I would like to ask the experts on this forum a question regarding ASME Low Temperature UCS66
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ASME UCS66 specifies that the minimum MDMT must be analyzed for each section of the container. However, the control thickness is determined based on the welds; for example, in the case of a shell connected to a flange, the control thickness for the flange is usually the thickness of the shell at the connection point. Why isn’t the thickness of the flange itself used? Controlling thickness is really confusing; I don’t understand this concept. Thank you
Classified by component type, into mating components； Jointed components ； Non-welded components ； Different components are covered by different provisions. The diagram referred to as UCS-66.3 (c) in the question represents the butt-welded components, with the thickness controlled according to the thickness at the butt weld.
The entire UCS66 standard is formulated based on the principles of fracture mechanics. It is stated that there are two main factors that affect the low-temperature toughness of components: temperature and stress level. Controlling the thickness is, in fact, a way of regulating the stress level; therefore, the lower the thickness controlled on the curve, the lower the lowest allowable operating temperature. For welded parts, the focus of the specifications is actually on the weld area. According to the equal strength theory in the container industry, the strength of fillet welds is usually not lower than that of the base materials they connect to; therefore, the thickness control is determined based on the requirements of the shell.