Frangible Roof Joint Behavior of Cylindrical Oil Storage Tanks Designed to API 650 Rules

This paper presents the results of an investigation into the frangible joint behavior of tanks designed to API 650 rules. In such tanks, the roof-to-shell joint is intended to fail in the event of overpressurization, venting the tank and containing any remaining fluid. The reasoning behind present API design formulas is reviewed. Combustion analyses, structural analyses, and the results of testing are presented. Results show that higher pressures are reached before frangible joint failure than predicted by the present API 650 calculation. One consequence is that (for empty tanks) uplift of the bottom can be expected to occur more frequently than predicted using API 650. However, uplift does not necessarily mean bottom failure. Instead, the relative strength of the shell-to-bottom and roof-to-shell joints will determine failure. This ratio is larger for larger tanks. Recommendations are made as to possible changes in the design approach of API 650.