Analysis of the integrity of a single stud assembly with externally corroded nuts

Abstract

The aim of this study was to determine whether a single 7/8′′ nominal diameter stud bolt and nuts assembly would fail to perform its required function in a flange assembly when the nut was corroded. This work was undertaken to form a basis for further work in development of a fitness-for-service assessment for corroded nuts. No current quantitative assessments exist for this area. A failure criterion was defined as a minimum 206.8 MPa stress in the stud, based on the bolting requirements stated in ASME B16.5-2003 pipe flanges and flanges fittings. A simple test rig was manufactured for testing the compliance of the corroded nuts subjected to the 206.8 MPa stress level and the corrosion was simulated by removing uniform layers of material from the surface of the nut. Finite element analysis (ANSYS 12.0) was also performed to evaluate the test rig and to consider the interaction between the nut and stud at the thread roots. It was found that no failure occurred in the threading and that failure of the assembly only occurred when there was a >60% material loss from the nut. The failure mode experienced was deformation of the flange plate. This analysis proved that a nut can experience significant effective material loss without damaging the flange integrity.

Keywords

Fitness for service corrosion flange joints fasteners oil and gas petrochemical

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