Risk Assessment of Pressure Vessels by Using Fracture Mechanics and Advanced Ultrasonic Testing

Risk assessment of cracked cylindrical pressure vessels for compressed air by using basic fracture mechanics and advanced methods of non-destructive testing (NDT), such as Phased Array Ultrasound (PAUT) and Time-of-Flight Diffraction (TOFD), is presented. Basic fracture mechanics equations are used to calculate the stress intensity factor, KI, in the case of unacceptable defects found in the pressure vessel 970 in the Reversible Hydro Power Plant "Bajina Basta", and get the ratio KI/KIc, according to the minimum measured values for fracture toughness for welded joints, as the typical zones where crack-like defects are found. The ratio Snet/Sc, where Snet is the net stress in cross-section with a crack, and Sc is the critical stress, is then evaluated to define the operating point in the Failure Assessment Diagramme (FAD), and thus, to estimate the likelihood of failure of the pressure vessel 970. In combination with estimated high consequence for the pressure vessel 970, the risk matrix was used, as a simple tool to assess the risk. In this paper the focus is on one aspect of this procedure, being the NDT role, since it is of utmost importance to use as precise as possible method for detected defects in welded joints. In this paper advanced ultrasound methods, PAUT and TOFD, are used to get precise image of defects in pressure vessel 970 welded joints, which were previously detected by conventional NDT methods. It is shown that the use of PAUT and TOFD is of utmost importance for decision making process in this case.