B31.3 PIPING CURVE B vs ASMEVIII/API-579 CURVE B

[robsalv]

Folks,

Regarding the curve B in fig UCS 66 and curve B in B31.3 fig 323.2.2A - which cover equivalent materials from a piping point of view, there's a slight difference in allowable MDMT without impact tests in the 10 - 13mm (0.394" - 0.5")thickness.

Any thoughts as to why?

Cheers

Rob

 

[metengr]

I believe the variation of the thickness/MDMT Curve B in Figure 323.2.2A of ASME B31.3 and Curve B in ASME Section VIII, Div 1 Part UCS and UG have more to do with design margins or conservatisms used by the specific committee or subgroup in formulating rules. For low temperature applications, thickness plays a significant role in affecting impact values because of material constraint. Thicker materials will exhibit lower impact properties, and as such, need to be adjusted for this effect.

The thickness effects are less pronounced for materials in ASME B31.3 versus ASME Section VIII because Code/Standards committees are unique in that they set their own agenda regarding rules for design and fabrication based on past experience. If you review the various Code sections, one can find differences in how materials are endorsed especially between applications for Section I and Section VIII, and comparison with the ASME piping codes.

There is really nothing wrong with being slightly more or less conservative or having experience with material that allows rules to be formulated. One just needs to use the appropriate code for their application and follow the requirements. One should not attempt to "cherry pick" between various Codes and Standards to better suit their design needs.