Model Steam Boiler Welding

I was thinking about making a model steam engine. I have a few books which shows how to make the boiler using silver soldering, but I don't
want to use this.
I was thinking about using my Dad's MIG welder. Is this possible?
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My neighbour does model locos and has said that silver soldered you can do yourself and then get a safety certificate from a tester when it passes checks such as visual inspection and twice working pressure hydraulic test. For welded boilers you have to be a coded welder qualified for boiler work or you won't be allowed to use it. This of coarse applies to any use in public what you do yourself is up to you but the consequences of a boiler failure could be lethal.
Hoticon wrote:

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How big a boiler are you talking about? What pressure? What type?
I do engineering for an ASME/NBIC boiler shop and would recommend using a standard design, and constructing it EXACTLY as designed, even a very small model boiler. As you gain experience and education, then you might consider your own modifications. The small 'toy' boilers (most assuredly not toys), externally fired and about 6" long by 3" diameter and fired on alcohol, used with small model steam engines, are relatively safe if properly built. The power rating on these is miniscule, and the heating method makes it unlikely that heating will be fast enough to bring the shell to a high enough temperature for it to fail, even if run almost dry.
See the NBIC web site http://www.nationalboard.org/ for some interesting reading about boilers large and small.
A few notes as to why: a) since you need to ask, then I would seriously not consider using anything but the joining method in the design. Modifying a boiler design is something for a specialist. By asking, you indicate that you have the common sense to realize that you arn't one, so this same common sense will tell you not to do it. I spent a lot of my time doing engineering evaluations on boilers, and I worry about even very minor changes. For example: When silver soldering the boiler, the heat is reasonaby evenly distributed about the joint area, giving minimal (but not zero) stress when the joint cools. Welding puts much more heat into the joint, and does it at a point that travls around/along the weld. This traps much greater stresses than the brazing process. Not a problem if proper procedure is followed and the design accounts for the stresses. If not, you end up with a high likelyhood of failure. One of the recent live steam mags had photos of a guy TIGging a boiler together. I can't make any comment other than that he discussed the leaks at the joints in the article. It didn't worry him. It should have.
b) I wouldn't use MIG for doing joints on a boiler. There are very few commercial shops that do, and there have been some prominant failures in pipeline and component fabrication. It is a developing field, and still in the realm of specialists. It is way too easy to make a weld that looks good and isn't. TIG and stick, as well as silver braze, are much esier to detect flaws visually, during and after the joining process, especially for less experienced operators. A great deal of skill is needed to get a good root on the open joints typically seen in a boiler.
c) Depending on your location (jurisdiction) and the size of the boiler (heating surface, operating pressure, rated HP, construction method) you may or may not need to be certified to do the construction, may or may not need outside inspection, and may or may not need a licence. Where I am (NJ, a National Board state) I believe the cutoff for needing an operators licence is 10 square feet of heating surface, but I may be out of date. The rules get more complex when you start worrying about fabrication and repair, especially at higher pressures.
d) For vessels requiring certified fabrication/repair, THERE IS NO DISTINCTION between welding and other joining processes. The process must be performed using a certified procedure, and the operator must be certified (usually under the shop doing the work-in general, you change shops, you recertify in the new shop using their procedures) Most shops purchase prequalified procedures; large shops, especially specialty shops, often qualify their own.
If the boiler is very small and operates at low pressure, as many model boilers are, then you have few if any regulatory worries. Safety is another matter, and you do want to hydro the boiler before running it, and please be sure that the materials you use are ok for the heat and pressure you will operate at and the heating method you use.

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David Billington wrote:

Also, here in Britain, a coded welder will have to get certification for the steel plates/tubes used, before an Insurance Co. will cover it.
Dave.
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