Aeroderivative Gas Turbine Coupling Generator Redesign

A major failure event was experienced at a 44 MW plant powered by four aeroderivative gas turbines arranged in two units, property of the Federal Commission of Electricity (CFE). The failure consisted of total fracture in the shaft coupling between the generator and free-turbine. Unit 2 has a twin pack configuration with two aero derivative Pratt&Whitney 20 MW gas turbines coupled to one generator at both end sides. The “A” side generator coupling was completely damaged as well as the coupling configuration at the free turbine. Failure analysis showed as root cause, an abnormal configuration of the coupling systems between the free turbine to rotor generator at side “A”. This side had an additional shaft component to compensate a longer coupling distance between the turbine and generator. This was longer than the original distance, generating additional dynamic forces during operation leading to a fatigue failure mechanism. The replacement coupling configuration for the rotor generator was different than the Original Equipment Manufacturer (OEM). The new (non-OEM) spare rotor generator was shorter in the longitudinal direction than the original one, forcing the addition of a new shaft in one side of the generator. This work describes the rehabilitation process of the generator coupling by the replacement of the old configuration by a new redesigned coupling. This was done keeping the original configuration distances and components for both end shaft sides of the rotor generator. The paper includes the redesigned couple analysis by finite element method and the in-situ activities for the installation of the new couple in the rotor generator.