ASME code explicitly addresses design for fatigue due to pressure or temperature cycles. Protection against fatigue failure due to cyclic external mechanical loads (e.g. piping loads) is not tackled in depth.
This paper provides a less-tedious yet fit-for-purpose approach to evaluate the effect of cyclic external mechanical loads as well as the pressure fluctuations — as a result of piping slug flow — on nozzles fatigue life. The evaluation compares between two types of nozzles construction (configuration); separate reinforcement nozzle and readily radiographed (lip type) integrally reinforced nozzle. Within the analysis, a unity fatigue damage ratio or exceeding the ratcheting allowable limits was selected as the indication for the inadequacy of the reinforcement configuration.
COMPRESS® FEA software results comprehensively predict that the separate reinforcement nozzles can’t withstand the imposed cyclic loads since the accumulated fatigue damage are greater than 1 (one) implying that the nozzle will experience fatigue failure before the end of its life time. COMPRESS® FEA results were examined at four locations, namely “shell next to nozzle”, “nozzle next to shell”, “nozzle thickness transition” and “shell away from nozzle”. The maximum stress of the four locations was always at “shell next to nozzle”.
These results have been verified against SOLIDWORKS® simulation FEA results.
The results show that the separate reinforcement nozzle construction, although adequate for static loadings, has less fatigue life compared to the integral reinforcement nozzle construction. Moreover, progressive distortion of the non-integral (separate) reinforcement connection is predicted showing that the mating members may become loose at the end of each complete operating cycle which could eventually cause disengagement. Additionally, the results support ASME 2004, VIII-2 para. 5-112 recommendations in prohibiting fillet welds in joints of category D for components subject to fatigue service.
The paper concludes the advantage of integral reinforced readily radiographed nozzle construction in protection against fatigue failure and ratcheting and also provides a roadmap for simplified fatigue analysis using commercially available software.
