shields on the front of the probe. The radiation shielding is a royal
P.I.A.
Thanks
Steve
IMO the thread sealing problem will be very difficult due to
differential thermal expansion and low compliance. How about a
packing gland with Grafoil packing, possibly with bellville or other
spring loading? You might be able to eliminate the threads on your
tube entirely and retain the tube with the packing if forces tending
to pull the tube out are small.
That makes me think of a compression seal, or test port seal, or whatever
you want to call it. A port with a close-fitting o-ring capped by a
female nut. A smooth tube is inserted, and the nut tightened down to
squish the o-ring against the sides.
I can't think of any o-ring materials that can handle sustained 400C, but
cork gaskets are used in engines. I don't know how well cork squishes
out. It might be tightened down into a cone-shaped cup, as long as it
doesn't wrinkle. Or if you have a squish seal you might just use Viton or
Teflon or something and plan to replace the gaskets often.
I don't know the thread you have, NPT or straight thread. But maybe a
straight thread with a gasket against a flat-bottomed fitting, e.g. cork
or felt. I don't know how tight you can screw down a BN tube.
--
"Suppose you were an idiot... And suppose you were a member of
Congress... But I repeat myself." - Mark Twain
Despite us not reaching any breathtaking insights you and Al have both
helped me think clearly about the problem, and let me bitch about it
with folks who can also feel the pain.
I appreciate all your comments.
Steve
The trouble is I don't think they realised that what they want is not
routine off the shelf engineering. Push the button - poof, out comes an
answer, the instruments are for much more fundamental research .
We asked the customer for their origianl comments on the materials of
construction, to be told that Alumina was quite ok and there would be no
problem. On site of course, they announce that there is no way that we
can use the alumina parts.
The rest of my thoughts had better remain private on a public forum.
Steve
Not knowing any better I use Copper Anti-Seize
This is available in automotive stores and is dirt cheap. I use it on my
spark plugs, manifold and turbocharger parts
It is good to 980°C Since it is untended for use on metals I do not know
what it will do to ceramics.
http://www.caswellplating.com/permatex/permlubricants.html
There is also Nickel Anti-Seize that is good to 2400°F
Regards,
Boris Mohar
Got Knock? - see:
Viatrack Printed Circuit Designs (among other things) http://www.viatrack.ca
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