A failure analysis of exhaust systems for naval gas turbines. Part II: Design changes

Cracks have nucleated and propagated through the exhaust systems of naval gas turbines. The main cracks have propagated, along the circumferential direction, from the weld toe of some butt and fillet welded joints located near the lower supporting ring, where the temperature measured with a Cu–Ni thermocouple was about 350 °C. The exhaust system studied has more than 8 m long and the dimensions of the cross sections varies from 1.5 m × 1 m at the inlet section to 2.6 m × 2 m at the outlet section. The structure is thermally insulated and is made of AISI 316L stainless steel grade type plates, with a wall thickness of about 4 mm, which were bent in roll machines and mainly butt welded by MIG/MAG welding process. This paper presents finite element analyses (FEA) of the exhaust system outlined before. Several elastic boundary conditions were simulated and the nominal stresses induced by the relative pressure, the thermal stresses, the inertial loads and the self-weight were determined. Moreover, the theoretical stress concentration factors of the critical butt and fillet welds were also calculated. The first frequencies and modes of vibration were determined. Design changes to the original exhaust system were then analysed in order to diminish the stresses and prevent further failures.