A Comprehensive Review of Ultimate and Allowable Stress Design Methodologies for External Load Calculation of Petrochemical Equipment: Eurocode 8 Versus ASME Codes

Abstract

The purpose of this paper is to provide a review of design methodologies for process equipment under pressure subjected to seismic loads. Such equipment includes spherical and cylindrical tanks, pressure vessels including towers and reactors, and fired heaters. For this equipment, the wall thickness of the pressure retaining space is designed so that the hoop stress is a fraction of the yield stress, Sy, of the material of construction. This fraction of the yield stress is called the allowable stress, Sm, and it is used in the allowable stress design codes such as ASME and API. However the magnitude of the stresses due to external loads is not determined by code rules. The task of calculating the stresses due to external loads is left up to the designer. Furthermore, process equipment is often sufficiently massive so that anchorage is needed to avoid overturning and a potential fire hazard. The anchors or bolts are imbedded in concrete bases, which are designed using strength design codes such as UBC, ASCE or EUROCODES. The level of stress in such structures is allowed to reach the yield stress of the material of construction. The safety factor in structures sized using allowable stress design philosophy is taken in the allowable stress. The safety factor in structures sized using strength design philosophy is realized by using load factors or increased loads. Guidelines are provided to solve the problem of merging the two design philosophies while avoiding the application of safety factors twice in the mechanical design of process equipment subjected to external loads and pressure.