Design enhancements to a gas turbine's exhaust system used for naval propulsion

Abstract

The main objectives of the present research were to study the mechanical and the thermal behaviour of a gas turbine's exhaust system used for naval propulsion and to assess several design changes to be introduced in it in order to reduce the induced stresses, and thus to increase its fatigue life.Several welded plates of different thicknesses compose the exhaust system, ranging from 4 mm to 15 mm + 15 mm (corresponding to two bolted flanges); in addition, the high number of supports and reinforcements originate a highly restrained structure with non-uniform expansion capability under heating and cooling cycles. Hence, thermal shock, in conjunction with different thermal inertia of the structural parts, originated thermal fatigue crack nucleation and propagation from the weld toe of some fillet and butt-welded joints located near the intermediate and lower support ring.The critical region of the exhaust system was instrumented with ten J-type thermocouples that enabled the mapping of the temperature distribution and a uniaxial encapsulated strain gage was installed by spot welding on the exhaust system's surface, near the intermediate flange, where cracks were detected. Therefore, the original exhaust system was studied through Finite Element Analysis, and several design enhancements were assessed. The use of a 445M2 ferritic stainless-steel marine grade type resulted in lower stresses at critical regions (−30%), while its pitting corrosion resistance equivalent number is comparable to that of the AISI 316L stainless steel; in addition, the standardisation of the plate thicknesses, either using 12 mm or 4 mm, could induce a stress reduction at the critical regions of the exhaust system (−61% or −54%, respectively). Moreover, a redesigned exhaust system geometry can be considered to replace the original geometry and a reduction of about 35% was calculated for the maximum principal stress at the lower regions of the supports.Finally, periodic surveys should be carried out during service to prevent crack propagation at the weld toes, and high-quality fabrication is required to avoid the presence of initial defects in the structure.