Structural integrity evaluation of CNG pressure vessel with defects caused by heat treatment using numerical analysis

Abstract

Natural gas vehicles use natural gas as a raw material; thus, compared to conventional diesel vehicles, there is no soot or fine dust produced, the noise generated is half, and the amount of air pollution is one tenth. Despite a considerable increase in their demand, there is still much controversy in terms of their safety, and safety-related accidents have frequently occurred in recent years. According to reports, most compressed natural gas (CNG) bus accidents are caused by the rupture of the pressure vessel, and the reason for this accident is that the containers show a high number of manufacturing defects. In the case of a metallic liner, the mechanical properties and rupture characteristics of the pressure vessel vary greatly depending on the method and conditions of the heat treatment process. Several accidents where pressure vessels have ruptured have been reported, and all these ruptures occurred owing to brittle fracture of the container because the mechanical properties, such as the tensile strength and the fatigue strength of the CNG vessel changed in comparison with the normal vessel owing to poor heat treatment processes. In this study, we compared and analyzed the structural integrity and fatigue characteristics of the normal vessel and the poorly heat-treated vessel under charging conditions and under an atmospheric environment, where the vessel is mounted on an actual CNG bus. This comparison is made through a heat convection and conduction analysis using SINDA/FLUENT and a structure and fatigue analysis using ADINA and winLIFE.