Low cycle fatigue failure of pressurised storage tank

Various storage tanks are used by petroleum industries for storing crude and its fractions as the feed for final products. They are of different sizes and have dome shaped fixed roofs. These roofs are welded in segments and strengthened to withstand higher than atmospheric pressures in many cases. In case if the welding is not properly done, some voids and fissures along with microstructural anisotropy can persist. Particularly damaging is hydrogen pick up during welding. Since these tanks go through fluctuating pressures during filling in and releasing of the stored product, chances of low cycle fatigue during service are probable. This can result in the coalescence and progress of the voids and fissures. When critically of crack size is reached, an unstable and cataclysmic growth takes place and failures of the pressurised storage tank handling naptha fraction (60-90°C) as the feed for toluene and benzene separation units. Extensive microstructural and mechanical properties have been studied while investigating the failure caused by higher hydrogen present in the weld and also due to cyclic stress.