Because for firebox material, which has been normalized, at a thickness over 2 inches the MDMT is somewhere above 60 degrees F. We have to be sure the metal temperature does not drop below that (for brittle fracture analysis since the vessels will see full pressure at ambient), but the site will see temps as low as the high 30s. So we take these old vessels and lower the pressure (stress) and reduce the MDMT by a stress ratio approach. This sort of analysis has been done on monobloc vessels, and is being asked for layered ones now.
So I have a vessel with a 3.75 inch inner layer surrounded by 12 0.25 inch layers, of higher toughness. I can find the MDMT at 3.75, but I have difficulty in the stress ratio approach since the MAWP requires those other layers. If I use the equivalent thickness, based on toughness of the two materials, the vessel comes out to be 5.22 inch thick. I can work with that if I assume it's monobloc. It just doesn't seem physically correct, unless those layers somehow contribute to brittle fracture.
