Design and analysis of formed bellows for nuclear applications- Case study

Abstract

Bellows are highly engineered components which find large number of applications in nuclear industry. Formed bellows are the most commonly used type of bellows especially when stoke is less than one thirds of bellows free length. Conventionally, design and manufacturing of the formed bellows are extensively addressed in standards of Expansion Joint Manufacturers Association (EJMA). In nuclear applications, design of the bellows shall comply with standard nuclear design codes such as ASME section-III, RCC-MR etc, which follow design by analysis. Hence, the design of the bellows for nuclear systems needs detailed Finite Element Analysis (FEA) and fatigue life estimation as per the design curves given in the codes. In this work, a case study of design of formed bellows, made of SS316LN, operating below creep range for nuclear application is presented. The bellows are preliminarily designed by EJMA and the geometry arrived based on preliminary design is analyzed using FEA. The stress/ strain range in the bellows is estimated by both linear elastic and inelastic FEA. Non-linear isotropic-kinematic hardening constitutive model is employed for inelastic analysis of the bellows. The fatigue life of the bellows is estimated as per the guidelines given in RCC-MR. It is found that the fatigue life estimated by linear elastic analysis is more conservative as compared to inelastic analysis. In the design fatigue curves given in the nuclear design codes, factor of safety (FOS) of 2 on stress range is considered. By considering similar factor of safety on the equivalent stress given in EJMA, the cycle life of the bellows is estimated and compared with the design life obtained by detailed stress analysis. It is found that after including FOS of 2 on equivalent stress, the life predicted by EJMA design curve is conservative. Hence, FOS modified EJMA equation can be used for design of the nuclear bellows when operating temperature is less than creep range of the bellows material. Use of FOS modified EJMA design equation is easy and eliminates the need for detailed stress analysis by the bellows suppliers. However, other aspects such as manufacturing tolerances, inspection, testing and qualification of the bellows shall comply with standard nuclear design codes.