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Soldering flue tubes

Posted: Mon Jul 04, 2016 6:55 pm
by little giant
I have heard some boiler makers silver solder flue tubes. How does one silver solder copper tubes in a steel boiler?

Re: Soldering flue tubes

Posted: Mon Jul 04, 2016 11:29 pm
by daves1459
This is not difficult. I have done it several times. Of course there is more than one way to silver solder the copper tubes. This is the way I do it. The tube should extend 1/16" to 3/32" past each plate and the bore should be .003" to .005" larger than the tube and with a square file make 3 or 4 axial notches .005" to .010" deep evenly spaced around the bore. You'll want to stand the boiler on each end so the solder flows around the tube and through the bore to fillet on both sides of the tube sheets. Solder the firebox end first. Therefore you'll need some way to hold the tubes in place for the first soldering or the tubes may drop through. If the boiler is not completely assembled machine a .010" shoulder on the tube front end to rest on the inside of the front tube sheet. If the boiler is completely assembled you'll have to rig up some sort of spacer to rest on to hold the front end of the tubes in position. The joints must be clean and bright and use a free flowing solder that will fillet, 1/16" diameter SafetySilv 56 works well. Liberally flux all joints, the usual white borax flus is fine. Coil the solder around a rod so when relaxed the coil will slip over the O.D. of the tube. Cut the coil into closed end rings and place a ring over the end of each tube and cover each ring with flux. Your now ready to apply heat. The trick is to heat the joint and let the solder flow into the joint. Avoid putting the torch directly on the joint until, at least, the flux has melted into a clear liquid. You'll need a lot of heat. I use two torches. One is a large propane torch around the periphery to bring the entire area up to near melting temperature. The other is oxy-cytelene(sp.) with medium rose bud tip for localized areas and the tube sheet center. A welding tip is too small and heat concentrated. I hope all of this is some help. Depending upon your mode of construction some of the above will have to be adjusted. Such as if you build your fire box first and slide it into place with the tube soldered. Even with that the actual fits and soldering process remains the same.

Re: Soldering flue tubes

Posted: Tue Jul 05, 2016 8:12 am
by Fred_V
Why go to all the trouble? Just roll them in. Tubes expand at a different rate than the boiler and allowing for some slip is a good idea.
Just my opinion.

Re: Soldering flue tubes

Posted: Tue Jul 05, 2016 9:48 am
by NP317
Opinions and options:
Rolled tubes are traditional, work well, can develop leaks requiring re-rolling, require special rollings tools, and make replacement simpler.
Silver brazed tubes take LOTS of heat to properly solder, will never leak, and make replacement more difficult.

Fortunately, copper tubes will outlast steel tubes from a corrosion standpoint, unless scaling is an issue; true for all tubes.
Opinions and Options.

Re: Soldering flue tubes

Posted: Tue Jul 05, 2016 11:31 am
by little giant
The tube sheets were reamed .020 oversized, they should be less than .010 max. I rolled the copper tubes in and had 2 tubes that would slide causing the boiler to leak when hot, then reseal when cold. I removed the 2 tubes and rolled new ones in. Now some of the other tubes are starting to do the same thing. When I tried to reroll the tubes the tube roller maxed out and can not go any farther. I also tried a 1/2 of a degree taper pin and still no luck. Next option would be to roll in .065 wall copper tubes, and if that fails solder in the tubes.

I was planning to use .005 x 1/2 wide solder wrapped around the tube with a ring on top with black flux hoping to seal everything up. If that fails then get a boiler from Marty Knox, been having trouble far too long.

Re: Soldering flue tubes

Posted: Tue Jul 05, 2016 4:12 pm
by James Powell
Could you swage out the ends of the tubes to give a slightly larger diameter at the ends? Rolled in copper tubes are my dad's favored way of dealing with smaller models (~2-3") with steel welded firebox/barrel/front tube sheet, and the copper is far easier to roll than the steel, so it works well. I have a row where they have crept quite a ways in the plate, they are very well tightened via expander, but at some point they need to be redone. It is all a question of trying to get the maximum life out of what is a consumable in a normal boiler- tubes lasted about the 7 years used as the replacement period (hence WHY the 7 year figure for FRA/ect) when in regular service. Copper tubes will outlast steel, but the affixing method is a question mark. If the boiler was new, I would contemplate silver soldering, but on an older boiler you are very unlikely to get it clean enough to get good take up of the silver solder.

Let us know how you make out with this,


Re: Soldering flue tubes

Posted: Tue Jul 05, 2016 5:13 pm
by Marty_Knox
The problem could be the holes, not the tubes. For an effective seal the hole needs to be round, but not smooth. You want a slightly rough surface so the tube will grip. I have found the best preparation is to ream the holes after welding. I ream the holes dry, without any cutting oil. You do need to clean the swarf off after each hole.

Re: Soldering flue tubes

Posted: Tue Jul 05, 2016 6:31 pm
by Greg_Lewis
In what's left of my memory I recall someone who has bult a number of boilers who just drills the sheets, leaving them at that, and then rolls the tubes in. He claims no leaks. Unfortunately I can't remember who that was.....

Re: Soldering flue tubes

Posted: Tue Jul 05, 2016 8:46 pm
by little giant
James- I had tried swaging the ends of 1 tube by using 4 pins .005, .010, .015, and .020 bigger than the ID annealing the tube between each pin. It brought the OD to within .003 of the tube sheet and then the roller maxed out and then I used the 1/2 degree tapered pin to get it sealed for a cold hydro test. That tube seems to be holding so far, just some of the other tubes are starting to leak a little.

Marty, Greg- The bore of the tube sheet was very smooth with the edges of the hole not sharp, but a slightly rounded corner less than a .010 radius. I thought it was the corners that held the tube in place, but I can agree that the tube can and will move inside of a smooth bore.

Now the question becomes how to roughen up the bore without making the hole out of round or any larger than it is?

Re: Soldering flue tubes

Posted: Tue Jul 05, 2016 8:50 pm
by Bill Shields
Listen to Marty.

He has rolled more tubes in more boilers than all of you (and me) together...and I have done more than a few....1000 of many sizes

We both learned as 'big boiler' people, (which includes industrial heat exchangers), and what he says is gospel truth.

Dave is correct...why would you want to silver solder them in when rolling / expanding is so easy?

Other than 'rolling' with a proper roller, on occasion I have expanded with drift pins and a rubber expanding plug made from faucet washers.

In reality, the boiler is the most $$$ part of the loco, and spending a couple of extra $$ on a good roller is small change.

I cannot say that I have ever had to re-roll a copper tube that passed a hydro.

Yes, I have had a few that leaked initially but never one that wouldn't seal.

Re: Soldering flue tubes

Posted: Wed Jul 06, 2016 7:51 am
by Fred_V
What size tube are you using? I know that you can get 3/4" OD tubes and 3/4" ID/.820 OD tubes. Maybe you could go up to the next size and try again.

Re: Soldering flue tubes

Posted: Wed Jul 06, 2016 11:15 am
by little giant
Here is my plan of attack:
1) Remove all tubes
2) Ream holes to roughen up surface
3) Buy .065 walled copper tube
4) Swage the ends to fit closer to hole diameter
5) Roll tubes in
6) Report outcome

Thanks to everyone who helped me out