ASME NTB-3:2020 pdf free download – Gap Analysis for Addressing Adequacyor Optimization of ASME Section lll,Division 5 Rules for Metallic Components
1.1.1 Summary
O’Donnell, Hull and Malik [1] summarized this issue in Item OG-10 as follows:
Appendix T in NH [7] provides three expressions for determining strain range [8] usingelastic analysis and， if these rules cannot be satisfied,additional rules are provided,presumably less conservative, based on the results of inelastic analyses which requiredetailed constitutive models of material behavior under time varying loading conditions.For the CRBR,these behavioral models were a contractual provision based on RDTStandards.These applicable standards are no longer maintained and there have beennumerous technical developments in this area since then [9].Appendix T rules cover strain,deformation, creep and fatigue limits at elevated temperatures for 304SS/316 SS(816°C),Alloy 800H(760°C),2.25Cr-1Mo (593°C),9Cr-1Mo-V(649°C). Development of materialmodels for materials not currently covered or for temperatures beyond their original rangeof verification will be a considerable effort. Modifications in Appendix T rules for highertemperatures and additional materials (e.g.,Alloy 617, Hastelloy X/XR) may be needed.1.1.2General Assessment
Appendix HBB-T of BPVC Section IlI, Division 5 provides procedures to evaluate strain limits and creepfatigue damage using elastic analysis. Alternatively, for some Class A materials, recent code cases provideprocedures based on Elastic-Perfectly plastic(EPP) analysis. If these rules cannot be satisfied, additionalrules are provided which are based on results of inelastic analyses. However,inelastic analysis requiresdetailed constitutive models of material behavior under time varying loading conditions. For the CRBR,these behavioral models were based on Nuclear Standard NE F9-5T. This standard is no longer maintained,and numerous technical developments have been made since. However, ASME has established a workinggroup to develop inelastic analysis methods and constitutive models for Class A materials for incorporationinto BPVC Section lll, Division 5. Models for several materials have been developed and are ready forASME Code action [ 10], and the development process continues in the working group for the remainingmaterials.
1.2.2General Assessment
The current strain accumulation rules have geometric and service level transient design restrictions. Therules are also complicated to apply. The BPVC Section IlI, Division 5 elastic rules for strain limitsevaluation are based on the decoupling of creep and plasticity. For temperatures above a certain cut off, thedecoupling of creep and plasticity can no longer be justified and a unified viscoplastic model is necessaryto describe the deformation behavior. The recently developed EPP methodology for strain limits evaluationof Type 304 and 316 stainless steels does not depend on the decoupling of creep and plasticity. It has beendemonstrated by tests to be applicable to the full temperature range permitted code allowable stresses,including very high temperatures where creep and plasticity are no longer decoupled.Cut off temperaturesfor the Class A materials have recently been established [ 12].
1.4.2General Assessment
The conservatism of the current creep-fatigue rules was increased with a more conservative design factorbased on the results from the Eddystone Power Plant (supercritical steam plant) failure and subsequentthermal shock tests results and analysis through a Department of Energy (DOE)/United Kingdomcollaborative program.As a result, the current BPVC Section lII, Division 5 creep-fatigue design rules areconsidered to be overly conservative and very complex to apply.There are efforts by ASME to reduce thecomplexity by developing EPP technology.The over conservatism is being addressed by the developmentof integrated EPP and Simplified Model Test (SMT) creep-fatigue design methodology.