Duplex SS (UNS S31803) - Low Temperature 1

[gvn]

Per ASME B31.3, Duplex SS can be used without impcat test up to -60 deg F (irrespective of thickness).
However, per ASME VIII Div 1, para UHA-51(d)(3) impact test is required for nominal material thickness greater than 10mm even for -20 deg F and warmer.
Any clarification on the above anamoly will be appreciated.

 

[metengr]

ASME B31 Piping committee and the ASME B&PV Code committee are not one in the same committees, despite having the ASME banner. They each have their respective approaches regarding use of material and design philosophy.

To basically address your question, pressure vessels can be designed with very thick walls resulting in more thru-wall constraint on tensile strains induced by service stresses (plane strain conditions). Higher thru-wall constraint requires tougher material behavior to reduce susceptibility to brittle fracture, if flaws are present.

Piping, being thinner walled, has less of an issue with constraint. So, other approaches can be used in design.