Hi,

I have a non-standard weld neck body flange in a heat exchanger. Flange thickness is 123 mm whereas shell thickness is 19 mm. PV Elite 2011 calculated MDMT based on 19 mm thk at weld joint and not 123/4 = 30.75 mm. Is the governing thickness in this case not (flange tnk)/4? I could not find a clear guidance in the code. Fig UCS-66.3(c) appears to be ignoring flange thickness of the welded flange and is only taking the bolted blind into account. If that is what it is intended to be shown there, why whole UCS-66 uses word "flange" instead of blind flange in general?

PD5500 does take flange thickness of a weldneck flange into account. Refer Table D.4 Item 10. Can’t believe it should be different in ASME.

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.