Sustained Stress Indices (SSI) in the B31.3 2010 Edition

Abstract

The 2010 version of B31.3 introduced sustained stress indices (SSI's) in paragraph 320. Using methods in references [1],[2],[3],[4],[5], and [11], a test procedure was developed to evaluate these SSI's for standard metallic piping components. The test procedure has been incorporated into draft versions of B31J so that the sustained stress index can be produced at the same time stress intensification or flexibility factor tests are performed for a particular component. This paper describes the sustained stress index and the B31J test procedure used to determine the SSI. NOMENCLATUREde = elastic specimen displacement calculated from de = Fm / Ee. D = mean diameter of pipe Dnom = nominal diameter of pipe dn = displacement at the measurement point calculated from the elastic model of the test assembly used to find the elastic constant G 1

Stress intensification factor, sustained stress index and flexibility factor analysis of large D/T elbows

“…Despite their widespread use, sustained stress indices are scarcely investigated in the open literature. Although the authors in literature 21 indicate that test data pertaining to elbow sustained stress indices are given in the commentary of the draft ASME B31J code, these data are not available in the open literature. Calculation of sustained stress indices by testing should be obtained using limit-load analysis, as explained in literature, 6 by means of twice elastic slope (TES in further text) procedure.…”

“…As for the sustained stress indices, the design codes typically do not specify dedicated equations, but refrain to calculation of these factors based on the stress intensification factors. This approach is discussed in literature, 21 which presents a brief background in the approach adopted sustained stress indices in piping design codes alongside most important references. As for the approach adopted in the current piping design codes, literature [1][2][3] generally specify that the sustained stress index can be calculated using the stress intensification factor multiplied by 0.75 (calculated sustained stress index must always be greater than 1).…”

“…22 , 24 It should be noted that the indicated references specifies sustained indices ( B2) as defined in nuclear piping design code (ASME Section III). As explained in literature, 21,25 due to the difference in the definition in the allowable stress between the codes, this factor B2 needs to be divided by a constant of 1.5 before being used in calculation as defined in design codes. 1–3…”