Reliability of Fitness-For-Service assessment has become more important especially for the aged pressure equipment such as towers in process plants put in service operation over decades. The effects of partial metal loss on buckling strength of the towers subject to overturning moment due to seismic or wind load are one of the critical issues to be clarified. The present paper simulates the buckling strength of towers under overturning moment by means of finite element analysis considering the condition that the shell has suffered from partial metal loss, and evaluates the validity of the buckling stress formulae of API 579-1/ASME FFS-1, NASA, and Donnell. It has been demonstrated that the buckling strength predicted by API formulae shows fairly good agreement with that simulated by finite element analysis. Finite element analysis results have shown that the axial length of metal loss does not affect the buckling stress very much while the buckling stress depends on the circumferential width of metal loss. It has been revealed that the API formulae underestimate the buckling stress when the width of metal loss is smaller than about 30 deg. The paper proposes the modification to the API formulae in this condition that gives more accurate buckling stress than the original formulae.
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