Analyzing Fatigue Limits and Real-Life Performance of Life Limited Components in the Field of Power Generation

Recent developments in the energy sector, driven by concerns on global warming, have triggered the energy transition especially within fossil-fuel based energy systems. Within this framework, Baker Hughes (BH) is designing new equipment to operate more efficiently, cleanly, and sustainably. To better understand the product requirements, research within the company databases has led to a better and more contemporary understanding of the expected operating profile of the equipment in real use. Two representative components of a new machine under development have been selected for a deeper analysis. This master’s Thesis focuses on the fatigue limits of these two components with limited life, part of a complex machine in use in the field of power generation. Understanding their fatigue limits is crucial for ensuring the structural integrity and reliability of the machine in demanding operational conditions. This study involves a comparison approach between the fatigue life limits and the accumulation profile of hours and cycles in time in the field. Firstly, the stress and strain distributions, obtained by a numerical simulation, will be calculated in a normal startup/shutdown mission and a normal startup/emergency shutdown mission. Analyzing the results of emergency shutdown cycles is necessary since they are known to be more damaging and require a higher severity factor for determining the accumulated cycles of the parts. Since the results of the simulations include local stresses that exceed the Yield Strength (but remain below the cyclic plasticity threshold) the approach for the Low Cycle Fatigue (LCF) study will be based on the Pseudo-stress method, for 1st cycle Yielding. The LCF is based on experimental strain-life curves on the material. One of the two components is impacted by a thermal gradient, so it is necessary to evaluate the full missions. In this case, the additional Rain-flow counting method will be used to extract the stress cycles during the normal startup/shutdown and startup/emergency shutdown cycles. Then the life will be calculated for each stress cycle and accounted for using Miner's rule. For both components, the crack propagation study developed is based on the Linear Elastic Fracture Mechanics (LEFM) calculations. Through LEFM, the stress intensity factor and the crack propagation can be evaluated using the elastically calculated stresses in the FEM analysis. This approach is based on the guide to methods for assessing the acceptability of flaws in metallic structures. These results will be compared with the life data of similar components of a machine of older design. Secondly, the cycles results will be compared with the expected utilization in the field, to offer a wider perspective on the components’ operation over time. This second analysis focuses on the accumulation curves of cycles and hours based on the historical data within BH’s databases. This is done to account for the accumulated cycles and hours between maintenance intervals for different types of operating profiles and based on this data, the difference between the life limiting curves and the actual cycles accumulation curves. The fatigue limits of the two components have been considered adequate and exceeding the expected life requirements coming from the current requirements set by the market conditions. Also, results of the analysis shows that the machine’s maintenance intervals currently in use are extremely safe regarding the cycles accumulation in the field.