Your interpretation is correct. As per ASME Sec.VIII Div.1, while determining impact requirements, only Connected shell thickness is the basis of determining minimum temperature allowed.
While as per PD-5500, IF L>4e2, then governing thickness will be e2 i.e go else maxi.(e2, e1/4) for determining minimum temperature allowed.
In UCS(a)(1) a & c, it is clearly written that the governing thickness tg of a welded part for for flat heads or tubesheets will be the larger of (b) or the flat component thickness divided by 4.
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Thank You & With Regards-
Mayank Mittal
ISGEC Heavy Engg. Limited,
Haryana, India