Casing seal attachment

An improved transition seal arrangement for sealingly connecting a casing to a wall of a vapor generator, the wall being of the type having a plurality of parallel, laterally spaced tubes and a plurality of bars disposed between and weld united to adjacent tubes to define a wall surface with longitudinally extending circumferential surfaces and intervening planar surfaces, wherein the improvement comprises an elongated plate welded gas-tight to the wall along a lengthwise edge and weld connected gas-tight to the casing, the lengthwise edge having a plurality of arcuate saddles and protrusions formed in alternate succession along said edge, each of said saddles overlapping a circumferential portion of one of the tubes and wherein the saddles and the protrusions extend from the lengthwise edge, substantially parallel to the tubes, for a distance less than the width of the plate, the protrusions having a depth which diminishes as the distance from the lengthwise edge increases.