API650-2008 is one of the prominent codes consisting of seismic specifications to design steel storage tanks for earthquakes resistance. In spite of the code's broad application, there are some failure modes such as slide bottom, elephant-foot buckling, sloshing and uplift needing more evaluation. In this paper, 161 existing tanks in an oil refinery complex have been classified into 24 groups and investigated using both API650-2008 rules and numerical FEM models. Failure modes and dynamic characteristics of studied models have been calculated by numerical FEM analysis and compared with code requirements. The results demonstrate that, in some cases, there are some imperfections in the code requirements that require further investigation. ► Seismic performance of liquid storage tanks in an oil refinery complex investigated. ► Failure modes of studied steel storage tanks calculated by FEM analysis. ► Numerical FEM analysis results compared with API650-2008 code requirements. ► We report some imperfections of API650-2008 for seismic design requirements of tanks.