Boiler

Hi folks, 

I have rolled uo a boiler from a piece of flat sheet copper and have made a lap joint at bottom, this will be riveted and then silver soldered, is there any problem doing it this way ? The reason I done this is I had problems sourcing 31/4"" dia seamless tube I have found a source but of course there would be extra cost plus with tube extension pieces would have to be fitted at the firebox end  cheers Mick

I think the usual way is to have a butt strap inside the tube, this makes it easier to get a good seal with the smokebox.

HTH,

Rod

Butt strap outside is prefered as the inspector can see how the solder has flowed into the joint. You may also need to recalculate the thickness as the strength is derated if there is a joint over solid tube.

Best thing to do is ask your boiler inspector as at the end of the day it is him who will pass or fail the boiler and some have different views to others.

Edited By JasonB on 08/05/2017 12:35:15

Thanks, calculate thickness of what Jason?

Mick

The thickness of the barrel, solid drawn tube has a valuve of 1, single row of rivits 0.5, soldered or welded value of 0.8. *so either leave thickness as is and have a lower working pressure or up the thickness to compensate for the joint to keep the original working pressure.

J

Taken from Hainings "Countryman's Steam Manual"

Ive never checked this before but my 'industry standard' excel spreadsheet concurs with Jason's observation, there is a 20% reduction in operating max psi for a silver soldered joint (god only knows who comes up with these figures as if properly made it is at the very least as strong as the parent metal).

Okay thanks, think I may get the solid drawn tube, original won't be wasted as I can unroll it and use on other bits, thanks for the advice better be safe than sorry

Mick

I suppose the 20% allows for the fact that the joint may not be properly made, hard to be 100% sure that solder has flowed completly through the joint particularly if rivits are a bit too tight or there is the odd gap within the joint (out of sight) that is too large for the solder to bridge.

Mick,

I'd be tempted to check and then double check it before unrolling all your hard work. You might find that 1/16" thick or 16g sheet at 80% is still well inside the requirements and surely none of your work is lost if you need to put an external strap along the seam. Also, as you point out, the outer firebox extensions are a pain if you use tube.

The main question for me is which data do we use to check whether 80% of the sheet thickness is ok?

Phil H

A fair point Phil, I was quite pleased with it, will do some more scratching

Cheers Mick

16g tube at 3.25" OD is plenty thick enough for the 80 psi I suspect your boiler is designed for. I wouldn't use the joggled joint, as Roderick says the norm is to have a butt strap.

What you will have to watch out for is the stay pitch, which will be quite small for this thin material.

But it's no longer a (seamless) tube so at 80% it is effectively 18g tube.

As I said speak with your inspector as you are deviating from the published design, he may be happy for you to carry on, he may not. Best to find out now before investing more time and money into the boiler.

The problem with a tube is that the outer firebox wrapper has to have extension pieces butt strapped on the inside silver soldered in order to reach the foundation ring, does this make fitting the throatplate more awkward? Boiler steams at 80lb test pressure 160lb, so if I use 64lb working pressure and test to128ln will the engine still steam satisfactorily?

Mick

I put the numbers through what is probably the same spreadsheet that Fizzy refered to and you should still be OK with what you have and still be over a decent factor of safety BUT don't take my word for it check with the boiler inspector.

As Duncan says the stay spacing on the flat areas is the one to watch, what stay spacing do the drawings show and also what thickness is the firebox

Firebox wrapper 1/16" same as boiler barrel, throat,smokebox,firebox backplate, and backhead all 3/32"thick, stays 16 of on each flat side 3/4" apart, perhaps I could use a 3/32" buttstrap on barrel seam. Your advice on talking to a boiler inspector is the way to go

Thankyou Mick

Hi Jason and all - dont know if you have already seen it but the Ozzy boiler fed (cant remember their correct title) have published test results of different boiler end plates under pressure, from fully flanged to just a plate plonked over the end of a tube and poorly silver soldered on - result...no sign of any failure in any joint even when it was evident that there had been no visible signs of solder penetration. Reassuring if nothing else!

Looked in the model locomotive boiler book an hour or so back (Ill get the proper title etc tomorrow morning). Some of the data is a bit flimsy and wishy washy e.g., what safety factor you should use etc but when I put the numbers through, 16g seemed ok i.e., required thickness was 0.065" versus 0.064" for 16g and 1/16" is surely ok at 0.0625". ll type out what I found tomorrow.

Phil H

Just my own personal observation but all the 3.5"g miniature loco boilers I have come across have been 3/32" or 2.5mm outer wrapper and outer firebox. A joggled joint on the barrel was quite common years ago but is now frowned upon and as Jason says a seperate outside butt strap on same, but there are many miniature locos running around without this feature.

As others have said, check first with your club boiler inspector.

Cheers,

Julian

Edited By julian atkins on 09/05/2017 00:01:39

Just to back up what Julian says, with a 1/16" wrapper and 3/4" spacing of the stays punched into the spreadsheet the stayed surfaces come below the factor of safety used which is 5 and therfore below the desired working pressure.

And further to Phil's comment about 1/16" (0.0625) being surely OK in place of 16g (0.064" ),

1/16" material puts the stayed areas below the factor of safety

16g is enough to tip the balance putting headed stays just over the factor of 5, 5.01 to be exact!

I' m getting confused now, just miked the copper I have used for boiler and it is 0.065" , Martin Evans book for Rob Roy says 1/16"-- 16 g, copper and the stay details are from his book also, my mistake for saying 1/16"" instead of 16 g from Jasons calculations this just meets the safety requirements needed , I shall procede with caution thanks for the help gents it is much appreciated

Regards Mick