Some extracts from the UK Federation of Model Engineering Societies:
Notes on the Design and Construction of Steel Boilers:
Experience shows that it is difficult to design and build a steel boiler with a barrel diameter of less than 200mm (8”). It is equally increasingly difficult to make a copper boiler with satisfactory joints over this diameter. The economics also tend to favour the 200mm/8” divide as the high price of copper has to be offset with the costs of design approval and in build inspection for welded steel.
Steel is comparatively cheap and although the plates in steel boilers are stronger they are thicker to allow for corrosion. BS2790 specifies a minimum thickness of 6.5mm or ¼”.
The design and construction of welded steel boilers is covered by BS2790.
If you are obtaining a copy of this standard be careful to check that the revision does not exclude locomotive boilers. When referring to this standard on drawings or specifications the part and revision date must be quoted.
These notes all refer to BS2790 Part 2 1973. (Refer to LATEST version, as this has been superceded!).
The formulae in all the revisions are the same but the diagrams, formula numbers and examples change together with their number.
The Euro-norm Standard for welded steel boilers specifically excludes locomotive boilers.
BS 2790 is in metric and hence Lengths are in millimetres and Pressure, Stress and Tensile Strength are in Newtons/mm2 (N/mm2).
1 Newton = 0.225lb. 1 N/mm2 = 10 Bar = 145 lb/in2
The boiler barrel should, wherever possible, be made from ASTM A106 grade B. This is hot finished seamless refinery tube as used in the oil industry and is normally easy to obtain.
The welding must be carried out by a welder who has current approval to BS EN287:2011 for the joints used in the boiler. The welding rods must be specified on the drawings and be suitable for the materials used. Certificates of Compliance should be supplied for the rods.
Fire tube should be expanded in and made from either cold drawn seamless tube which has been bright annealed or seamless copper tubes.
A comment from "the inspector": "I have seen an 8mm fire box wrapper reduced to 2mm thick in about 15 years of occasional use on a boiler which was well looked after".
In the UK: Normal service repeat testing is an hydraulic test at 1.5 x Working Pressure followed by a Steam test to ensure safety valves, pumps, etc. all work at max FIRE, and max steam pressure. - e.g. if the safety valve works OK with full blower on the full fire, but any feed pump, injector or valve does not work, then that constitutes a failure of the steam test. The initial test (to confirm the DESIGN and Construction) shall be at 2 x the NWP.
In the US: the Hydraulic test of a steel boiler shall be at not less than 1.5 x the Maximum pressure (safety valve limit pressure). ASME for Copper boilers states the hydraulic test shall be at an equivalent pressure to the stress value limit at the steam pressure of the safety valve limit x 1.3: I.E. 1.3 x the Max pressure multiplied by the ratio of the stress limit at the temperature of the max steam pressure divided by the stress limit at 70deg.F.: e.g. At 100psi Max steam pressure (=393deg.F.) this is 2.77 times the max steam pressure = 277psi. (Max allowable stress is 6700psi at 70deg.F. but is only 3754psi at 393deg.F.).
ASME for safety valves: they shall not permit the pressure to rise more than 6% above the NWP for the boiler.
Hope this helps?