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.
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
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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