Materials, Fabrication, Installation, and Applied Mechanics Considerations in the Catastrophic Failure of a Flex Hose Bellows

摘要

This paper is the third in a series that describes the materials, fabrication, installation, and applied mechanics considerations surrounding the catastrophic failure of a bellows component within a metallic flexible hose. The subject flexible hose was utilized in a compressor piping system attachment juncture to a petro-chemical piping system designed in accordance with the ASME B31.3 Process Piping Code. Specifically, the ultimate failure mode issues related to the instability of the metal u-shaped bellows, from which the hose derives its overall flexibility and name, are reviewed and discussed in detail. In an effort to provide a comprehensive examination of the use of the flexible hose in the petrochemical industry, a discussion of the materials, fabrication methods, installation, and, applied mechanics associated with column buckling of the bellows (also known as "squirm") are presented. A metallurgical failure analysis is presented to identify and document the mode of failure and metallurgical condition of the wire braid and bellows components of the hose. In addition, material examination results, including the discovery of inherent flaws from the fabrication process, are presented and the significance of the findings is presented. The selection process for this particular type of flexible hose (and bellows component) for eventual installation in a vibratory service environment is reviewed in light of the published recommendations provided by the rules of the Expansion Joint Manufacturers Association Standards (EJMA) with regard to squirm are also reviewed and discussed. Finally, a summary of the elastic-plastic finite element analysis of the u-shaped bellows is briefly described and compared against previously published theoretical works on the instability of shells of revolution and most specifically, toroids. The results of the theoretical, empirical, and analytical forensic investigations into the squirm phenomenon are utilized to identify some very practical recommendations in an effort to minimize the probability of catastrophic failures of u-shaped bellows from column type instability.