Residual Stress Assessment in Japanese FFS Code for Pressure Equipment, HPIS Z101

Abstract

The High Pressure Institute of Japan published the first edition of Fitness for Service Assessment Procedure in 2001, HPIS Z101. Z101 is currently being revised and updated including the addition of Level 2 Assessment which involves more detailed evaluation regarding the structural integrity of cracked pressure vessels or piping systems than Level 1 Assessment. One of the notable revisions of Level 1 and the additions in Level 2 are the weld residual stress assessment of welded components. In the Level 1 assessment, the simplified residual stress distribution was defined in the current version. After publishing the current version, the committee has made an additional investigation into the residual stress assessment in existing FFS codes such as R6 and API 579 and has also compared the data in the technical literature on residual stress. The existent FFS code contains a certain degree of safety in residual stress assessment by making conservative structural integrity assessments for cracked pressure vessels and piping to prevent unexpected failure. However, the definition of residual stress profiles on the surface is controversial. In addition, some simplified definitions in the FFS codes do not always evaluate the measured weld residual stress safely. In this paper, the contradictions and controversial issues regarding weld residual stress distributions are discussed. Furthermore, the draft of revised Z101 is explained.