David Dewey wrote:
"At the point where the barrel course intersect the front of the firebox plates (knuckle? I can't remember the tech term) there is an applied patch. This is an area prone to cracking, so that's not so unusual, but the CA state boiler inspector doesn't like it. I have looked at the patch, and it appears very proffessionally done, but I don't have a clue as to what the stress calculations show, or what ultra sound might show. The rumor is that the Boiler Inspector doesn't like riveted boilers at all, so. . . "
The transition from the firebox sheet to the barrel course is called a knuckle. The determination of the thickness is made from the required thickness of the adjacent stayed area. The radius of the knuckle is based upon the pitch of the stays in the stayed area. The efficiency of the rivetted joint will also affect the plate thickness. If you want to get fancy you can always go back to the theory of elasticity to determine the strains and resultant stresses.
As far as likes and dislikes of repairs, if the repair plate strength and thickness is at least equal to the original design, then I fail to see what is deficient. Of course, there is always a question concerning the strength of the welds. But that would be addressed by the welding procedure and the NDT methods.
With respect to riveted boilers that are 50+ years old, I have less concern about their integrity than I would for a welded boiler of the same age.
If you look in the current edition of the B&PVC Section I, the design of riveted boilers is referenced back to the 1971 edition. If there is concern about cracks originating from the rivet holes that are concealed by the rivet heads, the strength can be evaluated by a fracture mechanics approach comparing the margin of operating stress to failure stress.
My $0.02
