Session: CS-12-01 High Temperature Codes and Standards

Paper Number: 123438

123438 - Simplified Elevated Temperature Service Elastic Methods for Section Iii, Division 5 Class a Metallic Pressure Boundary Components 

The goal of this paper is to provide simplified elevated temperature service elastic stress evaluation methods and criteria that are numerically identical to the current Section III Division 5 rules but in a form that is easier to understand and apply.

Elevated temperature service elastic rules for nuclear metallic pressure boundary components were originally developed in the late 1900s first in code cases and then as part of BPVC Section III Subsection NH for Class 1 components in 1995.  During the elevated temperature stand-alone code rules effort in the early 2000’s, the rules moved to Subsection HB Subpart B as Class A components in the new Division 5 portion of Section III in 2017.  For the most part the elastic rules and methods have remained the same since their original publication in 1995.

The current rules cover several elastic evaluation criteria based on gross inelastic deformation, progressive inelastic deformation, fatigue and creep damage as well as some limited buckling rules.

Gross Inelastic deformation rules are based on primary stress intensity limits and use-fractions. The use-fractions rules can be simplified by defining specific load stress intensity values used to lookup allowed times from the St-vs-time curves.

Progressive inelastic deformation rules are based on primary plus secondary stress intensity range limits. These rules include several stress limits for negligible creep and significant creep.  Significant creep ratcheting, A-1 and A-2, tests can be combined and simplified by comparing the sum of the primary core bending stress intensity range plus the secondary stress intensity range to a significant creep stress limit. The negligible creep ratcheting, A-3, test can be simplified by defining a creep ratcheting stress limit based on the sum of the portions of the stress limit associated with the temperatures at cold and hot extremes of the stress cycle.

Fatigue and Creep damage and interaction rules are based on cumulative damage usage factors.  Fatigue damage rules include three methods to determine the modified maximum equivalent strain range; the bounding factor method, stress indicator method and the stress range method. The bounding factor method can be simplified by defining a bounding equivalent stress concentration factor based on the primary plus secondary stress intensity range and the creep ratcheting stress limit obtained from the A-3 test simplification. The fatigue damage rules also include a plastic Poisson ratio adjustment factor that can be simplified based on the bounding equivalent stress concentration factor obtained from the modified maximum equivalent strain range bounding factor simplification. Creep damage rules include four methods to determine the stress-time history based on a combination of single cycle or enveloped stress-time history, and isochronous or adjusted uniaxial relaxation. Simplification of these methods are not necessary, but clarification is provided on the different combination possibilities.

In addition to the elastic rules there are several inelastic rules included in the current code rules and recently developed code cases.  It is acknowledged that the current elastic rules may be overly conservative and that inelastic rules may be used when elastic limits are exceeded.  However, elastic rules can be a valuable screening tool to assist the designer in determining what portions of the design and loading conditions may require further evaluation by inelastic methods.  Providing simplified elastic methods will make it easier for the designer to understand and apply the code rules.

Presenting Author: Derrick Pease Becht

Presenting Author Biography: Mr. Derrick Pease has over 25 years of experience in mechanical engineering design and analysis. He is an expert in stress analysis of complex structures that include seismic, fatigue, creep, heat transfer, pressure transient analysis, fluid-structure interaction, impact, and explosion responses. Mr. Pease specializes in advanced finite element modeling and simulation of complex problems. Code experience includes application of ASME BPVC Sections III Div.1 and 5, Section VIII Div. 1 and 2, ASME B31.3, API-579, and others.

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