Soldering front tube plates

[waggy]

Hello all,
Yet again I seek the advice of the forum. I've taken on the completion of a small boiler for a friend buliding a freelance loco. The barrel is 6" O/D with10" long tubes. I'm having problems with tube leakage after cooling down from silver soldering the front tubes. When the thing is hot and the solder flows I can see it's gone all round all the tubes, when it's cooled off, some of the joints are cracking and leaking. I presume this is a problem caused by expansion / shrinkage of the barrel and tubes?
On boilers I've done from scratch, I always complete the front end with the tube nest and inner box in place before touching the firebox, etc. Never had a problem. On this boiler the firebox is fully soldered up.
What do other boiler builders do / recommend?

Waggy.

[Shawki Shlemon]

Are you sure it is cracking and not lack of penetration ? Are the tubes reasonably loose in the respective holes , a clearance is needed for proper penetration of silver solder . I am assuming that all is very clean . Are you allowing it to cool before cleaning ? I have done similar job and I have done repair to this area and had no problems . I suspect lack of penetration , clean well , heat all the boiler to very warm temperature , apply plenty of flux , heat uniformly the front tube area and let the solder run , keep the heat until you feel all joints has full penetration ,let cool then clean . I hope this may help .

[steam4ian]

G'day Waggy

I'm no expert.

Working on my own boiler recently with an experienced friend we were at pains to heat the whole of the boiler. I had to repair leaks around the flues at the firebox end. My friend did the cleaning yet again after I had done it, Whilst he heated around the tube plate with an oxy flame I poured heat onto the outside of the firebox ith twp burners; we had the boiler enclosed in fire bricks apart from where I was heating.

So far the result is most pleasing. My suggestion, heat the barrel as well as the tube area and enclose the barrel in firebrick so it cools reasonably slowly. Hopefully the barrel and tubes will expand/contract about the same amount.

Regards
Ian

[klendo]

Assuming the tubes and the tubeplate are made of the same material, then they will expand and contract at the same rate, copper CTE is 17ppm @ 20°C.
assuming copper is heated to about 800°C using a tube of 1/8" the tube will expand and contract by just shy of approx 2 thou or more precisely 42.1µm's.
I would question the penetration of the silver solder myself.

[steam4ian]

G'day Klendo

I would not contradict your comment about solder penetration.

There will however be temperature differences between the tubes and the barrel such that differential expansion will occur unless some effort is taken to minimise the temperature differences. After the job is done the barrel will cool more quickly than the tubes.

If the joints are structurally sound would caulking with either Comsol or Loctite be an answer?

Regards
Ian

[klendo]

G'day Ian,

Why would they contract at a different rate?, they are effectively 1 mass when together, I dont follow you logic.

The only other thing which I think has already been said, is that there maynot be enough gap between the tube and the hole.

[waggy]

Hello all,
Penetration isn't the problem. I can see the top row of tubes from the backhead regulator bush, solder all around the tubes. I presume if the top row is OK, the rest should be too as they were all fitted with a gap and a nick to allow the solder to flow.
I can understand why the barrel will cool quicker than the tube nest, the tubes are encased in "an oven" for want of a better description. Much less air flow through the tubes than along the outside of the thing.
Anyway, it's just been thoroughly heated through and resoldered, been cooling for half an hour now and still too hot to handle. Fingers crossed.
Waggy.

[waggy]

Thanks to all for the tips and advice. The problem was lack of heat, I set the boiler up on a frame to bring it up to comfortable working height. (The "frame" was two car ramps on end with a plate across the top.) The whole thing was heated up for about ten minutes with a 2942 burner. The front tubes were then finally heated using oxy acetylene, all tubes had the shiny ring around them. The whole was then allowed to cool for a couple of hours, bingo! Not yet pressure tested but holds 30 psi of air for ever. Now back with the owner to arrange full testing.

Waggy.