In the present work, the structural integrity of a NGV tank type 1 – uncoated steel cylinder – and its relative tolerance to the presence of a probable active phase of growth of an initial semi-elliptical fissure located on the Cylindrical surface of the tank in two possible directions: circumferential and longitudinal is evaluated. The integrity evaluation employs the Level 3 procedure and the FAD Fault Diagram recommended by the API 579 (FFS) standard that evaluates the suitability of pressurized components in service. By means of the FAD Diagram, a graphical analysis of the behavior of the crack is carried out, which changes in size and determines the extent to which the integrity of the tank is compromised through the calculation of ratios, which depend on the load (internal pressure) and the Stress Intensity Factor (FIT). Also, the FIT, parameter of the fracture mechanics that characterizes the crack behavior, is determined by numerical simulation for the different sizes of propagation. Finally, the results confirm that under the operating conditions of the analyzed tank the growth of a crack due to a fluctuating pressure load reaches the leakage condition before the tank failure condition, in a time greater than the established period By Peruvian regulations for the technical review of these equipment.