Case study of a pressure vessel operating with unacceptable defects

Pressure vessels (PV) for compressed air were installed 45 years ago in RHPP Bajina Basta. After 18 years of exploitation, unacceptable defects in welded joints were found during regular monitoring by NDT. Initial assessment of structural integrity, based on conservative fracture mechanics analysis, indicated a possibility to use the vessel in regular operating mode. Monitoring in the meantime was intensified, with regular NDT examinations every 6 months, using conventional NDT methods, Ultrasonic (UT) and Radiographic testing (RT). Recently, more advanced NDT UT methods were introduced, Phased Array Ultrasonic Testing (PAUT) and Time of Flight Diffraction (TOFD), to evaluate defect position and size more precisely. Also, the finite element method was applied to evaluate more precisely the stress state in the PV with a crack-like defect. In this way structural integrity and risk of failure was assessed more reliably. Finally, after 27 years of exploitation these PVs were replaced and welded joints with defects examined once again to get the final verdict.