Assessment of the Perforation Risk for Atmospheric Storage Tanks Containing Solvents

The assessment of the integrity of the atmospheric storage tanks is needed to prevent releases of hazardous materials. In particular, the control of the localised losses of thickness, due to pitting or other phenomena, is essential for the bottom of the storage tanks, because these could lead to the perforation of the plates and leakages of materials with severe environmental consequences. Different techniques are available for the integrity measurement, but they always require the tanks is out of service, therefore, it has to be empty and reclaimed. Measurements can be repeated only after some years of further service. Nevertheless, the estimation of the perforation probability is important to assess the risk of major accidents as well as the environmental risk, associated with possible leakages. Unfortunately, discrete thickness measurements cannot determine with certainty the maximum corrosion depth of the bottom, where the materials usually exhibit localised corrosion in the form of pits. For this reason, to assess the perforation probability of the storage tank, a probabilistic approach based on the extreme value theory must be used. This work describes a model predicting the maximum expected damage with respect to the deterioration mechanisms affecting the bottom of the atmospheric storage tanks containing naphtha based solvents. The model is based on an analysis of the bottom damage, obtained by collecting thickness measurements. The application to a real case-study is presented.