Operating Pressure for Vacuum Brazing Tube to Tubesheet joints

[InspectorWilly]

I'm currently looking into a process improvement so we can increase throughput on one of our product lines. This is as close to an off-the-shelf item as we have, most everything is custom engineered. We're looking at potential increase in orders of these and I've been tasked with determining methods to increase throughput. In the attached I've shown what we have to do. It is a type of tube to tubesheet joint. However, the joint is on the inside of the manifold. So, currently we're cutting our manifolds in half, welding the ID joint, then re-welding the manifold and performing RT on the joint. So the most cost-effective solution I'm seeing is vacuum brazing.

However, some of these items could see 10k plus PSI operationally so we'd be hydro testing at around 13k PSI. The question I've got from engineering is whether or not the joint will withstand this pressure. To be honest, I've got minimal experience with any sort of brazing at high pressures. Most I've seen is 150 PSIG max, and that was mostly monel and bronze. Here, we're looking at 300-series stainless steel and maybe some nickel-based alloys.

I'm looking at a vacuum brazing oven that can produce vacuum down to 10^-3 Pa at a max temp of 2250F and pressure of 1 Bar for inert gas quenching.

I'm assuming that so long as we're passing the bends and tensiles to qualify the procedure we should have no issue with the pressures so long as they're properly designed. But I just want to confirm that the practice matches the theory.

“When doing a job — any job — one must feel that he owns it, and act as though he will remain in that job forever.”
-Admiral Hyman G. Rickover

 

[EdStainless]

For a typical braze joint you will need a lot of surface area, and a very controlled fit (the gap depends on the braze).
Perhaps a machined socket welded on the manifold that the coil and braze can be fit into.
In some cases people use a braze foil, in some they a machine a groove or to to fit braze rings. Either way delivering the correct amount of filler is key.
That is about a 10micron vacuum, should work.
You won't want to go to 2250 because of sagging and distortion.
Ideally 1950-20500F is a decent range for austenitic stainless alloys.
Cooling is problematic as the coil will see very slow cooling.
Make sure that you have good L grade material that is well annealed to start with, or else you will bet sensitization.
I take it that all of the parts in one unit are the same alloy?
You will likely need a different braze for SS and Ni.
And you may need to also think about the corrosion resistance of the braze.

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P.E. Metallurgy, consulting work welcomed

 

[InspectorWilly]

Ed - Thanks for the info, I appreciate the reply.

First - yes, we used machined holes or sockets, and the tubes are standard exchanger tubes, either seamed or seamless.

Foil or groove - both is considered "pre-placed filler", correct?

Temp is just the max - in case we want to braze Hastelloy if we get that request for some reason (not out of the realm of possibility)

I was thinking about the cooling rates which is why I was considering a unit rated for 1-2 bar so we could perform a gas quench but I'm not sure how that would affect gas usage. We've got a bulk system but it's not piped to that building. And with the volume to quench the oven I assume that would be quite a bit.

Our most common unit requested is 304L or 316L. However, we do sometimes get alloy combinations.

“When doing a job — any job — one must feel that he owns it, and act as though he will remain in that job forever.”
-Admiral Hyman G. Rickover