Proposed Design Criterion for Vessel Lifting Lugs in Lieu of ASME B30.20

Abstract

This paper describes a method for evaluating the structural adequacy of various lifting lugs utilized in the erection and up righting of large pressure vessels. In addition, the analysis techniques are described in detail and design guidelines for vessel lifting are tendered. The statutory and provincial regulations in both the United States and the province of Alberta, Canada are also reviewed and discussed with respect to the too often utilized phrase “factor of safety” (FOS). The implied implications derived from the chosen FOS are also outlined. A discussion is presented as to the applicability of the ASME safety standard B30.20 entitled, “Below the Hook Lifting Devices” (1999, ASME, New York) and as to the severe shortcomings of the safety standard in its attempt to delve into the design of lifting devices, especially when applied to lifting lugs on large and heavy-weight pressure vessels. Exemplar lugs on vessels are defined and the finite element analyses and closed form Hertzian contact problem solutions are presented and interpreted in accordance with the proposed design criteria. These results are compared against the very limited design information contained within ASME B30.20. Suggestions for the revision and applicability of the safety standard are presented and discussed in light of the examples and technical justification presented in the following paragraphs. In addition, the silence of this safety standard on the very large contact stresses that are well known to exist between a lifting pin and clevis type geometry is also discussed. Because of the limited number of repetitive loading cycles that vessel lifting lugs actually experience during the service life of a vessel, a recommendation is made to either clearly exclude vessel lifting lugs from the scope of ASME B30.20 or to specifically include a separate design and analysis section within this standard to properly address the mechanical and structural design issues applicable to pressure vessel lifting lugs.