Stainless steel boiler/exchanger?

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rkcarguy
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Re: Stainless steel boiler/exchanger?

Post by rkcarguy » Fri Apr 12, 2019 11:55 pm

I've found that moving fast and stuffing rod is the key to TIG welding SS, as it keeps heat down. If your bead starts turning brown its too hot, I shoot for a silver/gold color with a little blue being the "warning". For something like this I would probably tack the tube sheets in, then cap the tube holes with cover plates and some threaded rod and purge the inside while seal welding the tube sheet to the shell. Then I think I'd countersink the holes about 1/3rd of the way through the tube sheet and silver solder them in.
The picture of the SS boiler looks well done, it appears the shell was TIG welded to the tube sheets and the tubes themselves were silver soldered?

My background is based in industrial fabrication, and one of the recent projects we bid was a large cooling exchanger about 30' long, basically a pipe bundle submerged in a big open top steel tub. The bundle had been replaced several times and they had tried stainless and it cracked, but it was also in a bare steel tank with chlorinated city water just dumped into it. We didn't make the SS bundle so I have no idea if it was assembled correctly, purged, passivated, etc.

Another wrench to throw into the works is SIB tig rod, I've used this on some work recently and been pleased with the way it welds at a lower temperature than stainless and steel melt. Because this will be a 10" pipe boiler I'll have some space to work and likely use 1/2" to 3/4" tubing with .065" wall, so I'll have a bit more meat to work with in the weld department.

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Bill Shields
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Re: Stainless steel boiler/exchanger?

Post by Bill Shields » Sat Apr 13, 2019 6:51 am

Actually, if you REALLY FEEL THE NEED to build a boiler from stainless, DUPLEX stainless is the best of the materials to use.. (2205?)
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Fender
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Re: Stainless steel boiler/exchanger?

Post by Fender » Sat Apr 13, 2019 7:46 am

Most copper boilers are silver-soldered together, with a few rivets or screws for locating the pieces. Why don’t I hear about silver-soldering a stainless boiler? Seems that this would avoid the weld zone issues, since no welding would be used.
Note: This is a question, not a recommendation.
Dan Watson

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Bill Shields
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Re: Stainless steel boiler/exchanger?

Post by Bill Shields » Sat Apr 13, 2019 7:08 pm

Because of the low coefficient of thermal conductivity and relatively large coefficient of thermal expansion, silver soldering of large chunks of stainless (like a boiler) is a tough go.

getting everything up to the proper uniform temperature without warping (especially of screwed together), is tough.
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fredrosse
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Re: Stainless steel boiler/exchanger?

Post by fredrosse » Fri May 03, 2019 9:38 pm

The ASME Boiler and Pressure Vessel Code specifically prohibits the use of common stainless steels for the wetted parts of steam boilers, with very few, and difficult to achieve, exceptions. The ASME Code is generally accepted throughout the USA, and the world, as the governing document for boilers.

Much work and experience goes into this code, so it is best to obey its rules. Sure, there are examples of evidently successful stainless steel boilers, but that doesn't justify the clear violation of the rules, based on the armchair engineering conclusions.

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