PSI - Issue 14

2

S.C.S.P. Kumar Krovvidi/ Structural Integrity Procedia 00 (2018) 000–000

S.C.S.P. Kumar Krovvidi et al. / Procedia Structural Integrity 14 (2019) 855–863

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1. Introduction Bellows are highly engineered components and find extensive applications in nuclear systems. Typical applications of bellows in nuclear industry include the bellows used in bellows sealed valves as primary leak tight barriers, bellows in piping systems to absorb differential movements and bellows used in reactor mechanism to prevent leakage of the active coolant/cover gas through the clearances in the mechanisms. Kumar Krovvidi (2017) presented typical applications of the bellows in nuclear applications as shown in Fig. 1.

Fig. 1 Application of bellows in (a) bellow-sealed valve and (b) piping system Conventionally bellows are designed as per standards of EJMA (2015). Design of the bellows for nuclear applications shall be in compliance with design codes such as ASME boiler and pressure vessel code, section-III, RCC-MR etc. Standards of EJMA have high operating experience of the bellows for operating temperatures below creep range of the bellows material. The design curve given in EJMA is a best fit curve generated based on testing of bellows. In the design curve given in standards of EJMA, neither separate factor of safety is considered nor a lower bound curve is recommended. In design codes for nuclear applications consists of factor of safety (FOS). Design of the bellows as per RCC-MR or ASME section-III require detailed finite element analysis and cycle life estimation using design fatigue curves given in the codes. In general formed bellows operate in the plastic zone and hence elasto-plastic analysis of the bellows predicts the life of the bellows more accurately. In this paper, the life of the bellows as per RCC-MR is estimated and compared with the same estimated based on standards of EJMA. The life of the bellows by analysis is estimated by simplified inelastic analysis, inelastic analysis using cyclic stress stain diagram and detailed inelastic analysis using non-linear isotropic-kinematic hardening model. The simplified inelastic analysis given in RCC-MR requires simple linear elastic FEA and the elasto-plastic stain is obtained by enhancing the elastic strain by suitable techniques [3]. In all the three methods of the analysis, the design fatigue curve given in RCC-MR is considered for estimating the fatigue life. The maximum value of the fatigue life of the bellows estimated among the three methods is considered as the fatigue life as per RCC-MR and the same is compared with the fatigue life estimated as per standards of EJMA. A factor is introduced in the design curve given in standards of EJMA to generate a lower bound design curve so that the life predicted by modified design curve is in agreement with the life predicted as per RCC-MR.