Re: HIGH temperature thread sealant.

From: Glen Walpert (gwalpert_at_notaxs.com)
Date: 02/18/05 

Date: Fri, 18 Feb 2005 23:04:44 GMT

On Fri, 18 Feb 2005 21:12:57 +0000 (UTC),
glhansen@steel.ucs.indiana.edu (Gregory L. Hansen) wrote:

>In article <42162247$0$8748$db0fefd9@news.zen.co.uk>,
>Steve Taylor <steve@ravenfield.com> wrote:
>>Gregory L. Hansen wrote:
>>
>>> This is kind of out of my ass, but what about using the BN itself as a
>>> sealant. Is there some kind of goop that would dissolve the BN but not
>>> attack the stainless, that could be applied to the threads and dried out
>>> before use?
>>
>>Dunno, its incredibly inert - acid proof, alkali proof, non stick with
>>liquid aluminium, steel and magnesium, calcium fluoride, calcium oxide.
>>
>>But breaks if you over tighten the %£^£&$* screws.
>>
>>Steve
>>
>
>
>You might also want to check out what these guys can offer.
>
> http://www.helicoflex.com/
>
>Metal seals with a spring core, and they can be made with a soft metal
>knife-edge so that the gasket material squishes into the peice, rather
>than the usual arrangement of a knife edge on the peice biting into the
>gasket.

Hollow metal O-rings might work, but they require a fair bit of load
to seal effectively, the mfgr will provide data on this and likely be
happy to discuss your problem with you.

Packing in a packing gland is still what I would regard as the most
foolproof method for getting a consistently tight seal that will stay
leakproof with thermal cycling, but it requires some skill to assemble
properly and you have very little space available in your probe
assembly (now that I see it has two tubes installed). There are a
number of different packing materials available which are potentially
suitable besides Grafoil, check any packing mfgr for recommendations.
I think that Grafoil may be the softest and easiest to remove after
heating however.

Back in the 1970's I worked for the late Dr. Philip Stein at the Stein
Seal Company, testing and then designing mechanical seals. Dr. Stein
was the first to solve a number of tricky sealing problems including
the first jet engine shaft seal design which was not the limiting
factor in engine life back in the 1940's and numerous others. His
approack to seal design always involved a complete analysis of
temperatures, pressures, forces and deflections in the seal structure.
I suggest you do the same; just how much thermal expansion do you need
to deal with? Microns count here! What forces will result from the
means of flexure you are going to design in to accommodate the thermal
expansion? What forces can your piece of chalk, er, BN, tolerate?

Looking at the pictures of your probe I might be inclined to provide
the compliance from the cold end. Install the probes in threaded
holes as usual, but rather than attempting to seal the threads have a
removable tube slide all the way through the probe to directly engage
a seal on the back of the BN tube. The seal could be a preformed
Grafoil ring on in a grove on the outside of the BN tube or any number
of other workable arrangements, as long as the tube is spring loaded
to maintain an appropriate and nearly constant seal load with
temperature changes and seal aging/shrinkage. This way the spring is
cold, there is nothing complex or big on the hot end, and you can
screw in the BN tube with very low torque and still get a good
relaible seal.

A gasket on a flat or tapered surface between the spring loaded gas
supply tube and the BN would also be workable, possibly even a knife
edge biting into the back of the BN tube would do it. If porosity of
the BN is a problem consider a coating of Acheson Aquadag colliodal
graphite.

The problem looks to me to be amenable to a miriad of solutions, just
a matter of finding a cheap and easy one without wasting too much time
on it.

Regards,
Glen

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