Reliability study of liquid ammonia storage tank based on failure assessment diagram

Liquid ammonia storage tanks are subjected environmental factors and stresses during operation, which may lead to localized corrosion or stress corrosion cracking. These corrosion accidents can result in significant environmental damage and financial losses. In this study, finite element analysis (FEA) was used to conduct a stress analysis on the longitudinal weld of a 50,000 m³ liquid ammonia storage tank, identifying regions with higher stress. The Failure Assessment Diagram (FAD) method, as outlined in the BS7910 standard, was utilized to evaluate the acceptability of cracks in these high-stress regions, and the remaining life of these cracks growing to critical size was estimated. The results indicated that the peak stress in the longitudinal weld was 366.87 MPa, occurring at the outer boundary of the junction between the longitudinal weld and the base plate of tank. The critical half-depth of a crack originating at this location was determined to be 10.5 mm. By using the Paris equation, it was estimated that the number of cycles required for a crack in a high-stress region of the longitudinal weld to propagate to its critical dimension was 8.69×10⁵ cycles.