Viability of brazing a tubesheet?

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Viability of brazing a tubesheet?

Post by TriangleTom » Fri Jun 19, 2020 11:24 am

Obviously it would be poorly suited to high pressure service, but for lower pressure (~100PSI), would a rolled and brazed boiler be viable? Given the relatively lower temperature of brazing compared to welding, it seems like it may be a good way to seal rolled tubes/prevent hot spots without affecting the structure or properties of the tube or tubesheet.

Obviously with higher boiler temperatures there would be issues with the brazing compound melting, but for a lower pressure water tube boiler where the brazed joints would be kept at a more or less constant temperature by the circulation of the boiler water would this be viable?
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Re: Viability of brazing a tubesheet?

Post by RGSP » Fri Jun 19, 2020 3:29 pm

I haven't done it myself, but model locomotive builders often use brazing for their boilers, and they tell me that getting everything hot enough is really rather difficult, even on boilers for the smaller models. I've done a fair bit of vacuum brazing of industrial pieceparts, and that makes a lovely job using copper as the braze metal, or more conventional brazing alloys, but hiring a vacuum chamber (and induction heater) big enough for a whole launch boiler would be expensive.

Brazing in air needs a Borax flux (or something equivalent) and minimising time at temperature is usually a good thing, especially if multiple joints are being done. I rather cringe from the thought of trying to braze a whole VFT boiler tube plate, but with an hour's practice I know I could make a nice job of it using TIG welding and the right filler rod. That might be an easier and more effective way to go, with no immediate restriction on maximum steam pressure.
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Re: Viability of brazing a tubesheet?

Post by RNoe » Fri Jun 19, 2020 3:38 pm

My commercially (USA) made locomotive boiler (8" diameter) is all welded except for the copper tubes which are silver brazed to the steel front and firebox tube sheets. It is not an ASME certified boiler.
It is certified by Washington State, USA, for operating pressures up to 150 psi, and initially hydrostatically pressure tested to 150%, per State requirements.
After 15 years of intermittent service it remains in excellent condition inside and out, and has never leaked at the tube sheets.
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Re: Viability of brazing a tubesheet?

Post by Lopez Mike » Sat Jun 20, 2020 4:44 pm

I have done two all copper locomotive boilers using brazing. And I will not do it again! The high conductivity of copper meant that I had to essentially immerse all of the boiler in vermiculite in order to get the working area hot enough. The whole experience was like sticking my head in a firebox. And I spent more time chasing leaks than I did on the original work.

That said, I have have had an excellent series of experiences brazing steel. The much lower thermal conductivity of the steel made working in a small area of the boiler quite doable. I have used both separate brazing flux, water based, and the rod with the flux coating. With fire tubes I would think that not rolling them in would be the way to go. You need the penetration of the joint.

As an example of the flexibility of brazing, the smoke stack assembly on my 2.5" to the foot locomotive was intended to be a casting with a fillet between the cylindrical stack and the saddle shaped base. I cut out a saddle shaped base out of the same diameter tubing as the smoke stack and machined the stack to fit. I then brazed them together with a very generous fillet and then used a round file to clean it all up. With a coat of paint it looks just like a casting.

I think that some arithmetic might show that concerns over the heat strength of brazing are not well founded. If one were using tin-lead solder, yes. But high silver content compounds, silicon/phosphorous rod (sil-phos brand) or straight brass I think hold their heat strength to a safe temperature for our boilers. I have not done these calculations however!

A rough calculation of the loads at one end of a 1" tube during a 225 p.s.i. hydro test show that a sheer load of around 180 lbs. is taken by a bit over 3" of brazed seam. Seams moderate. Now at 150 p.s.i. safety valve release point the temperature is about 350 degrees F. Does brass weaken significantly at that point? I've generally taken a machinist's view that brass is about half the strength of mild steel but I have no feel for it's decline in yield point with temperature. Metallurgy is strange stuff.
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Re: Viability of brazing a tubesheet?

Post by DetroiTug » Sat Jun 20, 2020 7:56 pm

Some Cuckoo-Ness from the steam vehicle community. My buddy brazes copper tube into steel pipe headers and runs at 400 psi
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Re: Viability of brazing a tubesheet?

Post by fredrosse » Sun Jun 21, 2020 3:04 am

ASME Boiler Code has rules for the construction of power boilers by brazing, defined as filler metal with a melting point above 840F (450C). Several brazing materials are permitted, the great majority are limited to 400F (205C) service, which corresponds to a steam saturation pressure of about 225PSIG.

Allowable stress vs. temperature limits are defined for each brazing material, and there are about a dozen listed brazing filler metals. The allowable stress values for brazing materials, usually Copper based alloys, falls off rapidly with increasing service temperature, however there are many applications within the realm of our steamboat boilers that can be reasonably designed with brazed pressure parts.

The ASME Code also lists several additional requirements, brazed joint gap/clearances, qualification of brazers, etc. The rules generally require assurance of full joint penetration, either through procedures or tests or visual examination.
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Re: Viability of brazing a tubesheet?

Post by Lopez Mike » Mon Jun 22, 2020 2:37 pm

Fred,

How about silver solder. I stick carbide bits to mild steel shanks for lathe tooling once in a while. When I'm impatient I get the tool smoking hot when machining, maybe even red, and haven't had one come loose yet. I do use a very high silver content solder though.

On reflection about my earlier post, I feel I must re-emphasize the need for penetration. I would not roll in the tubes if I were planning on brazing and especially silver soldering. My shop rule of thumb for a silver soldered joint is to use my automatic center punch to place three punches on one of the surfaces. Seems to give me a thou or so for the silver solder to wick in adequately.

I'm sure there is plenty of written advice available on such techniques.
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Re: Viability of brazing a tubesheet?

Post by fredrosse » Thu Jun 25, 2020 3:20 pm

"Fred, How about silver solder."

Silver solders meet the ASME definition for a boiler constructed by brazing, "melting point above 840F (450C)" and could be used for boiler construction. The costs would be much lower with other brazing materials. I think Silver solder for boiler construction is technically excellent, but in my experience used only in the smaller hobby boiler sizes because of the cost.
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Re: Viability of brazing a tubesheet?

Post by Mike Rometer » Thu Jun 25, 2020 7:03 pm

Some considerable years ago, when building a small loco boiler in copper, I was advised to price the copper and then double it to allow for the silver solder. I don't think the advise was far wrong! :o
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Re: Viability of brazing a tubesheet?

Post by Lopez Mike » Fri Jun 26, 2020 4:41 am

I have a stock of brazing rod called Sil-Fos. No flux needed with copper. A somewhat less expensive alternative to Silver solder. But I haven't tried it with ferrous materials. It's intended for only copper to copper I think.
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