Steam locomotive boilers made in the US since 1900, almost without exception, were made with radial stayed fireboxes. By definition, a radial stayed firebox has the crown sheet rolled to a constant radius transversely, and the staybolts are equally spaced, perpendicular to the crown sheet intersection, with the staybolt centerlines converging to the common point on the firebox longitudinal centerline.
A crown bar supported crown sheet is an example of a firebox that is not radial stayed.
The maximum allowable working pressure of a radial stayed wrapper sheet of a locomotive boiler has been addressed in the ASME Code at least back to the late 1920's by the rather convoluted equation:
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This equation applies specifically and ONLY to RADIALLY STAYED LOCOMOTIVE TYPE BOILERS. As exemplified by the accompanying drawing Fig. P-24 from the 1940 ASME Code.
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If the crown stays are NOT in this radial configuration, the staybolts must be placed in the wrapper sheet at a maximum pitch not exceeding that calculated as if the wrapper sheet was a flat surface. For 3/8" thick SA285 Gr. C with 55,000 tensile strength @ 200 PSI the maximum pitch is about 4-1/2".
More than a few "modern" welded locomotive boilers have been fabricated not in compliance with this Code requirement, which by the way, exists in the Baldwin Standard Practice also. The result of an inadequately stayed wrapper sheet is outward bulging of the wrapper along the longitudinal centerline resulting in excessive and permanent downward deformation of the upper wrapper sheet. This results in downward deformation of the crown sheet leading to excessive premature stress corrosion cracking of the crown/rear tube sheet juncture and crown stays. The upper row of side sheet staybolts, due to the excessive load, suffer accelerated cracking and corrosion.
Just because it has a Code stamp does not guarantee Code compliance. (Unfortunately).
Here is the full text of P-212 from the 1940 ASME Code.
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